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ElectrochemistryDictionary
Simple and brief definitions of words and phrases used often inelectrochemistry. In some cases, a second paragraph provides furtherinformation for the "more scientifically minded."(http://electrochem.cwru.edu/ed/dict.htm) Revision date: November 8, 2001.
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There are presently more than 600 entries in the dictionary, but it is far from complete. More terms are added at irregular intervals. The selection process to include terms is purely random, and it usually results in a chain reaction. Suggestions of terms to be included are welcome, together with corrections of the inevitable errors. Send them to:
Zoltan Nagy
Materials Science Division
Argonne National Laboratory
Argonne, Illinois 60439-4845, USA
E-mail:nagy@anl.gov
There are also numerous educational WWW sites where more detailed information can be found about many of the terms listed in this dictionary:http://electrochem.cwru.edu/estir/inet.htm#educ.
See also a listing of popular science books and articles in popular magazines:http://electrochem.cwru.edu/estir/pop.htm
Visit also the sister sites of the Dictionary:
Electrochemistry Encyclopedia (http://electrochem.cwru.edu/ed/encycl/)
Electrochemical Science and Technology Information Resource (ESTIR) (http://electrochem.cwru.edu/estir/)
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Symbol and abbreviation ofampere.A process "to take in and incorporate." E.g., light can be "absorbed" by a material. In chemistry, a term often used to describe the dissolution of a gas into a liquid or solid. The dissolving gas is said to be "absorbed." Or a liquid substance can be "absorbed" by a solid. This is a bulk process, not to be confused withadsorption.Seerechargeable battery.A compound thatdissociates to produce hydrogen (H+)cations when dissolved in water. See alsopH.SeepH.Theoverpotential associated with thecharge-transfer reactionelementary step in the overallelectrode reaction.The activity of a dissolved species in solution is the "effective"concentration of that species.In an "ideal" solution, themolecules in the solution do not interact with each other and the concentration and the activity are identical. This is the case for very dilute solutions. In a "real" solution, there is a certain interaction between the molecules resulting in a diminished "activity" of the molecules toward the outside world, and the solution behaves like it would contain lower concentration of the dissolved species than it actually does. The activity can be expressed as the product of an "activity coefficient" and the concentration.
Seeactivity.A material that isadsorbed.An increase of theconcentration of asolute in the vicinity of a solid surface, over that in the bulk of the solution, due to the attractive interaction between the solid immersed into the solution and the solute. Adsorption on a solid from a gaseous phase also occurs. It is a surface process, not to be confused withabsorption. Opposite:desorption.Symbol and abbreviation ofampere-hour.Seebase.SeepH.Seebrine electrolysis.SeeEdison battery.A modern version of theLeclanche cell containingbasic (potassium hydroxide)electrolyte. It has considerably improved characteristics and it is slowly replacing the Leclanche cell.Seecurrent.Aluminum metal is produced byelectrolysis of aluminum oxide dissolved in a high-temperature molten-saltelectrolyte. Aluminum is deposited as a liquid metal on thecathode of theelectrolytic cell (the aluminumcations arereduced to liquid metal). This is the only large-scale industrial process for the production of aluminum. See also anEncyclopedia Article.
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An alloy of mercury and another metal.Instrument used for the measurement ofcurrent.Symbol and abbreviation ofampere.Measurement unit ofcurrent. Abbreviation: "A" or "amp".An alternative unit of electrical charge. One ampere-hour = 3,600coulumbs. Symbol: "Ah".Anelectroanalytical technique based upon the measurement of thecurrent flowing through theworking electrode of anelectrochemical cell.Seegalvanostat.A substance whose chemical composition is to be determined by chemical analysis.A substance that does not contain water. The opposite ofhydrous.A negatively chargedion.Theelectrode whereoxidation occurs in anelectrochemical cell. It is the positive electrode in anelectrolytic cell, while it is the negative electrode in agalvanic cell. Thecurrent on the anode is considered a positive current according to international convention; however, inelectroanalytical chemistry the anodic current is often considered negative.A condition in anelectrolytic cell that produces an abrupt increase incell voltage and a decrease incurrent flow. It is usually caused by the temporary formation of aninsulating layer on theanode surface. It occurs almost exclusively in molten saltelectrolysis, such as inaluminum production.The insoluble residue that derives from theanodicdissolution of an impure metal such as copper duringelectrorefining. Also called "anode slime."Seeanode mud.Seepartial current density.A process forcorrosion protection of a metal or alloy achieved by impressing upon the metal ananodiccurrent of sufficient magnitude to cause the formation of apassive film. Anodic protection is effective only for metals that are prone to passivate, such as stainless steel and titanium. See also anEncyclopedia Article.A process to produce an oxide film or coating on metals and alloys byelectrolysis. The metal to be treated is made theanode in anelectrolytic cell and its surface is electrochemicallyoxidized. Anodization can improve certain surface properties, such ascorrosion resistance, abrasion resistance, hardness, appearance, etc. One metal very often anodized is aluminum, all the above properties improve, furthermore, since the surface film is porous, the aluminum metal can even be colored by the application of pigments or dies in the pores.Theelectrolyte on theanode side of anelectrochemical cell that isdivided into compartments.A solution with water as the solvent.The smallest physical unit of achemical element that can still retain all the physical and chemical properties of that element. Atoms combine to formmolecules, and they themselves contain several kinds of smaller particles. An atom has a dense central core (the nucleus) consisting of positively charged particles (protons) and uncharged particles (neutrons). Negatively charged particles (electrons) are scattered in a relatively large space around this nucleus and move about it in orbital patterns at extremely high speeds. An atom contains the same number of protons as electrons and thus is electrically neutral and stable under most conditions.The average relative weight of achemical element as it occurs in nature referred to some element taken as a standard.Seecounter electrode.
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Seeresidual current (density).A compound thatdissociates to produce hydroxyl (OH-)anions when dissolved in water (also called "caustic" or "alkali"). See alsopH.SeepH.Anelectroplating solution is often called "bath."A device thatstoreselectrical energy usingelectrochemical cells. Chemical reactions occur spontaneously at theelectrodes when they are connected trough an external circuit, producing an electricalcurrent. The physical construction of the battery is such that it does not permit the intermixing and consequent direct reaction of the chemicals stored in it. See alsorechargeable battery andnon-rechargeable battery.Strictly speaking, a battery should consist of several, internally connected,electrochemical cells. (The individual cells in a battery can beseries orparallel coupled, or a combination of both.) However, in present usage all storage devices (singlecell and multiple cell) are called batteries.
Seecharger.Electrochemistry of biological systems and biological compounds.Anelectrode that is shared by twoseries-coupledelectrochemical cells in such a way that one side of the (usually planar) electrode acts as ananode in one cell and the other side acts as acathode in the other cell. In storagebatteries andfuel cell stacks many cells are usually connected internally, and it is a very efficient design feature to use a single planar structure for electrodes in two neighboring cells and also as the electrical interconnection between them.Seebrine electrolysis.Seehydrodynamic boundary layer.Small amounts of (usually organic) compounds added to anelectroplating solution that changes the mechanism of the plating to produce "bright" (mirror like) metal deposits.Electrolysis of anaqueous solution of common table salt (sodium chloride), also called "brine," results in the production of chlorine gas at theanode and hydrogen gas at thecathode. Since the hydrogen is produced by breaking up watermolecules, the solution is becomingbasic around the cathode and a solution of sodium hydroxide (also called "caustic" or "alkali") is produced. If the electrolysis is carried out in adivided cell, the products are chlorine, caustic, and hydrogen. If the electrolysis is carried out in an undivided cell and the chlorine gas and the caustic are allowed to mix and react with each other, sodium hypochlorite (bleach) is produced if the cell operates close to room temperature, and sodium chlorate is produced if the cell is operated near the boiling point of water. Electrolysis is the only large-scale industrial process for the production of chlorine gas and these chlorine chemicals. Smaller scale cells are also used to produce chlorine-based disinfectants in municipal water-treatment plants and for swimming pools. See also anEncyclopedia Article.The overall cell reaction is:
2NaCl + 2H2O = Cl2 + H2 +2NaOH
Chlorine gas and sodium hydroxide react to form sodium hypochlorite and sodium chloride:
Cl2 + 2NaOH = NaOCl + NaCl + H2O
Sodium hypochlorite will react further at high temperature to form sodium chlorate and sodium chloride:
3NaOCl = NaClO3 + 2NaCl
(although, sodium chlorate can also form by direct electrochemical oxidation).
A solution with a constant, specifiedpH. The pH of the solution "resists" any change: addition of small amounts of solvent or evenacid orbase will not appreciable change the pH. This is called "buffer capacity."Anoninsulatedconductor used to carry a largecurrent or to make a common connection between several circuits.
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Symbol and abbreviation ofcoulomb.A commonly usedreference electrode. It is very similar to thesilver/silver-chloride electrode both in construction and in theory of operation. The silver metal is replaced by mercury (electrical connection is made by an inert metal wire), the salt is mercury chloride, and the solution issaturated potassium chloride.Theequilibrium electrode potential is a function of the chlorideconcentration of theinternal electrolyte ("filling solution"). The electrolyte is practically always saturated potassium chloride (hence the name: "saturated calomel electrode," SCE, "calomel" is an old name for mercurous chloride), producing apotential of 0.244 volt against thestandard hydrogen electrode at 25oC.
The capacitance value expresses the ability of acapacitor tostore electrical charge. The unit of capacitance is thefarad.Thecurrent (orcurrent density) flowing through anelectrochemical cell that is charging/discharging theelectrical double layer capacitance. This current does not involve any chemical reactions (charge transfer), it only causes accumulation (or removal) of electrical charges on theelectrode and in theelectrolyte solution near the electrode. There is always some capacitive current flowing when thepotential of an electrode is changing, and the capacitive current is generally zero when the potential is constant. Also called "non-faradaic" or "double-layer" current. Contrast withfaradaic current.Capacitive current can also flow at constant potential if the capacitance of the electrode is changing for some reason, e.g., change of electrode area or temperature.
An electrical device which serves tostore electricity orelectrical energy. It has three essential parts: two electricalconductors, which are usually metal plates, separated and insulated by the third part called the dielectric. The plates are charged with equal amounts of positive and negative electrical charges, respectively. This is a "physical"storage of electricity as compared with the "chemical" storage in abattery.Seecapacitance. The term "capacity" is also used in a somewhat different meaning forbatteries: it expresses the total amount of electrical charge a battery is able to hold. It is usually expressed inampere-hours.A tube with very small bore.Alternative name for "electrophoresis." Seeelectrokinetic effects.Theelectrode wherereduction occurs in anelectrochemical cell. It is the negative electrode in anelectrolytic cell, while it is the positive electrode in agalvanic cell. Thecurrent on the cathode is considered a negative current according to international convention; however, inelectroanalytical chemistry the cathodic current is often considered positive.Seepartial current density.Theelectrolyte on thecathode side of anelectrochemical cell that isdivided into compartments.A positively chargedion.Seebase. Sometimes it specifically refers to sodium hydroxide.Seebrine electrolysis.Stands forclosed-circuit voltage.Seeelectrochemical cell.Seeconductivity cell.SeeseparatorAseries-coupled assembly of cells, a term used primarily in industrialelectrolysis usingelectrolytic cells.The overall chemical reaction occurring in theelectrochemical cell. It is the sum of the twoelectrode reactions.Theelectrical potential difference between the twoelectrodes of anelectrochemical cell. In case of athree-electrode cell, the potential difference between theworking electrode and thecounter electrode."Cell voltage" usually refers to nonequilibrium conditions, that is whencurrent is flowing through the cell (although this convention is not always followed). The "cell voltage" differs from theelectromotive force (emf) (oropen-circuit voltage (ocv)) of the cell by the amount of theovervoltage. The term "voltage" is usually reserved for the case when anelectrochemical cell is under consideration, while the term "potential" is usually reserved for the case when anelectrode is considered. (Of course, the latter case is still an "electrochemical cell" consisting of the electrode under consideration and areference electrode.) Unfortunately, the terms "voltage" and "potential" are sometimes used interchangeably.
The process of spontaneous reduction of the ions of a metal by another metal above it in theelectromotive series. For example, a piece of iron immersed in copper sulfate solution will be immediately covered by a thin film of copper. The iron is beinganodicallydissolved while copper iselectroplated on its surfacecathodically. Also called "metal displacement reaction."
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Charge referred to the unit area of theelectrode. Charge divided by electrode area.Seeactivation overpotential.A chemical reaction where an electrical charge (usually anelectron) is transferred from onereactant to another. See alsoheterogeneous charge-transfer reaction andhomogeneous charge-transfer reaction. In case of anelectrode reaction, theelectrode itself is considered one of the "reactants." Anelectrode reaction is always a heterogeneous charge-transfer reaction.A characteristic quantity for anelectrode reaction indicative of its inherent speed: a large charge-transfer resistance indicates a slow reaction. See alsonon-ohmic resistance.The phenomenon of movement (transportation) of electrical charge from one part of the system to another, occurring throughelectromigration.Seemass transport for a discussion of the decoupling of mass and charge transport.
An electrical device used tocharge arechargeable battery using household electricity.A process to "fill" arechargeable battery with electricity by applying acurrent to itsterminals. The process will causeelectrochemical reactions to occur in the battery,storing the electricity in chemical form. In contrast, during the charging of acapacitor the electricity is stored as electrical charges, without causing any chemical reactions to occur. Opposite:discharging.Seekinetics.Seebrine electrolysis.Seebrine electrolysis.Seebrine electrolysis.Seebrine electrolysis.An electrochemical measuring technique used forelectrochemical analysis or for the determination of thekinetics andmechanism ofelectrode reactions. A fast-risingpotential pulse is enforced on theworking electrode of anelectrochemical cell and thecurrent flowing through this electrode is measured as a function of time. See alsoCottrell equation.An electrochemical measuring technique used forelectrochemical analysis or for the determination of thekinetics andmechanism ofelectrode reactions. A fast-risingpotential pulse is enforced on theworking electrode of anelectrochemical cell and the electrical charge passing through this electrode is measured as a function of time.An electrochemical measuring technique used forelectrochemical analysis or for the determination of thekinetics andmechanism ofelectrode reactions. A fast-risingcurrent pulse is enforced on theworking electrode of anelectrochemical cell and thepotential of this electrode is measured against areference electrode as a function of time.In an unstirred solution, the potential will rise to theelectrode potential of thereaction requiring the least amount of energy to proceed, and it will increase in time due to theconcentration overpotential developing as theconcentration of thereactant is exhausted at theelectrode surface. If thecurrent is larger than thelimiting current, eventually thediffusional process will not be able to provide the required flux for the current, and the electrode potential will sharply rise (at thetransition time) until it reaches the electrode potential of the next available reaction in the solution, and so on. See alsoSand equation.
Anamperometric sensor assembly used for the measurement of dissolved oxygenconcentration in water oraqueous solutions. It is a two-electrodeelectrochemical cell with theworking electrode (typically positioned at the end of a tubular structure) separated from the test solution by a thin membrane permeable to oxygen. The oxygendiffusing through the membrane isreduced at the electrode and thecurrent produced is proportional to the concentration of the dissolved oxygen (calibration required).Thevoltage of abattery when it isdischarging (on-load condition).
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An assembly that combines anion-selective electrode and areference electrode in one physical structure (typically in a tubular form).This can be conveniently used for the determination ofionicconcentrations in test solutions. It is most often used forpH measurements.Seefuel cell.See theHelmholtz model of the double layer.Alternative expression forideal polarizable electrode.The maximum value of thecurrent andvoltage that a control instrument (e.g.,galvanostat orpotentiostat) is capable to provide.The measure of the amount of dissolved material (solute) in a solution. It can be expressed in a variety of ways. Expressions in weight percent, and grams of solute per liter of solution are common. A more fundamental way to express concentration is used in chemistry: the "molar" concentration. A solution is considered one molar (1 M) if it contains as many grams of solute per liter of solution as is themolecular weight of the solute (the so calledgram-mol). This provides an atomistically fundamental expression because one gram-mol of any material will contain the same (and very large) number ofmolecules. One gram-mol of hydrogen gas contains the exact same number of molecules as one gram-mol of table salt (sodium chloride), even though the latter is much heavier. In this dictionary, the term "concentration" always designates "molarity" unless otherwise specified.Agalvanic cell in which the chemicalenergyconverted intoelectrical energy is arising from theconcentration difference of a species at the twoelectrodes of the cell. An example is adivided cell consisting of two silver electrodes surrounded by silver nitrate solutions of different concentrations. Nature will tend to equalize the concentrations. Consequently, silvercations will be spontaneouslyreduced to silver metal at the electrode (cathode) in the higher concentration solution, while the silver electrode (anode) in the lower concentration solution will beoxidized to silver cations.Electrons will be flowing through the external circuit (from the anode or negative electrode to the cathode or positive electrode) producing acurrent, and nitrateanions willdiffuse through theseparator. This process will continue till the silver nitrate concentration is equalized in the two compartments of the cell.Theoverpotential associated with thediffusional transport of thereactants to theelectrode surface from the bulk of theelectrolyte and the reverse transport of the products. The diffusion is anelementary step in the overallelectrode reaction. Also called "diffusion overpotential" or "mass-transport overpotential."Seecapacitor.Seeconductivity.A polymeric material (e.g., plastics) havingelectronic conductivity.The measure of a material's capability to carryelectrical current.The reciprocal ofresistivity.
A cell specially designed for the measurement of theconductivity of anelectrolyte solution. It is a small vessel containing two metallicelectrodes, the cell is filled with the solution to be measured. Also called "conductance cell."The measurement of the conductivity of an electrolyte solution is more complicated than a similar measurement with a metallic conductor. When measuring withdc current, one would have to take into consideration theelectromotive force of theelectrochemical cell, and thepolarization of the electrodes. Therefore, the measurements are typically carried out with high frequencyac current and the electrodes in the conductivity cell are typically made ofplatinized platinum to avoid these complications. The cell geometry usually does not ensure that exactly and only one cubic centimeter of solution will carry the current; therefore, the cell has to be calibrated to obtain thespecific conductance of the solution. The calibration is usually carried out with high purity potassium chloride solutions, and the resulting calibration constant is often called the "cell constant."
Anelectroanalytical technique based upon the measurement of theconductivity anelectrolyte solution.A material that is capable to carry anelectrical current. See alsoelectronic conductor andionic conductor.A technique used inelectroanalytical chemistry or in the determination of thekinetics andmechanism ofelectrode reactions or a process carried out in anelectrolytic cell that operates at constantcurrent. See alsochronopotentiometry.A technique used inelectroanalytical chemistry or in the determination of thekinetics andmechanism ofelectrode reactions or a process carried out in anelectrolytic cell that operates at constantpotential. See alsochronoamperometry andchronocoulometry.Adsorption with the adsorbedmolecule orion being in direct contact with the solid surface.Amass-transport mechanism that involves bulk movement of a solution (contrast withdiffusion that involves individualmolecules orions). We differentiate "forced" convection from "natural" convection. The simplest example of forced convection is mechanical stirring. If a non-uniform solution is stirred, thesolute is "transported" from the highconcentration parts of the solution to the low concentration parts till the solution becomes completely uniform. Other examples of forced convection are the "flow" of a solution through a pipe or a porousseparator driven by pressure difference. "Natural" convection is very important in electrochemistry. It always occurs at the surface of anelectrode carryingcurrent in the absence of "forced" convection. Aselectrode reaction is proceeding, the buildup of reaction products and the consumption ofreactants changes the density of the solution layer close to the electrode surface compared to that of the bulk solution. Eventually, this density difference will force the surface solution layer to sink or rise, setting up a "natural stirring" action close to the electrode surface which will tend to equalize the surface and bulkconcentrations. As a "rule of thumb," natural convection starts after after about a minute of current flow.Seeenergy conversion.A chemical (often electrochemical) process that destroys structural materials. Typically it refers to corrosion of metals, but any other material (e.g., plastic or semiconductor) will also corrode. The simplest example of metallic corrosion is the rusting of iron in air. Iron is spontaneouslyoxidized by the oxygen in air to iron oxides (while the oxygen is beingreduced). Metallic corrosion is very often an electrochemical process. It is always electrochemical when the metal is immersed in a solution, but even in atmospheric corrosion a thin film of condensed moisture often covers the surface. The metal in the corrosive solution essentially acts as a short-circuitedgalvanic cell. Different areas of the surface act asanode andcathode, at the anodic areas the metal isoxidized to an oxide while at the cathodic areas the dissolved oxygen is beingreduced. The spontaneous complementary oxidation/reduction processes of "rusting" are spatially separated while an electricalcurrent is flowing "internally" from one part of the corroding metal to another; the current is totally "wasted" as it produces no useful work but only generates heat. (A cell arrangement like this is often called a "local cell.") Seecorrosion current andcorrosion potential. See also anEncyclopedia Article.Corrosion products are typically oxides, but other products (e.g., sulfides) can also form depending on the environment. Corrosion always involvesoxidation of the corroding material in the general sense of the term.
Thecurrent flowing in acorrosion "local cell" (often, but not always, understeady-sate conditions). Theanodic andcathodic currents must be equal, but thecurrent densities may be different depending on the area ratio. The corrosion current is closely related to the concept ofcorrosion potential. See also anEncyclopedia Article.A chemical that stops (or at least decreases the rate of) acorrosion process. The inhibitor can be added to an otherwise corrosive solution (often a very smallconcentration will accomplish the goal) or it can be incorporated in a coating applied to the metal surface. See also anEncyclopedia Article.Theelectrode potential of acorroding metal. It is a "mixed potential" with a value that is in between theequilibrium potentials of theanodic andcathodic corrosion reactions. See also anEncyclopedia Article.The corrosion is a spontaneous, dynamic phenomena withelectrode reactions taking place and acurrent flowing. Consequently, both reactions arepolarized and their potentials approach each other; as a matter of fact, they must become equal to preserve a single potential for the metal.However, the two reactions are not necessarily equally polarized. Theoverpotential of the two electrode reactions will be generally different, and their values will be dictated by the requirement that theelectrode potentials be equal (at the "corrosion potential") at one, uniquely defined current (thecorrosion current). See theTafel equation for a relation between overpotential and current. (Their drops in the solution and the metal are ignored in the above discussion, this is justified by the close proximity of the anodic and cathodic areas on the corroding metal.)
A relation betweendiffusion limited current density and time in achronoamperometric experiment, assuming that thepotential excursion is sufficiently large to immediately result in limiting current. The equation is valid only for planarelectrodes in unstirred solution.The diffusion current density is inversely related to the square root of time, or expressing it differently: the product of i(t) × t0.5 is a constant. The constant is proportional to theconcentration of thereactant and to the square root of thediffusion coefficient of the reactant. Because the equation was derived for an unstirred solution, it ceases to be valid oncenatural convection starts.
Measurement unit of the electrical charge. Symbol: "C".The charge passing a given point during one second when thecurrent. is oneampere.
Instrument used for the measurement of electrical charge.Anelectroanalytical technique based upon the measurement of the amount of electrical charge passed through theworking electrode of anelectrochemical cell.An electrochemical measuring technique forelectrochemical analysis or for the determination of thekinetics andmechanism ofelectrode reactions based on the control of the amount of charge flowing through the system.Instrument used to count the number of small particles (e.g. biological cells) in a given volume of a suspension by monitoring decreases inelectrical conductivity through a small orifice caused by the particles passing through the orifice.Anelectrode in athree-electrode cell that is used only to make an electrical connection to theelectrolyte so that a current can be applied to theworking electrode. The processes occurring on the counter electrode are unimportant, it is usually made of inert materials (noble metals or carbon/graphite) to avoid its dissolution. This is the case for cells used for research or forelectroanalytical purposes. Of course, for many practically used cells, the processes occurring on both electrodes can be very important. Also called "auxiliary" electrode.The mobileion inion exchange. The ion with opposite charge to that of the fixed site on the ion-exchange resin. Contrast withfixed ion.
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Theoverpotential associated with the crystallization step inelectrocrystallization. The crystallization is anelementary step in the overallelectrode reaction.The movement of electrical charges in aconductor; carried byelectrons in anelectronic conductor and byions in anionic conductor. "By definition" the electrical current always flows from the positivepotential end of the conductor toward the negative potential end, independent of the actual direction of motion of the differently charged current carrier particles. Two kinds of currents must be distinguished: "direct current (dc)" and "alternating current (ac)." Direct current is the unidirectional continuous flow of current, while alternating current is the oscillating (back and forth) flow of current. In electrochemistry, we almost always use direct current. Consequently, the term "current" always designates "dc" in this dictionary unless specifically stated to be "ac." The normal household current is an alternating current.As mentioned above, the "defined" current flows from the positiveterminal of thecurrent source, trough theload, to the negative terminal of the source. Consequently, inside the "source" (whether it is electromechanical or electrochemical) the current must flow from the negative terminal to the positive terminal since there must be a complete circuit. This concept is especially important in electrochemistry because anelectrochemical cell can be either a current "source" (galvanic cell) or a "load" (electrolytic cell). Furthermore, arechargeable battery operates as a "source" duringdischarge and as a "load" duringcharge.
Current flowing through anelectrochemical cell is usually the sum of thecapacitive current and thefaradaic current.
A structural part of a complicatedelectrode assembly. Its primary purpose is to conduct the electricity between the actual working (reacting) parts of the electrode and theterminals.Seecompliance limits.The ratio between of thecurrent flowing through a compartment of anelectrochemical cell and the volume of that compartment (e.g.,anodic orcathodic current concentration). It is an often-used parameter in cell-design engineering.Current referred to the unit area of theelectrode. Current divided by thetrue electrode area.The localcurrent density on anelectrode as a function of position on the electrode surface. Most processes operate best when the current distribution is "uniform." That is, when the current density is the same at all points on the electrode surface. See alsoprimary,secondary, andtertiary current distribution.The fraction, usually expressed as a percentage, of thecurrent passing through anelectrolytic cell (or anelectrode) that accomplishes the desired chemical reaction. Inefficiencies may arise from reactions other than the intended reaction taking place at the electrodes, or side reactions consuming the product. The expected production can be theoretically calculated and compared with the actual production.A common characterization of anelectrode or anelectrochemical cell. Thecurrent (or more often thecurrent density) is plotted against theelectrode potential orcell voltage. See alsoTafel equation.Seeelectrical source.Seecurrent-potential plot.Seecurrent efficiency.Stands for "cyclic voltammetry," seevoltammetry.Involtammetry: a complementary pair of forward and reversepotential sweeps. Forrechargeable batteries: a complementarydischarging and charging processes.The number of times arechargeable battery can be "cycled" (charged anddischarged) before it loses its ability to accept charge. The processes occurring in the battery are not completely "chemically"reversible, and after repeated charging/discharging the battery will accept less and less charge till it becomes useless as a practicalenergy storage device. Some batteries can be recharged hundreds to thousands times.Seevoltammetry.
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A very earlynon-rechargeable battery. It consisted of a glass jar containing copper and zincelectrodes,each immersed in their respectiveacidic sulfate solutions. The two solutions were separated by a porous clay cylinderseparator. It was agalvanic cell in which the spontaneouselectrodissolution of zinc andelectroplating of copper provided the electricalcurrent. It was one of the earliest practical "laboratory"electrical sources; but, of course, it was not much use outside the laboratory.Theelectrode potential (cell voltage) at which a "measurable"electrolysiscurrent begins to flow. This is a qualitative parameter since "measurable" is rather subjective.Seereserve battery.Process for removal of grease, oil, etc from metal surfaces in preparation forelectroplating. Typically, the metal is immersed in hot, stronglybasic solution or in organic solvents to remove and dissolve these coatings. See alsoelectrolytic degreasing.Seedesalination.Seedesalination.A crystalline shape produced by skeletal growth ("dendritic growth") resulting in "tree-like" appearance (often with many branches) inmetal deposition.An archaic expression (hardly used any more) for a material added to abatteryelectrode for reducing thepolarization upon application of acurrent. It usually completely changed the nature of theelectrode reaction.Seemetal deposition/dissolution reactions.For arechargeable battery: the fraction, usually expressed as a percentage, of the totalelectrical energystored in a battery bycharging that was recovered bydischarging at a certain point of time. Contrast withstate of charge.A process to produce clean (potable) water from brackish or seawater.Electrodialysis is an electrochemical technique often used for this purpose.The opposite process ofadsorption. The removal of the excessconcentration of theadsorbate from the vicinity of the solid surface.Corrosive removal of zinc from a brass surface, leaving rough copper.
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Seeseparator.See theGouy-Chapman model of the double layer. Often simply called the "diffuse layer." (The diffuse layer is not to be confused with thediffusion layer.)Seeliquid-junction potential.Seediffuse double layer.The movement of chemical species (ions ormolecules ) under the influence ofconcentration difference. The species will move from the high concentration area to the low concentration area till the concentration is uniform in the whole phase. Diffusion in solutions is the most important phenomenon in electrochemistry, but diffusion will occur also in gases and solids.Thre rate of diffusion (diffusional flux) is proportional to the gradient of theconcentration in the solution, with the proportionality constant called the "diffusion coefficient."
A thin liquidboundary layer at the surface of anelectrode that is immobile. This is part of a rather simplified and not strictly correct model (originally proposed by the early electrochemist Nernst, and is often called the "Nernstian hypothesis") that works surprisingly well in most cases. Theelectrolyte solution is divided into three distinct parts: the bulk solution and the two diffusion layers at the surfaces of the electrodes. The bulk solution is assumed to be so well stirred that theconcentration of all species is uniform throughout. In this region,mass transport occurs only throughconvection. While in the diffusion layers mass transport occurs only throughdiffusion.Charge transport occurs throughelectromigration everywhere. The thickness of the diffusion layer can vary typically between the order of 0.01 centimeter in a stagnant solution and the order of 0.0001 centimeter in very well-stirred solution. (The diffusion layer is not to be confused with thediffuse layer.)While the concept of the diffusion layer is related to the concept of thehydrodynamic boundary layer, the two are not identical and neither is their thickness. The structure of the diffusion layer can be assumed to be relatively simple in the presence of a large excess ofsupporting electrolyte, which is usually the case inelectroanalytical applications and inelectrode kinetics research. Under these conditions, practically all the electricalcurrent is carried by theions of the supporting electrolyte, and thetransport number of thereactant and the product is practically zero. When the current is initially turned on, the ions of the supporting electrolyte willmigrate in the diffusion layer to/from theelectrode (depending on their charge). However, since they do not take part in anyelectrode reaction, theirconcentration will increase/decrease at the electrode surface compared to that of their concentrations in the bulk solution. This will start thediffusion of these ions in a direction opposite to their migration. Aftersteady-sate is reached, the diffusion will completely cancel the migration, and the net flux of these ions will be zero. They do not contribute to themass transport in the diffusion layer; however, they are responsible for all thecharge transport, that is, theresistance of the diffusion layer depends on theconductivity of the supporting electrolyte. On the other hand, the reactant and the product will diffuse to/from the electrode surface, and they will carry all the mass. The situation remains the same even if either the reactant or the product is an electrically neutralmolecules. It is usually assumed that the concentration of all species changes linearly between the electrode surface and the edge of the diffusion layer. An example is the diffusion layer at thecathode surface duringelectroplating of copper from a solution containing a small amount of copper chloride and a large concentration of sulfuricacid. All the current is carried by theions of the sulfuric acid (hydrogencations and sulfateanions) but the only possibleelectrode reaction is thereduction of the copper ions since the reduction of hydrogen ions cannot occur at the prevailingelectrode potential (copper is lower in theelectromotive series than hydrogen).
The situation is much more complex in the absence of supporting electrolyte. Then both the electromigrational and the diffusional flux of the reactant and product must be considered.
Seelimiting current density.Seeconcentration overpotential.Seeliquid-junction potentialA pair of equal and opposite electrical charges separated by a small distance. A dipole will align itself, if possible, in the presence of other electrical charges according to the attraction of opposite and repulsion of like charges. Externally neutral chemicalmolecules can have a dipole inside. E.g., water is a triangular molecule with the oxygen at one corner and the two hydrogens at the other two corners. The internal charge distribution is such that the hydrogen side has a slight excess of positive charge and the oxygen end is correspondingly negative. A dipole is characterized by its "dipole moment," the product of the charge and the separation distance (coulomb times centimeter).Seedipole.Seecurrent.The opposite process ofcharging. In this process thebattery orcapacitor supplies electricity to aload (e.g., motor, light bulb). The term discharging is also used to describe the neutralization of anion during anelectrode reaction. E.g., a metalcation is said to be "discharged" to a neutral metalatom duringelectroplating.Seebrine electrolysis.The process that may occur when a chemical compound is dissolved in a solvent (e.g., water). Themolecules of the compound will break up ("dissociate") into two or moreions resulting in anionically conductingelectrolyte solution. E.g., the common table salt (sodium chloride) will dissociate into a single charged sodiumcation and a single charged chlorideanion.SeeClark electrode.Anelectrochemical cell in which theelectrolyte is divided into two or more compartments byseparators. Such separation may be necessary for two reasons. The solutions around theanode and thecathode may be different and it may be desirable to keep them from intermixing. Alternatively, it may be desirable to keep the products of the reactions at the anode and the cathode separated.Seeseparator.Stands fordropping-mercury electrode.
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SeeDonnan potential.Theelectrical potential difference between two solutions separated by anion-exchange membrane in the absence of anycurrent flowing through the membrane.The concept of the Donnan potential is analogous to that of theequilibrium electrode potential. Consider two table salt (sodium chloride) solutions of differentconcentrations separated by acation-exchange membrane. The concentration difference will set up adiffusional force driving the sodium chloride from the higher concentration solution into the lower concentration solution through the membrane. However, theion-exchange membrane will permit only the passage of the positively charged sodiumcations. Consequently, excess positive electrical charges will accumulate on the low concentration solution side of the membrane, while excess negative electrical charges will accumulate on the high concentration side because of the negatively charged chlorideanions that are left behind. This charge separation will induce anelectrical potential difference that will drive theelectromigration of the sodium ions in the direction opposite to that of the diffusion. The overall result will be that the net movement of the sodium ions into the lower concentration solution will slow and eventually stop when the two opposing forces are equal end the two opposing fluxes are equal. In this so called "Donnan equilibrium," the diffusional flux of the sodium ions in one direction will be equal to the electromigrational flux in the opposite direction, resulting in net zeromass transport and zerocharge transport. The electrical potential difference across the membrane under these equilibrium conditions is the "Donnan potential."
Alternative name for "sedimentation potential." Seeelectrokinetic effects.Areference-electrode assembly (for example asilver/silver-chloride electrode) that is encased into a secondary containment vessel (typically a glass tubing) filled with an electrolyte not containing chlorideions (often a highconcentration potassium nitrate solution). This second"internal electrolyte" of the reference electrode assembly and theexternal electrolyte into which the whole assembly is immersed are in ionic contact through a secondseparator (e.g., a porous ceramic plug). The purpose of this arrangement is the avoidance of chloride ion contamination of the test solution (manyelectrode reactions are stronglycatalyzed by chloride ions) at the price of increasedliquid-junction potential.Seeelectrical double layer.The measure of the ability of anelectrical double layer tostore electrical charge as acapacitor.Seecapacitive current (density).Theelectrode potential range where anelectrode behaves as anideal polarized electrode. In this potential range, the electrode potential is not positive enough that any species in the solution could beoxidized and the potential is not negative enough that any species could bereduced. In practical situations, there is almost always a smallresidual current flowing that is faradaic in nature.Aworking electrode arrangement forelectroanalytical techniques, such aspolarography. Mercury is flowing continuously through acapillary tubing forming a small droplet (typical diameter about 0.1 cm) exposed to the solution. The old drop falls off and a new drop forms typically every few (3-6) seconds. The advantage of this self-renewingelectrode is that the effect of impurities in the solution is minimized. Typically a new drop will form before impurities have a chance toadsorb on the surface of the old drop to such an extent as to influence thecharge-transfer reaction. Abbreviated as "dme."An early name for thenon-rechargeable battery that is still used occasionally. The early non-rechargeable batteries were laboratory devices (see, e.g. theDaniell cell). To produce a practical device, theelectrolyte solution was "immobilized" by some gelling agent, and the whole cell was sealed to permit its use in any position. Hence the name: "dry cell." See alsoLeclanche cell.Seereserve battery.Seeexchange current density.
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Arechargeable battery developed by Edison. In thecharged state, the active material of the positiveelectrode is nickel oxide while that of the negative electrode is metallic iron, with abasic (potassium hydroxide)electrolyte. Duringdischarging, the nickel oxide is converted to a loweroxidation state oxide, while the iron is converted to iron oxide. It is still used today.Seecurrent.The structure of charge accumulation and charge separation that always occurs at the interface when anelectrode is immersed into anelectrolyte solution. (For a simple example seeequilibrium electrode potential.) The excess charge on the electrode surface is compensated by an accumulation of excessions of the opposite charge in the solution. The amount of charge is a function of theelectrode potential. This structure behaves essentially as acapacitor. There are several theoretical models that describe the structure of the double layer. The three most commonly used ones are theHelmholtz model, theGouy-Chapman model, and theGouy-Chapman-Stern model.A form ofenergy. It expresses the ability of anelectrical source to carry out useful work or generate heat. E.g., this energy can be used to drive an electrical motor and carry out some mechanical work, or to generate heat with an electrical heater. The electrical energy is usually expressed in units of watt-hour, symbol: "Wh". See alsoelectrical power.The electrical potential difference between two point in a circuit is the cause of the flow of acurrent. It is somewhat analogous to the difference in height in a waterfall that causes the water to fall, or the difference in pressure in a pipeline that causes the gas to flow. In electrochemistry we typically cannot measure "absolute" potentials, only the "difference" of potential between two points. For similar concepts, seeelectromotive force (emf) andvoltage. These terms are sometimes used interchangeably. However, in electrochemistry "emf" usually refers to the potential difference between the twoelectrodes of anelectrochemical cell when there is nocurrent flowing through the cell, "voltage" refers to same with current flowing, and "potential" is usually used in connection with electrodes (seeelectrode potential).The rate at which anelectrical source can supplyelectrical energy. E.g., abattery may be able tostore a large amount of energy, but if it has a small power capability it can provide the energy (do some work) only slowly, and it will take a long time to discharge. Another battery with the sameenergy storage capability but larger power will provide the energy (do work) faster, but will also be discharged faster. Electrical power is expressed usually in units of watt, symbol: "W". Unfortunately, the terms "power" and "energy" are often used interchangeably in everyday language (and sometimes also in the technical literature) even though they are quite distinct concepts, e.g., when we talk about "energy source" or "power source," we usually mean the same thing. Not only electrical sources but alsoloads are characterized by a power rating, e.g., an electrical motor or a light bulb is characterized by the power it needs to operate it.The power of asource (or the power need of aload) can be calculated as the product of thecurrent andvoltage (watt =ampere ×volt). One watt means that one watt-second (coulomb × volt)energy is provided (used) every second. In more practical units, one watt means that one watt-hour (ampere-hour × volt) energy is provided (used) every hour.
A source ofelectrical power (electrical energy), a device that supplies electricalcurrent. It can be electrochemical (battery orfuel cell) or an electromechanical device (dynamo) or a specialized electronic instrument. Specialized sources can be called "voltage source" or "current source," indicating the characteristic of the electrical power that can be controlled by that device.Theelectrokinetic effects arising when soundwaves cause oscillation of small particles suspended in a liquid; particularly, effect analogous to sedimentation potential.A species in solution that can take part in anelectrode reaction or that can beadsorbed on theelectrode.Seeelectrosorption.The application ofelectrochemical cells and electrochemical techniques for chemical analysis. Theanalyte is dissolved in theelectrolyte of the cell, and one can perform either "qualitative" analysis (determination of the type of constituents present) or "quantitative" analysis (determination of the amount of a given constituent).
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The phenomenon of increasing the rate of anelectrode reaction by changing theelectrode material. The rate of the electrode reactions (the magnitude of theexchange current density) can strongly depend on the composition and morphological structure of the electrode surface. This is called the "electrocatalytic effect."A material that can causeelectrocatalysis.A device thatstoreselectrical energy usingelectrochemical cells. Large surface areaelectrodes are used resulting in largedouble layer capacitance, and much of the storage capacity is due to thecharging/discharging of the double layer. Some surfaceoxidation/reduction also occurs, but in contrast to reactions occurring inbatteries, this is limited to a monolayer or two on the electrode surfaces. Consequently, the device behaves more like acapacitor than a battery. It is also called "supercapacitor" and "ultracapacitor".Electrochemical capacitors typically have much largerpower density but much smallerenergy density than batteries.
A device thatconverts chemicalenergy intoelectrical energy or vice versa when a chemical reaction is occurring in the cell. Typically, it consists of two metalelectrodes immersed into anaqueous solution (electrolyte) withelectrode reactions occurring at the electrode-solution surfaces. See alsogalvanic cell andelectrolytic cell.It consist of twoelectronically conducting phases (e.g., solid or liquid metals, semiconductors, etc) connected by an ionically conducting phase (e.g.aqueous ornon-aqueous solution, molten salt, ionically conducting solid). As an electricalcurrent passes, it must change from electronic current to ionic current and back to electronic current. These changes of conduction mode are always accompanied byoxidation/reduction reactions. An essential feature of the electrochemical cell is that the simultaneously occurring oxidation-reduction reactions are spatially separated. E.g., in a spontaneous "chemical reaction" during the oxidation of hydrogen by oxygen to water,electrons are passed directly from the hydrogen to the oxygen. In contrast, in the spontaneouselectrochemical reaction in agalvanic cell the hydrogen isoxidized at theanode by transferring electrons to the anode and the oxygen isreduced at thecathode by accepting electrons from the cathode. Theions produced in theelectrode reactions, in this case positive hydrogen ions and the negative hydroxyl (OH-) ions, will recombine in the solution to form the final product of the reaction: water. During this process the electrons are conducted from the anode to the cathode through an outside electrical circuit where the electrical current can drive a motor, light a light bulb, etc. The reaction can also be reversed,water can be decomposed into hydrogen and oxygen by the application ofelectrical power in anelectrolytic cell.
Seeelectrolytic degreasing.Seeelectrolytic degreasing.Seeelectrical double layer.Seaelectrochemical machining.The weight of a substance (in grams) produced or consumed by the passage of onecoulomb in anelectrochemical reaction.Thegram-equivalent weight divided by theFaraday constant.
A combination ofelectrochemical machining and mechanical grinding. Used when the products of electrochemical dissolution are not easily soluble and must be removed physically from the surface. Used with a metal-bonded and diamond-impregnated grinding wheel. Also called "electrolytic grinding" and "electrogrinding."Seeirreversible electrode reaction.A process to produce metallic objects with a technique that is essentially precisionelectrodissolution. The metal to be machined is made theanode in anelectrolytic cell while thecathode (or "tool") is made of inert material and is machined to be the "mirror image" of the desired shape. A very small gap (typically, less than 0.1 cm) is maintained between theelectrodes and a largecurrent density is applied with a fast flowingelectrolyte. One of the advantages of this production technique is that very complicated shapes can be produced with a single operation from very hard alloys that would be very difficult, if not impossible, to machine with any other metal cutting technique. Some typical applications are the production of turbine blades and the drilling of holes with very large depth-to-diameter ratio. Thecathodic reaction is typicallyhydrogen evolution.An instrument which comprises a sampling system, an array of chemical/electrochemical gas sensors with differing selectivity, and a computer with an appropriate pattern-classification algorithm, capable of qualitative and/or quantitative analysis of simple or complex gases, vapors, or odors. See also anEncyclopedia Article.Seeelectrolytic pickling.Seeelectropolishing.Anoxidation/reduction reaction that occurs in anelectrochemical cell. The essential feature is that the simultaneously occurring oxidation-reduction reactions are spatially separated. E.g., in a spontaneous "chemical reaction" during the oxidation of hydrogen by oxygen to water,electrons are passed directly from the hydrogen to the oxygen. In contrast, in the spontaneous electrochemical reaction in agalvanic cell two separateelectrode reactions occur. The hydrogen isoxidized at theanode by transferring electrons to the anode and the oxygen isreduced at thecathode by accepting electrons from the cathode. Theoverall electrochemical reaction is the sum of the twoelectrode reactions. Theions produced in the electrode reactions, in this case positive hydrogen ions and the negative hydroxyl (OH-) ions will recombine in the solution to form the final product of the reaction: water. During this process the electrons are conducted from the anode to the cathode through an outside electrical circuit where the electronic current can drive a motor, light a light bulb, etc. The reaction can also be reversed,water can be decomposed into hydrogen and oxygen by the application ofelectrical power in anelectrolytic cell.Seereversible electrode reaction.Seeelectromotive series.A variety of electrochemical techniques used to "shape" metal objects. These include:electrochemical machining,electrochemical drilling,electrochemical grinding, andelectropolishing.An electrochemically switchablemolecule displays a different reactivity toward some other chemical species depending whether the switchable molecule isoxidized or reduced. Consequently, the reactivity of the molecule can be controlled by electrochemical oxidation/reduction. This phenomenon is primarily important inbioelectrochemistry.Light emission by excited species produced in anelectrode reaction. Also called: "electrogenerated chemiluminescence."A "chromatographic" separation method with the "mobile," liquid phase forced through the "immobile" phase by the application of anelectrical potential difference, that is, byelectroosmosis. In some cases, the separation is enhanced byelectrophoresis. Chromatography is an analytical (seeelectroanalytical) separation technique based on the different attraction of the sample components to an immobile/stationary phase through which the sample solution is forced through by a flow of solvent. The sample components areadsorbed/desorbed on the surface of the stationary phase as they are flushed through by the solvent; consequently, they move with speeds inversely proportional to their adsorption strengths and become separated: the least-strongly adsorbed component is flushed out first and the most-strongly adsorbed last. A variety of stationary phases can be employed; the most common ones are: paper, thin layer of gelatinous material, or a column (orcapillary) packed with small particles.Seeelectrolytic degreasing.Seeelectrophoretic deposition.Process for increasing theconcentration of a trace component in a sample. It can be achieved by a variety of techniques, e.g.:electrochromatography,electrodialysis,electroplating,electroosmosis, andelectrophoresis.Seeelectroplating. Electroplating typically will result in a crystalline metal deposit; therefore, the two terms can be used interchangeably. The term "electroplating" is mostly used in technological applications, and the term "electrocrystallization" is often used in research studies.
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The twoelectronically conducting parts of anelectrochemical cell. See alsoanode andcathode. These can be simple metallic structures (rods, sheets, etc) or much more complicated, composite structures. E.g., the electrodes in arechargeable battery will also "contain" the chemicals being converted during its operation. The term "electrode" is also used to denote complex assemblies that include an electrode in a small vessel, which contains anelectrolyte and is equipped with anion-permeableseparator.Reference electrodes are such assemblies.The application ofkinetics toelectrode reactions. Not to be confused withelectrokinetics.A simple metalelectrode immersed in a solution containing its ownion (e.g., silver immersed in a silver nitrate solution). Theequilibrium potential of this electrode is a function of theconcentration of thecation of the electrode metal in the solution (seeNernst equation). Contrast withelectrode of the second kind andelectrode of the third kind.A metalelectrode assembly with theequilibrium potential being a function of theconcentration of ananion in the solution. Typical examples are thesilver/silver-chloride electrode and thecalomel electrode. Contrast withelectrode of the first kind andelectrode of the third kind.The assembly consists of a metal, in contact with a slightly soluble salt of this metal, immersed in a solution containing the sameanion as that of the metal salt (e.g., silver---silver chloride---potassium chloride solution). Thepotential of the metal is controlled by the concentration of itscation in the solution, but this, in turn, is controlled by theanion concentration in the solution through thesolubility product of the slightly soluble metal salt.
A metalelectrode assembly with theequilibrium potential being a function of theconcentration of acation, other than the cation of the electrode metal, in the solution. These have been used, with limited success, in sensors for metalions for metals that are not stable inaqueous solutions, e.g., calcium and magnesium. Contrast withelectrode of the first kind andelectrode of the second kind.The assembly consists of a metal in contact with two slightly soluble salts (one containing thecation of the solid metal, the other the cation to be determined, with both salts having a commonanion) immersed in a solution containing a salt of the second metal (e.g., zinc metal---zinc oxalate---calcium oxalate---calcium salt solution). Thepotential of the metal is controlled by the concentration of its cation in the solution, but this is controlled by theanion concentration in the solution through thesolubility product of the slightly soluble metal salt, which, in turn is controlled by the concentration of the cation of the second slightly soluble salt. These electrodes are very sluggish and unstable due to a series of equlibria to be established to produce a stable potential.
Theelectrical potential difference between anelectrode and areference electrode. We cannot measure the "absolute" potential of an electrode; therefore, the electrode potential must always be referred to an "arbitrary zero point," defined by the potential of the reference electrode. Consequently, it is very important always to note the type of reference electrode used in the measurement of the electrode potential. See alsoequilibrium electrode potential.A chemical "half" (or "partial") reaction occurring at theelectrode surface. It is called a "half" (or "partial") reaction because only theoxidation or thereduction part of the overallcell reaction occurs at any one electrode. See alsoelectrochemical reaction. Many electrode reactions can proceed either as oxidation or as reduction, depending on the direction of thecurrent flowing through theelectrode/electrolyte interface. See, e.g.metal deposition/dissolution orredox reactions.An electrode reaction always occurs in several series and parallelelementary reaction steps. Even in the simplest case there are three steps in series: (1) thereactant must be transported to the electrode surface from the bulk of theelectrolyte (usually predominantly bydiffusion, but it can also occur byelectromigration), (2) acharge-transfer reaction occurs, and (3) the product must be transported from the electrode surface to the bulk of the electrolyte.
Seeelectroplating.The part of electrochemistry that deals with phenomena occurring at the surface ofelectrodes, particularlycharge-transfer reactions.The reverseelectrode reaction ofelectroplating. See alsometal deposition/dissolution.A process to moveions from one solution into another using anelectrolytic cell. An example is the electrochemicaldesalination of seawater. In its simplest form, the cell is separated into three compartments by appropriateion-exchange membranes withelectrodes placed in the two outer compartments, and all compartments are fed seawater. As an electricalcurrent is forced through the cell,anions will move from the central compartment through an anion-exchange membrane into theanode compartment and thecations will move through an cation-exchange membrane into thecathode compartment. Since theion-exchange membranes are appropriately ion selective, the ions cannot move from the edge compartments to the central compartment, resulting in a desalinated effluent from that compartment. In practice, more than one cell will be connected inseries,, and the process will be carried out in several stages since it would not be efficient to remove all the salt in one step. This process is also used to remove industrial pollutants from waste streams.Seeelectroosmosis.Seeelectrowinning.A process to produce metallic objects with a technique that is essentially precisionelectroplating. The metal is deposited onto a "mandrel" or "former" of suitable shape to a desired thickness, followed by the removal of the mandrel to produce a free standing metal object. One of the advantages of this production technique is that very complicated shapes can be produced with a single operation. It is often used to produce very precise optical elements, and solid-state electronic devices (integrated circuit boards, computer chips). Other applications are the production of flat or perforated metal sheets, seamless perforated metal tubes, and metal bellows. Two very prominent past applications of this technique were the production of "stampers" for the old-fashioned musical (phonograph) records and "electrotypes" for the printing industry. Practically any metal or alloy that can be electroplated can also be used for electroforming. The preparation of the removable mandrel is an important step in this process. One example is the use of machined copper or brass that is surface treated to permit electroplating that will closely follow the mandrel surface but will not permit strong adhesion of the electroformed piece.Seegalvanizing.Seeelectrochemiluminescence.A chemical species produced at anelectrode surface by acharge-transfer reaction.Anelectroanalytical technique in which the substance to be determined (usually a metal) isplated (deposited) out on anelectrode which is weighed before and after the experiment. Thepotential of the electrode must be carefully chosen to ensure that only the metal do be determined will deposit. Under favorable conditions, two or more metals can be determined by successive depositions at different potentials.Seeelectrochemical grinding.Phenomena that arise due to a charge separation caused by the relative motion of a solid and liquid phase. A portion of theGouy-Chapman diffuse layer is sheared off as the two phases move relative to each other, resulting in a charge separation. Thehydrodynamic boundary layer remains attached to the solid surface while the rest of the liquid moves separately; consequently, electrokinetic effects arise when the "diffuse double layer" is thicker that the "hydrodynamic boundary layer." Theelectrical potential difference between the bulk solution and the "shear plane" is the "electrokinetic potential," often called the "zeta potential." Two types of effects arise: an electrical potential difference will arise between the two phases if they move relative to each other due to an external force (streaming potential and sedimentation potential) or a movement of the two phases will arise relative to each other if an electrical potential is applied parallel to the phase boundary (electroosmosis and electrophoresis). Accordingly, "electrokinetics" includes the following four "electrokinetic effects:"
Streaming potential arises when liquid is flowing by a solid surface, e.g., when liquid is forced through acapillary tubing or porous solid by a pressure differential.
Sedimentation potential arises when small suspended particles move through a liquid (e.g., forced by gravity). This can occur in "dispersions" (suspended solid particles) or "emulsions" (suspended immiscible liquid droplets). Also called "eletrophoretic potential" or "Dorn potential."
Electroosmosis is the movement of a liquid through a capillary tubing or porous solid driven by anelectrical potential difference. Also called "electroendosmosis."
Electrophoresis is the movement of small suspended particles in a liquid driven by an electrical potential difference. This can occur in "dispersions" (suspended solid particles) or "emulsions" (suspended immiscible liquid droplets). Also called "cataphoresis."
Electrokinetics should not be confused withelectrode kinetics.
Alternative name for "zeta potential." Seeelectrokinetic effects.
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A process that decomposes a chemical compound into itselements or produces a new compound by the action of an electricalcurrent. The electrical current is passed trough anelectrolytic cell andoxidation/reduction reactions occur at theelectrodes. E.g.,water can be decomposed into hydrogen and oxygen by electrolysis.A chemical compound (salt,acid, orbase) thatdissociates into electrically chargedions when dissolved in a solvent. The resulting electrolyte (or electrolytic) solution is anionic conductor of electricity. Very often, the so formed solution itself is simply called an "electrolyte." Also, molten salts and molten salt solutions are often called "electrolyte" when used inelectrochemical cells.Seeelectrolyte.Astorage device similar to any other type of electricalcapacitor. However, only one of its conducting phases is a metallic plate, the other conducting phase is a solution containing a chemical compound or electrolyte. The dielectric is a very thin oxide film on the surface of the metal (typically aluminum or tantalum) that constitutes one conducting phase of the capacitor. There is also another metal immersed in the solution, which serves only as the electrical contact to the solution.Anelectrochemical cell thatconvertselectrical energy into chemicalenergy. The chemical reactions do not occur "spontaneously" at theelectrodes when they are connected through an external circuit. The reaction must be forced by applying an external electricalcurrent. It is used tostoreelectrical energy in chemical form, seerechargeable battery. It is also used to decompose or produce (synthesize) new chemicals by application ofelectrical power. This process is calledelectrolysis, e.g.,water can be decomposed into hydrogen gas and oxygen gas.The free energy change of the overall cell reaction is positive.
Seeelectrolytic degreasing.Process for removal of grease, oil, etc from metal surfaces in preparation forelectroplating. The metal is made thecathode in anelectrolytic cell containing stronglybasic (sometimes hot) solution that dissolves these coatings. The stronghydrogen evolution occurring on the cathode mayreduce some of the coatings, and the strong bubble evolution removes the coatings mechanically, while the agitation of the solution helps the chemical dissolution of the coatings by the base. Also called: "electrolytic cleaning," "electrochemical cleaning," "electrocleaning," and "electrochemical degreasing." See alsodegreasing.Seeelectrochemical grinding.Seewater electrolysis.Seewater electrolysis.Seeelectrochemical machining.Process for removal of oxide scales from metal surfaces in preparation forelectroplating. The metal is made thecathode in anelectrolytic cell containing stronglyacidic (sometimes hot) solution that dissolves the oxide scales. The stronghydrogen evolution occurring on the cathode mayreduce some of the oxides, and the strong bubble evolution removes the scales mechanically, while the agitation of the solution helps the chemical dissolution of the scales by the acid. See alsopickling.Seeelectropolishing.Seeelectrorefining.Seeelectrolyte.
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Seeelectrochemical machining.Branch of metallurgy (science dealing with the production of metals) using electrochemical processes, that iselectrowinning.Avoltmeter with very large inputresistance. A typical modern voltmeter has an input resistance of around ten millionohms, an electrometer can have ten million times more. Electrometers are used to measure theelectromotive force ofelectrochemical cells that can be easilypolarized bycurrent. The voltmeter always draws a small current, the magnitude depends on the ratio of the resistance of the cell and the voltmeter. High resistance cells (e.g., one containing aglass electrode, must be measured with an electrometer.The movement ofions under the influence ofelectrical potential difference.Thecell voltage of agalvanic cell measured when there is nocurrent flowing through the cell. In other words, theequilibrium electrode potential difference between the twoelectrodes of the cell.A tabulation on which various substances, such as metals or elements, are listed according to their chemical reactivity or standard electrode potential. It is usually ordered with increasing standard electrode potentials (most negative on top). For metals, the order indicates the tendency to spontaneously reduce the ions of any other metal below it in the series (see cementation). Duringelectrolytic reduction ofcations (e.g.,electroplating) an element lower in the series (more positive) will deposit first, and an element higher in the series (more negative) will deposit only when the solution is practically depleted of the ions of the first element. Also called "electrochemical series" and "galvanic series."Seeatomic structure.The expression of nature's tendency to keep any system electrically neutral, that is, if it contains electrically charged particles the total sum of negative charges will be equal to the total sum of positive charges. Applying this condition to a solution ofelectrolytes implies the equality of the total positiveionic charges to the total negative ionic charges. This equality should hold even as we subdivide the solution into smaller and smaller volume elements.This condition results from the statistical distribution of the ions around each other considering the attractive tendency of oppositely charged particles and the repulsive tendency of similarly charged particles. Consequently, there is a statistical limit of the size of the volume element to which it applies. At the extreme, a volume small enough to contain only a single ion obviously cannot be electrically neutral.
A material that conducts electricity withelectrons as charge carriers.Seeelectrochemical nose.Seecharge-transfer reaction.Relating to organic electrochemistry.The movement of a liquid through acapillary tubing or porous solid driven by anelectrical potential difference. Seeelectrokinetic effects. Also called "electroendosmosis."Compaction of slurries byelectroosmosis.Oxidation carried out with anelectrochemical reaction.As a phenomenon: the movement of small suspended particles or very largemolecules in a liquid driven by anelectrical potential difference. Also called "cataphoresis." Seeelectrokinetic effects. As anelectroanalytical technique: a separation method for very large organic molecules (usually of biological origin) based on their different electrophoretic velocities through an "immobilized" liquid phase. The liquid can be immobilized by a variety of "supports", e.g.: paper, gelatinous material,capillary tubing.Deposition of particles carried to a surface byelectrophoresis. The loosely formed deposit layer typically needs compacting that can partially occur byelectroosmotic removal of the liquid, or by other means.Electrode reactions occurring on the substrate surface can take part in "binding" the coating. Practical applications are surface coating and paint deposition (practiced on large scale in the automotive industry) and fabrication of ceramic products. Also called "electrocoating."Alternative name for "sedimentation potential." Seeelectrokinetic effects.The process that produces a thin, metallic coating on the surface on another metal (or any other conductor, e.g., graphite). The metal substrate to be coated is made thecathode in anelectrolytic cell where thecations of theelectrolyte are the positiveions of the metal to be coated on the surface. When acurrent is applied, theelectrode reaction occurring on the cathode is thereduction of the metal ions to metal. E.g., gold ions can bedischarged form a gold solution to form a thin gold coating on a less expensive metal to produce "custome" jewelry. Similarly, chromium coating is often applied to steel surfaces to make them more "rust resistant." Electroplating is also used in the production of integrated circuits on computer chips and for other modern electronic instrumentation. Theanode material can either be the metal to be deposited (in this case theelectrode reaction iselectrodissolution that continuously supplies the metal ions) or the anode can be of nonreactive material and the anodic reaction isoxygen evolution (in this case the plating solution is eventually depleted of metal ions). Also called "electrodeposition."The application of very brief, carefully controlled, pulsed, rotating electrical fields to human cells, a process that causes pores to open in the cell membrane and allows pharmaceuticals or genes to gain access to the cell's interior.A process that produces a bright, shiny surface on a metal. The metal isanodicallydissolved in anelectrolytic cell under conditions that projections in the surface are dissolved faster than the smoother areas.Reduction carried out with anelectrochemical reaction.An electrochemical process that produces a purified metal from a less pure metal. The metal to be purified is made theanode in anelectrolytic cell and it isdissolved by the application of acurrent into a usuallyacidicaqueouselectrolyte or a molten salt. At the same time, the pure metal iselectroplated on thecathode. The process is carried out under conditions that most impurities will either precipitate as "sludge" or remain dissolved in the electrolyte. Copper is one metal that is often electrorefined in aqueous solutions, and aluminum is electrorefined using a molten salt electrolyte. Also called "electrolytic refining" and "metal refining."Seeelectroviscosity.Adsorption atelectrode surfaces. Generally, adsorption at electrically charged interfaces.Production of chemicals in anelectrolytic cell.Seeelectroforming.The phenomenon of a change inviscosity due to the presence of charge on particles suspended in a solvent.An electrochemical process that produces metals from their ores. Most metals occur in nature inoxidized form in their ores. While numerous ways exist toreduce the ores, for many metals electrochemical reduction is the most practical. The ore is dissolved (often following some chemical purification or preprocessing) in anacidicaqueous solution or in a molten salt and the resultingelectrolyte solution iselectrolyzed. The metal iselectroplated on thecathode (either in solid or in liquid form), while theanodic reaction is usuallyoxygen evolution. Copper and zinc are two metals that are often produced by aqueous electrolysis. Aluminum, magnesium, and sodium are some metals that can be produced by molten salt electrolysis. For aluminum, this is the only practically used production process (see also anEncyclopedia Article). Also called "electroextraction."A substance that cannot be decomposed into simpler substances by chemical means.Chemical reactions usually take place in a number of simple ("elementary") reaction steps proceeding in series. The overall reaction is the sum of the elementary reactions. E.g., theelectroplating of copper on some metal involves three elementary steps: (1) aredox reaction in which the double positively charged coppercation reacts with anelectron from the metalelectrode to form a single charged copperion, followed by (2) ametal deposition reaction in which the single charged copper ion reacts with a second electron to form a copperatom on the surface of the metal, and finally (3) anelectrocrystallization step in which the copper atom becomes incorporated into the crystalline structure of the underlying metal. Therate-determining step in copper deposition is usually the first of these steps.Some complicated reactions can also involve parallel paths, each proceeding through a different series of elementary steps (different reactionmechanisms). The sum of the series elementary steps in each parallel path must add up to the same overall reaction.
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Stands forelectromotive force.The energy of a system expresses the ability of that system to do some useful work or generate heat. Energy can be in many forms; e.g., mechanical energy, chemical energy, heat energy,electrical energy, etc. The different forms of energy can beconverted into each other.It is a fundamental law of nature that energy can never be converted from one form to another 100%, some of the energy is always converted into heat energy during the conversion. Also, heat can never be converted 100% into any other form of energy.
A process in whichenergy is converted from one of its many forms to another. Thefuel cell is an electrochemical energy conversion device.Characteristic parameter of abattery indicating the amount ofelectrical energystored per unit weight or volume. The terminology is not strictly defined. Weight based energy density is often called "specific energy" or "gravimetric energy density." Volume based energy density is often called "energy density" or "volumetric energy density." The energy density is typically expressed as watt-hour/kilogram or watt-hour/liter.For arechargeable battery: the fraction, usually expressed as a percentage, of theelectrical energystored in a battery bycharging that is recoverable duringdischarging. For anelectrolytic cell: the fraction, usually expressed as a percentage, calculated as the theoretically required energy divided by the energy actually consumed in the process (production of a chemical,electroplating, etc). Inefficiencies arise fromcurrent inefficiencies and the inevitable heat losses.Seeelectrical source.A process in whichenergy is stored in some form, ready for future use on demand. The time scale of storage can extend to many years if needed. Thebattery is an electrochemical energy storage device.Seeelectrochemical nose.Anelectrode or anelectrochemical cell is said to be in "equilibrium" when there is no netcurrent flowing and there are no netelectrode reactions taking place in the system. (See, however,exchange current density.) In equilibrium, thepotential of the electrodes is theequilibrium potential and thecell voltage is theelectromotive force.Seeelectromotive force.Theelectrical potential of anelectrode measured against areference electrode when there is nocurrent flowing trough the electrode. In other words, theelectromotive force of anelectrochemical cell consisting of the electrode in question and a reference electrode. Also called: "open circuit potential (ocp)." See alsoequilibrium andstandard electrode potential.The concept of equilibrium potential is probably easiest to demonstrate with a simple metal/metal-ionelectrode system. When a metal (e.g., silver) is immersed in a solution containing itsion (e.g., silver nitrate solution) metalions will cross the metal/solution interface. They will pass from the phase where the "chemicalenergy" of the ion is large to the phase where the "chemical energy" of the ion is smaller. Depending on the system, this can occur in either direction. However only the positively charged (e.g., silver)cations can pass through the interface. The negatively chargedelectrons cannot pass into the solution, and theanions (e.g., nitrate) cannot pass into the metal. Consequently, charge accumulation occurs at the interface forming anelectrical double layer. Consider an example when the metal ions move preferentially from the metal into the solution: the metal surface becomes negatively charged because of the accumulation of the electrons left behind, while the solution layer near the metal surface becomes positively charged because of the accumulation of silver ions. This process produces a potential difference between the two phases that will slow and eventually stop the passage of the metal ions. At "equilibrium" the chemical driving force and the opposing electrical force are equal. The potential difference between the metal and the solution phases under these conditions is the "equilibrium potential difference." This potential difference cannot be measured because there is no way to make an electrical connection to the solution phase without setting up another electrode potential. Consequently, electrode potentials are always measured against areference electrode whose potential is known on an arbitrary scale. Seestandard hydrogen electrode.
Seeequilibrium electrode potential.Seeelectromotive force.A characteristic weight of a substance relating to a specific reaction the substance participates. Inelectrochemical reactions, themolecular weight oratomic weight divided by the number ofelectrons transferred during the reaction. Consequently, the equivalent weight of a substance can be different for different reactions. E.g. the equivalent weight of the cuprousion (singly charged copper ion) is equal to its atomic weight, independently whether it isoxidized to cupric ion (doubly charged copper ion) or it isreduced to (neutral) copper metal. On the other hand, the equivalent weight of the cupric ion is one half of its atomic weight if it is reduced to copper metal, but it is the atomic weight if it is reduced only to cuprous ion.In a wider sense, the molecular weight or the atomic weight divided by the valence change occurring during the reaction. Foracid/base reactions, the molecular weight divided by the number of hydrogen ions produced or consumed during the reaction.
At theequilibrium potential there is no netcurrent flowing through theelectrode. However, theequilibrium is a dynamic one, that is, theelectrode reaction proceeds at "equal rates" both in the forward and in the reverse direction, resulting in a zero "net" reaction rate and a zero "net" current. The rate of the electrode reaction can be expressed as an equivalentcurrent density and the "exchange current density" of a reaction is the current density flowing "equally" in both directions in equilibrium. A large exchange current density indicates a fast reaction (see alsonon-polarizable electrode), while a small exchange current density indicates a slow reaction (see alsopolarizable electrode)Theelectrolyte solution in theelectrochemical cell into which thereference electrode is immersed. Contrast with theinternal electrolyte of the reference electrode.
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Symbol and abbreviation offarad, and symbol and abbreviation of theFaraday number. (Often a "bold-face" letter is used for the latter, but this is not a general practice.) It is usually obvious from the context which meaning is appropriate.Measurement unit ofcapacitance. Symbol: "F," which is the same as the symbol of theFaraday Number. It is usually obvious from the context which meaning is appropriate.Acapacitor has a capacitance of one farad when onecoulombcharges it to onevolt.
Some of the most fundamental laws of electrochemistry discovered by Faraday in the 1830's. They are usually stated as: (1) In any electrolytic process the amount of chemical change produced is proportional of the total amount of electrical charge passed through the cell. (2) The mass of the chemicals changed is proportional to the chemicals'equivalent weight. The proportionality constant being theFaraday Number.The Faraday Number (or constant) gives the amount of electrical charge needed to change onegram-equivalent of substance byelectrochemical reaction. Its value is 96,485.34coulombs or 26.80ampere-hours. This charge is often simply called one "Faraday." Symbol: "F," which is the same as the symbol of thefarad. It is usually obvious from the context which meaning is appropriate.Thecurrent (orcurrent density) that is flowing through anelectrochemical cell and is causing (or is caused by) chemical reactions (charge transfer) occurring at theelectrode surfaces. Contrast withcapacitive current.Seeheterogeneous charge-transfer reaction.Seeinternal electrolyte.Seeelectrode of the first kind.The permanently attached charged fragment in anion-exchange resin. Contrast withcounterion.A method of maintaining arechargeable battery in a fully charged condition by continuous, long-term, constant-voltagecharging, at a level sufficient to balanceself-discharge. See alsotrickle charging.Thevoltage required for retaining arechargeable battery in fullycharged condition. Seefloat charging.Anelectrode that permits theelectrolyte to flow through it, e.g., aporous electrode or apacked-bed electrode. This type of electrode is especially useful for removing small traces of impurities from the solution byelectrolysis (e.g., waste treatment) because the solution contacts a large surface of the electrode material.Seepacked bed electrode.Similar to astandard electrode potential except that both theoxidized and the reduced species are present in unitconcentration instead of unitactivity. It is not as well defined as the standard potential but it is useful in cases when the activities are unknown.Essentially the same as themolecular weight, but it can be used more generally, e.g., also forions.A device thatconverts chemicalenergy intoelectrical energy. It is different from abattery in that theenergy conversion continues till fuel and oxidizing agent are fed to the fuel cell; that is, in principle indefinitely. (A battery is manufactured with a limited amount of chemicals, and it is exhausted when all the chemicals have reacted.) It is agalvanic cell, where spontaneous chemical reactions occur at theelectrodes. The fuel isoxidized at theanode, and the oxidizing agent (almost always oxygen or air) isreduced at thecathode. Presently, the most commonly used fuel is hydrogen. More conventional fuels (e.g., gasoline or natural gas) must be converted ("reformed") into hydrogen before they can be utilized in a fuel cell. Fuel cells that can burn hydrocarbon fuels directly are in the development stage.A single fuel cell has a rather small (typically less then one Volt)cell voltage. For practical applications a large number of them are assembled,series coupled, in what is called a "fuel cell stack." (A term essentially analogous to the original meaning of thebattery.)
The expression of "activity" for a component in a mixture of gases. It has the same physical meaning as theactivity for a component in a solution.
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Anelectrochemical cell thatconverts chemicalenergy intoelectrical energy. A cell in which chemical reactions occur spontaneously at theelectrodes when they are connected through an external circuit, producing an electricalcurrent. E.g., in afuel cell hydrogen isoxidized at theanode by transferringelectrons to the anode and the oxygen isreduced at thecathode by accepting electrons from the cathode. During this process the electrons are conducted from the anode to the cathode through an outside electrical circuit where the electrical current can drive a motor, light a light bulb, etc. In contrast, in anelectrolytic cellelectrical power must be supplied to force the non-spontaneous reverse reaction, theelectrolysis of water. See alsoelectrochemical reaction,battery, andfuel cell. Also called "voltaic" cell.The free energy change of the overall cell reaction is negative.
Seeelectromotive series.Process for coating iron or steel with a thin layer of zinc forcorrosion protection. It can be carried out electrochemically byelectroplating (called "electrogalvanizing") or by "hot-dip" galvanizing consisting of immersing the substrate into molten zinc.A very sensitiveammeter that can be used to measurecurrents in the range of one millionth to one billionth of anampere.A somewhat archaic, today very seldom used, term forelectroforming.An electronic instrument that controls thecurrent through anelectrochemical cell at a preset value, as long as the neededcell voltage and current do not exceed thecompliance limits of the galvanostat. Also called "amperostat."An electrochemical measuring technique forelectrochemical analysis or for the determination of thekinetics andmechanism ofelectrode reactions based on the control of thecurrent flowing through the system.Anyelectrode with one of thereactants or products in the gaseous phase. The solution surrounding the electrode is typicallysaturated with the gas.Essentially aredox electrode with the "dissolved" gas as one of thepotential determining species. E.g., theoxidized species for the "chlorine electrode" is the dissolved chlorine gas, while thereduced species is the chlorideion in solution. Under equilibrium conditions, the chemical potential of the gaseous chlorine is the same as that of the dissolved chlorine. This may not be the case whencurrent is flowing through the electrode: during the electrolytic production of chlorine, the solution may become supersaturated in chlorine if the nucleation of the gas bubbles requires considerable activation energy.
Current density calculated with thegeometricelectrode area.The surface area of anelectrode calculated from its geometrical dimensions. Contrast withtrue electrode area.Amembrane electrode with a thin glass membrane (usually in the form of a bulb at the end of a glass tubing) sensing element. It is most often used as apH electrode, but some glass compositions can also be sensitive to theconcentration of othercations (e.g., sodium).An amount of a substance equal in grams to itsequivalent weight.A concept similar togram-mol except it relates toatoms rather thanmolecules.An amount of a compound equal in grams to itsmolecular weight. E.g., the molecular weight of water is 18, so 18 grams of water is called a gram-mol of water. This provides an atomistically fundamental unit because one gram-mol of any material will contain the same (and very large) number ofmolecules. One gram-mol of hydrogen gas contains the exactly same number of molecules as one gram-mol of table salt (sodium chloride), even though the latter is much heavier.A model of theelectrical double layer. According to this model, the excessions are non-uniformly distributed in the vicinity of theelectrode, theirconcentration is the largest at the surface of the electrode, decreasing non-linearly till they reach bulk concentration. The "thickness" of this so called "diffuse layer" is variable, but it is typically around the order of magnitude of a millionth of a centimeter.A model of theelectrical double layer. TheGouy-Chapman model predicts an unrealistically large surfaceconcentration, because it assumes that theions are infinitely small and can get infinitely close to the surface of theelectrode. The "Stern modification" is essentially a combination of theHelmholtz and Gouy-Chapman models. It assumes a "plane of closest approach" where a portion of the excess ions reside and attaches to this a Gouy-Chapman type "diffuse layer."
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SeeLuggin capillary.A somewhat archaic term, indicating a structure that contains anelectrode and the surroundingelectrolyte.Electrochemical cells are oftendivided, containing two separate electrolytes (one surrounding each electrode, e.g., theDaniell cell). In these cases the electrode and its electrolyte can be considered "half" of the cell. Commercially availablereference electrodes can be considered "half cells."A not incorrect, but somewhat archaic term forelectrode reaction.Seeelectrode reaction.See theHelmholtz model of the double layer.The simplest model of theelectrical double layer. The excessions in the solution side of the double layer line up in one plane ("Helmholtz plane") very close to theelectrode surface. In a somewhat more complex model there are two planes of closest approach of the ions. Ions in the "outer Helmholtz plane" are about two solvent-molecule diameters away from the electrode surface because both the ions and the electrode surface aresolvated. Ions in the "inner Helmholtz plane" have shed their solvation layer (these are usually the weekly solvated, largeanions) and penetrated the solvent layer on the electrode; these, so calledcontact adsorbed, ions are sitting directly on the electrode surface. The ionic portion of the Helmholtz model is often called the "Helmholtz layer" or "compact layer."See theHelmholtz model of the double layer.Stands forhydrogen evolution reaction.Acharge-transfer reaction with the charge transferred across a phase boundary, typically between a solid and a liquid phase. Contrast withhomogeneous charge-transfer reaction.Acharge-transfer reaction with both reactants present in the same phase. Typically both reactants are dissolved species in a solution while the charge is transferred from one to the other. Contrast withheterogeneous charge-transfer reaction.Solvation occurring in anaqueous solution.The number of watermolecules associated with anion in the process ofsolvation inaqueous solutions.A thin immobile layer of fluid that always exists at a solid/moving-fluid interface. Whether the movement of the fluid is due to "forced" or "natural"convection, a thin layer of fluid will always remain completely immobile at the surface of the solid due to the solid-liquid interactive forces.Voltammetry under conditions ofconvective mass transport to/from the surface of theworking electrode.A proposed, newenergy distribution system based on hydrogen gas as the energy carrier and hopefully on a renewable energy supply. Hydrogen could be generated (using e.g., solar energy) in a variety of ways, one of them beingwater electrolysis. Hydrogen would be distributed to the end users through a system similar to today's gas pipelines. The hydrogen could be used either by burning to generate heat or byfuel cells to generate electricity.Aredox electrode with dissolved hydrogen gas being thereduced species and hydrogenions theoxidized species. Hydrogen gas (or a gas mixture containing hydrogen) is bubbled through theelectrolyte to keep a desired dissolved hydrogen content. The inert metallic electrode is usuallyplatinized platinum. Theequilibrium potential of this electrode depends on theconcentration (strictly speaking,activity) of both the hydrogenions and the dissolved hydrogen gas (controlled by the hydrogen gas pressure), seeNernst equation. The electrode can be used as a measuring electrode in a sensor to determine the hydrogen ion concentration (pH), or it can be used as areference electrode if all the concentrations are known and constant. It is used equally often for both purposes. It is also the most fundamental reference electrode as thestandard hydrogen electrode.Anelectrode reaction in which hydrogen gas is produced at thecathode of anelectrolytic cell by thereduction of hydrogenions or the reduction of the watermolecules of anaqueous solution. Often abbreviated as "her." See alsowater electrolysis.Seewater electrolysis.Seestandard hydrogen electrode.A chemical reaction in which water reacts with another substance and gives decomposition or other products, often a reaction of water with a salt to create anacid or a base.A substance that contains water. The opposite ofanhydrous.Seebrine electrolysis.
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Alternative, and somewhat archaic, expression forideal non-polarizable electrode.Anelectrode that is practically notpolarizable. That is, thepotential of the electrode will not change from itsequilibrium potential with the application of even a largecurrent density. The reason for this behavior is that theelectrode reaction is extremely fast (has an almost infiniteexchange current density). Also called "ideal depolarized electrode." Contrast withideal polarizable electrode.Anelectrode is called "ideal polarizable" if noelectrode reactions can occur within a fairly wideelectrode potential range. Consequently, the electrode behaves like acapacitor and onlycapacitive current ( nofaradaic current) is flowing upon a change of potential. Many electrodes can behave as an ideal polarized electrode but only within an electrode potential range called the "double-layer range." Also called "completely polarizable electrode." Contrast withideal non-polarizable electrode.Stands for "inner Helmholtz plane." See theHelmholtz model of the double layer.Impedance is the analogue of theresistance or resistivity when applied toalternating current. That is, it is a measure of a material's inability to carry the electrical current. In many materials, the impedance varies as the frequency of the appliedelectrical potential changes, due to the properties of the liquid or solid. In electrochemistry, the impedance of theelectrode reaction is also frequency dependent.Seeworking electrode.Seesupporting electrolyte.The production of chemicals in anelectrolytic cell through intermediate electrolysis products. It is often used in theoxidation/reduction of organic compounds that would otherwise react very slowly at theelectrode surface. An intermediate oxidizing/reducing agent is produced at the electrode surface and the agent reacts with the organic in the bulk solution. The agent is continuously regenerated by the electrolysis. A typical oxidizing agent is the ferric (tri-valent iron)ion, and an example of the reducing agent is the cerous (tri-valent cerium) ion. The reactive intermediate is often called a "mediator," and the overall reaction a "mediated reaction."Seeindirect electrolysis.Seesupporting electrolyte.A chemical that stops (or at least decreases the rate of) a chemical reaction. See alsocorrosion inhibitor.See theHelmholtz model of the double layer. Abbreviated as "IHP."Acharge-transfer reaction with thereactants in direct contact with each other, without any intervening solventmolecules. Note that a "reactant" can also be anelectrode. Contrast withouter-sphere charge-transfer reaction.A material that will not carry anyelectrical current. It has zeroconductivity and infiniteresistivity.Anelectrically conductive structural part that connectsseries-connected cells in afuel cellstack.Theelectrolyte solution inside areference electrode assembly such as thesilver/silver-chloride electrode. (Also called "filling solution.") Internal electrolytes are used also inmembrane electrodes.Areference electrode used inside amembrane electrode assembly as an electrical contact, with stable potential, to theinternal electrolyte.
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An electrically charged chemical particle (atom,molecule, or molecule fragment). "Anions" are negatively charged, and "cations" are positively charged.A plastic sheet formed fromion-exchange resin. The utility of suchmembranes is based on their property that they are permeable preferentially only to either positiveions (cation-exchange membrane) or to negative ions (anion-exchange membrane).A polymeric resin that contains electrically charged fragments ("fixed ions") permanently attached to the polymer backbone, electrical neutrality is achieved by attached mobile "counterions" in the solution phase the resin is immersed into. A practical use of such resin is the removal of unwantedions from a solution by replacing them with other ions. E.g., acation exchange resin containing fixed negative charges with attached mobile sodium ions can be used to remove "hardness" from water if the calcium and magnesium ions are more strongly attracted to the resin and therefore will replace the sodium ions. Eventually all the sodium ions will go into solution and the ion-exchange process terminates. The resin can be regenerated by soaking in a highconcentration sodium salt solution. Such process can also be used to remove unwanted ions from polluted water streams.A material that conducts electricity withions as charge carriers. See alsoelectrolyte.A quantitative measure of anion's ability to move under the influence of apotential difference in solution. (See alsoelectromigration.) It is the speed of movement under the influence of unit potential difference.While the mobility is defined in terms electromigration, it also affects the speed ofdiffusion.
Part of electrochemistry that deals with the behavior ofions in solutions (liquids in general) and also in solids (called "solid state ionics").Seeion-exchange membrane.Anelectrode or electrode assembly with apotential that is dependent on theconcentration of anionic species in the test solution and is used forelectroanalysis. Ion-selective electrodes are oftenmembrane type electrodes.Seeion-selective electrode.Somepotentiostats are equipped with an optional ir compensation. The potentiostat electronically corrects for thesolution ir drop and thepotential of theworking electrode is controlled (at least in principle) at the correct value. Unfortunately, most potentiostats become unstable at full compensation, so one can only make a partial compensation, resulting in anuncompensated ir drop and an error in the potential control. The user must provide the solutionresistance value, though some potentiostat setups will measure it automatically.A numerical correction of measuredpotential of theworking electrode for thesolution ir drop. (One must know the value of thecurrent and the value of theresistance of theelectrolyte between theworking and thereference electrodes.) It cannot be simply stated whether this correction is positive or negative because of the contradictory conventions used for theanodic andcathodic currents. In either case, the absolute value of the corrected potential must be smaller than that of the uncorrected potential.Theelectrical potential difference between the two ends of a conducting phase during acurrent flow. It is the product of the current (i) and theresistance (r) of theconductor. In electrochemistry, it refers to thesolution ir drop, or to the total ir drop in anelectrochemical cell. SeeOhm's law.Anelectrode with anirreversible electrode reaction.A qualitative term for a slowelectrode reaction. An electrode reaction having a smallexchange current density. Opposite:reversible electrode reaction. See alsoquasi-reversible electrode reaction.Stands forion-selective electrode.A variation of theelectrophoretic separation technique. The separation ofmolecules occurs in a combination ofpotential anpH gradients resulting in sharper separations compared to simple electrophoresis.
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Seeliquid-junction potential
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kilo
kiloohm
kinetics
kΩ
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The letter "k" when used as a prefix before a unit symbol indicates a multiplier of 103. Symbol of "kilo". E.g.,kΩ = 103ohm, one kiloohm, one thousand ohms. (The symbol is the letter "k" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)When used as a prefix before a unit name it indicates a multiplier of 103. E.g.,kiloohm = 103ohm, one thousand ohms. Symbol: "k".103ohm, symbol: "kΩ" (one thousand ohms). (The symbol is the letter "k" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)Chemical kinetics is a scientific discipline dedicated to the study of the rates of chemical reactions. How fast is a reaction proceeding in time, and what is affecting the rate.Symbol and abbreviation ofkiloohm (= 103ohm, one thousand ohms). (The symbol is the letter "k" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)
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Arechargeable battery. Duringdischarging, the reaction on the positiveelectrode is the conversion of lead dioxide to lead sulfate, while on the negative electrode it is the conversion of metallic lead to lead sulfate. The reactions are reversed duringcharging. Thecurrent collector can be lead in both electrodes. Theelectrolyte is sulfuricacid. While it is one of the earliest practical storage batteries (1866), it is still very widely used today, e.g. as automobile starter battery.One of the earliest practicalnon-rechargeable batteries (Georges-Lionel Leclanche, 1866). It uses a zincanode (negativeelectrode) and a manganese dioxidecathode (positive electrode) with ammonium chloride solution aselectrolyte. The initially liquid electrolyte was later "immobilized," and this system became the firstdry cell. It is still widely used.Small amounts of (usually organic) compounds added to anelectroplating solution that changes themechanism of the plating to produce a metal deposit smoother than the original substrate.The maximumcurrent density that can be achieved for anelectrode reaction at a givenconcentration of thereactant in the presence of a large excess ofsupporting electrolyte.Themass transport occurs exclusively throughdiffusion in thediffusion layer, driven by theconcentration difference of thereactant between the edge of the diffusion layer and theelectrode surface. As the current density is increased (usually by changing theelectrode potential), the surface concentration of the reactant must decrease so that the concentration difference driving the diffusion can increase and provide the required flux of the reactant. However, the surface concentration obviously cannot decrease below zero, thereby a situation is reached when further change of the electrode potential cannot increase the reactant flux, and correspondingly the current density. The concept of "limiting current density" is valid even in the absence of supporting electrolyte. However, the situation is more complex in this case becauseelectromigrational effects must also be taken into consideration.
Seevoltammetry.Apotential difference between two solutions of different compositions separated by a membrane typeseparator. The simplest example is the case of two solutions containing the same salt in differentconcentrations. The salt willdiffuse from the higher concentration side to the lower concentration side. However, the diffusion rate of thecation and theanion of the salt will very seldom be exactly the same (seemobility). Let us assume for this example that the cations move faster; consequently, an excess positive charge will accumulate on the low concentration side, while an excess negative charge will accumulate on the high concentration side of the junction due to the slow moving anions. This sets up a potential difference that will start anelectromigration of theions that will increase the net flux of the anions and decrease the net flux of the cations. Insteady-sate conditions, the two ions will move at the same speed and a potential difference will be created between the two solutions. This "steady-sate" potential difference seems constant, but this is misleading because it slowly changes as the concentrations between the two solutions equalize. The diffusion process will "eventually" result in equal concentrations of the salt in the two solutions separated by the membrane, and the liquid-junction potential will vanish.A device that consumeselectrical power, e.g. a motor or a light bulb.Seecorrosion.Stands for "linear-sweep voltammetry," seevoltammetry.Asalt bridge with a thin,capillary tip at one end. This can be useful for minimizing thesolution ir drop by placing the fine capillary tip very close to the surface of the working electrode, when used to connect theworking andreference electrode compartments of athree-electrode cell. The solution distance causing the ir drop can be easily limited to a fewmillimeters; and, in specially designed cells, often to a much smaller distance.Stands for "linear-sweep voltammetry," seevoltammetry.
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The letter "m" when used as a prefix before a unit symbol indicates a multiplier of 10-3. Symbol of "milli". E.g.,mV = 10-3volt, one millivolt, one thousandth of a volt.The letter "M" when used as a prefix before a unit symbol indicates a multiplier of 106. Abbreviation of "meg" or "mega". E.g.,MΩ = 106ohm, one megohm, one million ohms. (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)The letter "µ" when used as a prefix before a unit symbol indicates a multiplier of 10-6. Abbreviation of "micro". E.g.,µV = 10-6volt, one microvolt, one millionth of a volt.Symbol and abbreviation ofmilliampere (= 10-3ampere, one thousandth of an ampere).Symbol and abbreviation ofmicroampere (= 10-6ampere, one millionth of an ampere).Electrochemical phenomena occurring under the influence of magnetic field. See also anEncyclopedia Article.Arechargeable battery which does not require periodic "topping up" (addition of water) to maintainelectrolyte volume. See alsosealed battery.The phenomenon of movement (transportation) of mass (e.g., chemical compounds, ions) from one part of the system to another. Most of the time this occurs throughdiffusion andconvection. Under special circumstances it can also occur throughelectromigration since the movement of the electrically chargedions also carries mass with them (e.g., ioniccurrent through anion-exchange membrane.The mass transport and thecharge transport are typically "decoupled" in electrochemistry; that is, the reacting species and the charge carrying species are not necessarily identical. E.g., one would intuitively assume that duringelectroplating of copper from a solution of copper sulfate all the mass and charge required for theelectrode reaction at thecathode would be carried by the coppercations in the solution. That is not the case at all. In the presence of a large excess ofsupporting electrolyte, all thecurrent is carried byelectromigration of theions of the supporting electrolyte (both in the bulk solution and in thediffusion layer) while all the mass is carried by the copper ions byconvection in the bulk solution and bydiffusion in the diffusion layer. In the absence of any supporting electrolyte, all the mass is still carried by the copper ions, while the current is divided between the coppercations and the sulfateanions according to the ratio of theirtransport numbers (both in the bulk electrolyte and in the diffusion layer). The situation is maybe more self-explanatory in the case of a neutralmoleculereactant that cannot carry any current.
Seeconcentration overpotential.Seeworking electrode.Seeindirect electrolysis.Seeindirect electrolysis.When used as a prefix before a unit name it indicates a multiplier of 106. E.g.,megohm = 106ohm, one million ohms. Symbol: "M".106ohm, symbol: "MΩ" (one million ohms). (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)Seeseparator.Anion-selective electrode assembly terminating in an ion permeable (e.g.,ion-exchange) membrane sensing element. The membrane separates the internal filling solution (that contains a fixedconcentration of theion to be detected) and the test solution. Thepotential across the membrane depends on the concentration ratio of the ion in the two solutions. The assembly also contains aninternal reference electrode immersed in the filling solution, serving as an electrical contact with a stable potential. The potential of this assembly is then measured against an externalreference electrode immersed in the test solution. See alsoDonnan potential.SeeDonnan potential.A class ofelectrode reactions involvingoxidation/reduction of a solid metal and its dissolvedion. E.g., if a silver metal rod is immersed in a silver nitrate solution, the univalent (singly positively charged) silver ions can becathodically reduced to silver metal (electroplating), or the silver metal can beanodically oxidized to silver ions (electrodissolution). Compare with aredox reaction where both the oxidized and the reduced species are in solution.Seecementation.Seeelectroplating.Seeelectrorefining.Seeelectrowinning.
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When used as a prefix before a unit name it indicates a multiplier of 10-6. E.g.,microvolt = 10-6volt, one millionth of a volt. Symbol: "µ".10-6ampere, symbol: "µA" (one millionth of an ampere).A smallelectrode, with dimensions not larger than a fewmillimeters, and typically with dimensions of a small fraction of a millimeter.10-6volt, symbol: "µV" (one millionth of a volt).Seeelectromigration.When used as a prefix before a unit name it indicates a multiplier of 10-3. E.g.,millivolt = 10-3volt, one thousandth of a volt. Symbol: "m".10-3ampere, symbol: "mA" (one thousandth of an ampere).10-3volt, symbol: "mV" (one thousandth of a volt).Theelectrode potential when twoelectrode reactions occur on the same electrode surface. The mixed potential has a value in between theequilibrium potentials of the two electrode reactions. The mixed potential is asteady-state phenomena, with thecorrosion potential being a good example.Symbol and abbreviation ofmegohm (= 106ohm, one million ohms). (The symbol is the letter "M" followed by the "Greek capital omega" letter, some browsers unfortunately do not support this.)Seeionic mobility.Seeconcentration.Seeconcentration.The weight of amolecule of a compound that may be calculated as the sum of theatomic weights of its constituentatoms.The smallest physical unit of a substance that retains all the physical and chemical properties of that substance. It may consist of a singleatom or of a group of atoms bonded together chemically.Instrument that can be used for the measurement of more than one parameter. Typically, it can be used to measurecurrent,potential, andresistance.Symbol and abbreviation ofmillivolt (= 10-3volt, one thousandth of a volt).Symbol and abbreviation ofmicrovolt (= 10-6volt, one millionth of a volt).
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The letter "n" when used as a prefix before a unit symbol indicates a multiplier of 10-9. Abbreviation of "nano". E.g.,nV = 10-9volt, one nanovolt, one billionth of a volt.Symbol and abbreviation ofnanoampere (= 10-9ampere, one billionth of an ampere).When used as a prefix before a unit name it indicates a multiplier of 10-9. E.g.,nanovolt = 10-9volt, one billionth of a volt. Symbol: "n".10-9ampere, symbol: "nA" (one billionth of an ampere).10-9volt, symbol: "nV" (one billionth of a volt).An equation defining theequilibrium potential of anelectrode. The potential is the sum of thestandard electrode potential and a correction term for the deviation from unitconcentrations of thereactant and the product of theelectrode reaction.
The correction term is the product of the "Nernst slope" and the logarithm of the ratio of the concentrations (strictly speaking,activities) of theoxidized species and thereduced species. At room temperature, the Nernst slope is 0.05916 volt divided by the number ofelectrons transferred during the reaction. E.g., for a simplemetal deposition/dissolution reaction the slope is 0.05916 for a single charged metalcation, 0.00296 volt for a double charged ion, etc.
SeeNernst equation.It is equal to the change ofequilibrium electrode potential when theconcentration (strictly speaking,activity) of a species involved in theelectrode reaction changes by ten fold.
Anelectrode is said to behave "nernstially" if theequilibrium electrode potential obeys theNernst equation when theconcentration (strictly speaking,activity) of a species involved in theelectrode reaction changes. Opposite:non-nernstian behavior.Seediffusion layer.Seereversible electrode reaction.Seeatomic structure.Stands for "normal hydrogen electrode," which is an alternative name for thestandard hydrogen electrode.A solution with the solvent anything but water (e.g., organic or inorganic liquid, molten salt).Seecapacitive current (density).Anelectrode is said to behave "non-nernstially" if theequilibrium electrode potential does not obey theNernst equation when theconcentration (strictly speaking,activity) of a species involved in theelectrode reaction changes. Opposite:nernstian behavior.A system or system element is behaving "non-ohmically" if it does not followOhm's law. That is, the value of theresistance depends on thecurrent or thepotential. Opposite:ohmic behavior.The resistance can be formally defined as the differential of the potential with respect of the current. In the case of Ohm's law, this is the constant value of the resistance. In electrochemistry, a typical "non-ohmic" element is thecharge-transfer resistance. Thecharge-transfer reaction can be considered a circuit element because it requires a certain amount ofoverpotential to force through a current. However, the pertinent relation here is theTafel law (at least at relatively large overpotentials), and the differential of the current (that is the resistance) is a function of the current itself.
Anelectrode that is not easilypolarizable. That is, thepotential of the electrode will not change significantly from itsequilibrium potential with the application of even a largecurrent density. The reason for this behavior is that theelectrode reaction is inherently fast (has a largeexchange current density). See alsooverpotential. Opposite:polarizable electrode.Abattery in which the chemical reaction system providing the electricalcurrent is not easily "chemically"reversible. It provides current until all the chemicals placed in it during manufacture are used up. It is discarded after a singledischarge. Also called "primary" battery or cell. Contrast withrechargeable battery.This battery always operates as agalvanic cell. Consequently, theanode is the negativeelectrode, while thecathode is the positive electrode.
Alternative name forstandard electrode potential.Alternative name forstandard hydrogen electrode.Seeatomic structure.Symbol and abbreviation ofnanovolt (= 10-9volt, one billionth of a volt).
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Symbol and abbreviation ofohm. (The symbol is the "Greek capital omega" letter, some browsers unfortunately do not support this.)Stands for open-circuit potential. Seeequilibrium potential.Stands foropen circuit voltage.Stands foroxygen evolution reaction.Measurement unit of the electricalresistance. Symbol: "Ω". (The symbol is the "Greek capital omega" letter, some browsers unfortunately do not support this.)A system or system element is behaving "ohmically" if it followsOhm's law. That is, the value of theresistance is independent of thecurrent and thepotential. Typically, metals andelectrolyte solutions are "ohmic." Opposite:non-ohmic behavior.The relation amongst thecurrent flowing through aresistor and thepotential difference between the two ends of the resistor. The potential difference is equal to the product of the current and the resistance (volt =ampere timesohm).
Seeir drop.Seeir drop.Instrument used for the measurement of electricalresistance.Stands for "outer Helmholtz plane." See theHelmholtz model of the double layer.Seereserve battery.Seeequilibrium potential.Thecell voltage under zero current conditions. For agalvanic cell seeelectromotive force (emf).Stands foroxidation/reduction potential electrode.Stands foroxygen reduction reaction.See theHelmholtz model of the double layer. Abbreviated as "OHP."Acharge-transfer reaction with thereactants separated from each other by some solventmolecules due to thesolvation of the reactants. Note that a "reactant" can also be anelectrode. Contrast withinner-sphere charge-transfer reaction.Seecell reaction.During thecharging of arechargeable battery, eventually enough electrical charge is supplied to convert all the active material stored in theelectrodes. If charging continues, the battery is said to be "overcharged." It very much depends on the battery system whether overcharging is detrimental to the battery or not.The difference in theelectrode potential of anelectrode between itsequilibrium potential and its operating potential when acurrent is flowing. The overpotential represents the extraenergy needed (an energy loss that appears as heat) to force theelectrode reaction to proceed at a required rate (or its equivalentcurrent density). Consequently, the operating potential of ananode is always more positive than its equilibrium potential, while the operating potential of acathode is always more negative than its equilibrium potential. The overpotential increases with increasingcurrent density, seeTafel equation. The value of the overpotential also depends on the "inherent speed" of the electrode reaction: a slow reaction (with smallexchange current density) will require a larger overpotential for a given current density than a fast reaction (with large exchange current density). See alsoovervoltage.Anelectrode reaction always occurs in more than oneelementary step, and there is an overpotential associated with each step. Even for the simplest case, the overpotential is the sum of theconcentration overpotential and theactivation overpotential.
The difference between thecell voltage (with acurrent flowing) and theopen-circuit voltage (ocv). The overvoltage represents the extraenergy needed (an energy loss that appears as heat) to force thecell reaction to proceed at a required rate. Consequently, the cell voltage of agalvanic cell (e.g., arechargeable battery duringdischarging) is always less than its ocv, while the cell voltage of anelectrolytic cell (e.g., arechargeable battery duringcharging) is always more than its ocv.The overvoltage is the sum of theoverpotentials of the twoelectrodes of the cell and the totalir drop of the cell. Unfortunately, the terms "overvoltage" and"overpotential" are sometimes used interchangeably.
Alternative expression foroxidizing agent.In a narrow sense, oxidation means the reaction of a substance with oxygen. Hydrogen can react with oxygen to be oxidized to water. Hydrocarbon fuels (gasoline, natural gas, etc) can react with oxygen to be oxidized to carbon dioxide and water. Iron can react with oxygen to be oxidized to "rust." During oxidation, the oxygen itself is being reduced. Oxidation and reduction always occur simultaneously.During these reactions,electrons are transferred from the substance that is oxidized to the oxygen. In a wider sense, allelectron-transfer reactions are considered oxidation/reduction. The substance gaining electrons ("oxidizing agent" or "oxidant") is oxidizing the substance that is losing electrons ("reducing agent" or "reductant"). In the process, the "oxidizing agent" is itself reduced by the "reducing agent."
A measure of theoxidation/reduction capability of a solution. It is aredox potential measured with an inert electrode. An oxidizing solution (e.g., onesaturated with oxygen) has a more positive potential than a reducing solution (e.g., one saturated with hydrogen).Ameasuring electrode used for the determination of theoxidation/reduction potential of a solution. Abbreviated as "ORP."A substance that is affecting oxidation by acceptingelectrons from another substance. Seeoxidation/reduction. Also called "oxidant."Anelectrode reaction in which oxygen gas is produced at theanode of anelectrolytic cell by theoxidation of hydroxyl (OH-)ions or the oxidation of the watermolecules of anaqueous solution. Often abbreviated as "oer." See alsowater electrolysis. It is the reverse reaction ofoxygen reduction.Seewater electrolysis.Anelectrode reaction in which oxygen gas isreduced at thecathode of anelectrochemical cell. The product of the reduction can be hydroxyl (OH-)ions or watermolecules (or occasionally hydrogen peroxide molecules). Often abbreviated as "orr." It is the reverse reaction ofoxygen evolution. It is a very important and much studied electrode reaction because it occurs at the cathode of practically allfuel cells and it occurs at the cathode of many (though not all)corrosion cells.
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Anelectrode assembly consisting of loosely packed small particles of the electrode material (e.g., some metal or carbon) with theelectrolyte flowing through the bed. This type of electrode is especially useful for removing small traces of impurities from the solution byelectrolysis (e.g., waste treatment) because the solution is well stirred and it contacts a large surface of the electrode material.Seeelectrophoretic deposition.Individualelectrochemical cells can be combined in assemblies by parallel orseries coupling (or a combination of the two). In case of "parallel" coupling, the positiveelectrode of every cell is connected together and the negative electrode of every cell is connected together, resulting in two externalterminals. Thevoltage of every cell must be identical in parallel coupled assemblies. The overallcurrent passing through the assembly is the sum of the individual cell currents, while the assembly voltage is identical to the individual cell voltage. Parallel coupling is often used inbatteries. Celllines andstacks can also be parallel coupled.The twocurrent densities at which theelectrode reaction is proceeding in theanodic andcathodic directions at anelectrode potential. The actual (net) current density is the algebraic sum of the two partial current densities (one is considered positive the other negative).Electrode reactions are typically "chemically"reversible, that is, they can proceed both in forward and reverse direction. Atequilibrium, the reaction is proceeding at equal rate in both directions (seeexchange current density): the "anodic partial current density" and the "cathodic partial current density" are equal and the net current density is zero. When theelectrode ispolarized, the partial current densities are unequal and the net current density is not zero. If the electrode is negatively polarized, the cathodic reaction speeds up (compared with its rate at equilibrium), while the anodic reaction slows down and a net cathodic current density results (and vice versa for anodic polarization).Seeelectrode reaction.The formation of a thin adherent film or layer on the surface of a metal or mineral that acts as a protective coating to protect the underlying surface from further chemical reaction, such ascorrosion,electrodissolution, or dissolution. The passive film is very often, though not always, an oxide. A passivated surface is often said to be in a "passive state." The surfaceoxidation can result from chemical or electrochemical (anodic) oxidation. During anodic passivation, in thecurrent-potential plots, the current, instead of increasing with potential, falls to a very small value. Seepassivation potential. See also anEncyclopedia Article.The most negativeelectrode potential at which apassivating film is formed electrochemically. It is equal to or more positive than theequilibrium potential of formation of the compound (usually oxide) constituting the passive film. Usually thecurrent goes through a maximum at the passivation potential. See also anEncyclopedia Article.The permeation of certainions in preference to other ions through anion-exchange membrane.A measure of the acidity/alkalinity (basicity) of a solution. The pH scale extends from 0 to 14 (inaqueous solutions at room temperature). A pH value of 7 indicates a neutral solution. A pH value of less than 7 indicates anacidic solution, the acidity increases with decreasing pH value. A pH value of more than 7 indicates abasic solution, the basicity or alkalinity increases with increasing pH value.The pH of a solution is equal to the negative, ten-based logarithm of theactivity of the hydrogenions in the solution. Neutral waterdissociates into equal amounts of hydrogen (H+)cations and hydroxyl (OH-)anions. As the product of theconcentrations (activities) of the two ions is always a constant 10-14, water has a pH of 7. In acidic solutions the hydrogen ions are in excess, while in basic solutions the hydroxyl ions are in excess.
Seebuffer solution.Anelectrode assembly with apH dependentpotential. A variety of different electrodes can be used for this purpose, the most common one is theglass electrode.Volt meter that measures theelectrical potential difference between apH electrode and areference electrode and displays the result in terms ofpH value of the sample solution in which they are immersed.Agalvanic cell in which usablecurrent andvoltage are simultaneously produced uponabsorption of light by at least one of theelectrodes. Also calledphotogalvanic cell.Chemistry resulting from the interaction of light with electrochemical systems. See alsophotoelectrochemical cell andphotoelectrolytic cell.Anelectrolytic cell in which the production of chemicals is caused by or speeded up by theabsorption of light by at least one of theelectrodes. The process occurring in such cell is called "photoelectrosynthesis."Production of chemicals in aphotoelectrolytic cell, where the production is caused by or speeded up by theabsorption of light by at least one of theelectrodes.Seephotoelectrochemical cell.Seebuffer solution.Process for removal of oxide scales from metal surfaces in preparation forelectroplating. Typically, the metal is immersed in hot, stronglyacidic solution that dissolves the oxide scales. The solution usually also contains somecorrosion inhibitor to avoid dissolution of the metal itself. See alsoelectrolytic pickling.An archaic name for abattery or otherseries-coupledelectrochemical cells. See, e.g. thevoltaic pile.Electrode structures inrechargeable batteries are sometimes called "plates."Seeelectroplating.A platinum metalelectrode that is covered with a rough, large surface area platinum coating. The purpose is to produce an electrode with a largetrue area that will be relativelynon polarizable. See alsoplatinum black.A rough, large surface area platinum coating usually deposited on a platinum metalelectrode. See alsoplatinized platinum electrode.
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Anelectrode that is easilypolarizable. That is, thepotential of the electrode will change significantly from itsequilibrium potential with the application of even a smallcurrent density. The reason for this behavior is that theelectrode reaction is inherently slow (has a smallexchange current density). See alsooverpotential and ideal polarized electrode. Opposite:non-polarizable electrode.The change ofpotential of anelectrode from itsequilibrium potential upon the application of acurrent. See alsooverpotential.The graphical representation of the result ofpolarography.An instrument used in carrying outpolarographicanalysis.A classicalelectroanalytical technique discovered in 1922 by J. Heyrovsky, for which he was awarded the Nobel Prize for Chemistry in 1959. Essentially, it islinear-sweep voltammetry using adropping-mercury electrode forworking electrode and a large mercury pool ascounter electrode.Anelectrode consisting of a highly porous solid. This is often used infuel cells with gaseousreactants. In this case, thecharge-transfer reaction proceeds mainly at the triple interface formed by the electrode material, theelectrolyte, and the gaseous reactant. The pores of the structure are partly filled by the electrolyte and partly by the gas. A porous electrode provides a much larger area for reaction than a solid electrode with the gas bubbled around it. A porous electrode can also be used as aflow-through electrode.An alternative name of an industrialelectrolytic cell used inaluminum production.Seeelectrical potential.Theelectrode potential where the charge in theelectrical double layer is zero.Can be used in more than one meaning:1. A continuously variableresistor. More precisely, a resistor with continuously variable tap. This can provide threeresistance values, a fixed resistance between the two end connectors, and two variable resistances, one between either end connector and the variable tap connector. The sum of the two variable resistances is the fixed resistance.
2. A somewhat archaic measurement system, based on a resistor with a continuously variable tap, that can be used to measure theelectromotive force ofelectrochemical cells that can be easilypolarized bycurrent. It uses a comparison technique to compare a "standard"voltage source to the unknown, under conditions of practically zero current. It is seldom used today because high input resistancevoltmeters andelectrometers are readily available.
Anelectroanalytical technique based on the measurement of theelectromotive force of anelectrochemical cell comprised of ameasuring and areference electrode. The simplest example of a measuring electrode is a metal electrode whosepotential depends on theconcentration of thecation of the electrode metal (seeNernst equation).An electronic instrument that controls the electricalpotential between theworking andreference electrodes of athree-electrode cell at a preset value. It forces whatevercurrent is necessary to flow between theworking andcounter electrodes to keep the desired potential, as long as the neededcell voltage and current do not exceed thecompliance limits of the potentiostat.An electrochemical measuring technique forelectrochemical analysis or for the determination of thekinetics andmechanism ofelectrode reactions based on the control of theelectrode potential.Seeelectrical power.Characteristic parameter of abattery indicating itselectrical power per unit weight or volume. The terminology is not strictly defined. Weight based power density is often called "specific power" or "gravimetric power density." Volume based power density is often called "power density" or "volumetric power density. The power density is typically expressed as watt/kilogram or watt/liter.Seeelectrical source.Seenon-rechargeable battery.Acurrent distribution that is completely controlled by theresistivity of theelectrolyte solution between theworking andcounter electrodes. Thecurrent always follows the least resistive path; consequently, a nonuniform current distribution will result if the geometry of the electrodes is such that the resistivity of the current path is not the same to every point on the working electrode. Every other effect that may influence the current distribution is ignored in this case, or assumed to be negligible. See alsosecondary andtertiary current distribution.Seeatomic structure.Stands forpotential of zero charge.
a -- b -- c -- d -- e -- f -- g -- h -- i -- j -- k -- l -- m -- n -- o -- p -- q -- r -- s -- t -- u -- v -- w -- zquasi-reversible electrode
quasi-reversible electrode reaction
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Anelectrode with aquasi-reversible electrode reaction.A qualitative term for an intermediate speedelectrode reaction. An electrode reaction having an intermediateexchange current density, between that of anirreversible and areversible electrode reaction.
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The rate of a chemical reaction is proportional to the product of theconcentrations of all thereactants taking part in the reaction, with the "rate constant" the proportionality factor. In other words, the rate constant is the rate of the reaction when all reactants are present in unit concentration.The rate constant of a chemical reaction is a function of temperature and pressure. For anelectrode reaction the rate constant is also a function of theelectrode potential.
The slowestelementary reaction step in the series of steps comprising the overall reaction. The slowest step will control the rate of the overall reaction. This is analogous to a traffic situation, the time required to drive from the suburbs to downtown may completely depend on the time spent in one traffic jam on the road. Abbreviated as "rds."Thepotential of anelectrode expressed against thepotential of zero charge (pzc) of the same electrode in the same solution. This provides a potential scale specific for each electrode, with the origin (zero potential) always at the respective pzc. Consequently, two electrodes having the same "rational potential" are typically not at the same "potential" (as measured against anyreference electrode). However, the potential of these two electrodes is displaced by the same amount from their respective pzc; therefore, thecharge density in thedouble layer of the two electrodes will be approximately equal. Consequently, the comparison of electrode behavior at the same "rational potential" is more meaningful for some purposes than a comparison at the same "potential on some arbitrary scale."The charge densities of two electrodes at the same rational potential are only approximately equal because thedouble-layer capacitance and its potential dependence varies from electrode to electrode.
Stands forrotating-disk electrode.Stands forrate-determining step.A chemical species that is taking part in a chemical reaction by reacting (sometimes by itself, but usually with other reactants) to form the products of the reaction.The totality of all theelementary reaction steps occurring in series or parallel that fully defines the overallelectrode reaction.Theoverpotential associated with a chemical reaction (withoutcharge transfer) step that is anelementary step in the overallelectrode reaction.Abattery in which the chemical reaction system providing the electricalcurrent is easily "chemically"reversible. Afterdischarging, it can berecharged by applying an electricalcurrent to itsterminals. Some batteries can be recharged hundreds to thousands times. See, e.g. thelead-acid battery. Also called "secondary" battery, and "accumulator." Contrast withnon-rechargeable battery.It operates as agalvanic cell duringdischarge and as anelectrolytic cell duringcharge. As a consequence, theanode is the negativeelectrode during discharge, while it is the positive electrode during charge; at the same time, thecathode is the positive electrode during discharge, while it is the negative electrode during charge. This can create a confusing situation, and it is preferable to refer to the electrodes of a rechargeable battery as "positive" and "negative," because this designation is independent of the operational mode. Unfortunately, this nomenclature is not always followed. Often the "negative" electrode is designated asanode and the "positive" electrode is designated ascathode. This naming convention is a carry-over from the convention of thenon-rechargeable battery.
An electrical equipment that converts alternating current intodirect current.Arechargeable battery with tworedox electrodes contained in compartments typicallyseparated by anion-exchange membrane E.g., a battery with iron and chromiumredox couples in the two compartments. Duringdischarge, "ferric" (trivalent iron)cations arereduced to "ferrous" (divalent iron)ions at one of the redox electrodes, while "chromous" (divalent chromium) ions areoxidized to "chromic" (trivalent chromium) ions at the other electrode. The reverse reactions occur duringcharging. An advantage of this system is that thepolarization losses are relatively small because the redox reactions are typically fast and no solid phases are being formed during the reaction. Also, the solutions containing the iron and chromium salts can be stored in separate large tanks and circulated to a small "battery," permitting thestorage of large amounts of energy.Seeredox reaction.An inertelectrode (e.g., platinum, gold, carbon) thepotential of which is controlled by aredox reaction in solution.This is somewhat of a misnomer since all electrodes involveoxidation/reduction. The distinction is that in the case of a redox electrode both oxidized and reduced species are dissolved in the solution surrounding the electrode.
Thepotential of aredox electrode.A class ofelectrode reactions involvingoxidation/reduction of two dissolved species. E.g., iron metal can exist in solution as a doubly positively charged ("ferrous")ion or a triply positively charged ("ferric") ion. Such a system is often called a "redox couple," such as the "ferrous/ferric" couple. The ferric ions can becathodically reduced to ferrous ions, or the ferrous ions can beanodically oxidized to ferric ions. With these reactions, an inertelectrode is used that does not take part in any reactions under the conditions of the oxidation/reduction of the ions. This electrode then acts only as a source or sink ofelectrons; examples are: carbon, graphite, platinum, gold. Compare with ametal deposition/dissolution reaction where one of the reacting species is a solid metal and the other species is in solution.A substance that is affecting reduction by donatingelectrons to another substance. Seeoxidation/reduction. Also called "reductant."Alternative expression forreducing agent.Seeoxidation/reductionAnelectrode that has a well known and stableequilibrium electrode potential. It is used as a reference point against which thepotential of other electrodes (typically that of theworking electrode or measuring electrode) can be measured in anelectrochemical cell. In principle it can be any electrode fulfilling the above requirements. In practice, there are a few commonly-used (and usually commercially-available) electrode assemblies that have an electrode potential independent of theelectrolyte used in the cell. For some common reference electrodes see e.g., thesilver/silver-chloride electrode,calomel electrode, andhydrogen electrode.Strictly speaking, there can be a small change in the potential of these electrodes depending on the electrolyte because the presence of aliquid-junction potential. This is very often (justifiably or not) ignored. The liquid-junction potential is also minimized by the use of highconcentration potassium chloride as the filling solution of the reference electrodes, because thediffusion rate of bothions is very closely the same in these solutions.
Anon-rechargeable battery that is stored in an "inactive" form until its intended, immediate use. E.g., the battery is stored "dry," and it can be activated by injection of theelectrolyte. Or, a battery operating with molten salt electrolyte is stored at a temperature below the electrolyte melting point, and it is activated by the sudden application of heat to melt the electrolyte. The advantages of such arrangements are an extremely longshelf life without practically any loss ofstored energy, and the possibility to produce a very powerful battery with very reactive chemicals that would otherwise cause a very fastself discharge. Most reserve batteries are made for military applications.A smallfaradaic current density flowing through anelectrode under conditions when zero faradaic current is expected (e.g., within thedouble-layer range). It is caused by traces of impurities in theelectrolyte. Also called "background current (density)."Seeresistivity.Seesolution ir drop.The measure of a material's inability to carryelectrical current. See alsoimpedance.The reciprocal ofconductivity.
An electrical circuit element with a fixedresistivity.Seeequilibrium potential.Anelectrode with areversible electrode reaction.A qualitative term for a fastelectrode reaction. There are, unfortunately, several meanings attributed to the term "reversibility," resulting in possibly confusing situation. An electrode reaction is considered reversible in the "electrochemical sense" if the reaction is fast, that is, if theexchange current density of the electrode reaction is large. In contrast, in the "chemical sense," reversibility indicates that the reaction can proceed both in forward and backward (reverse) direction. Also callednernstian reaction. Opposite:irreversible electrode reaction. See alsoquasi-reversible electrode reaction.Both of the above described meanings of reversibility are different from the meaning in the "thermodynamic sense."
A commonly usedreference electrode. Ahydrogen electrode immersed directly into theelectrolyte of theelectrochemical cell and usually (unless otherwise sated) operated with one atmosphere pressure hydrogen gas. Theequilibrium potential depends on the hydrogenionconcentration (strictly speaking,activity) of the cell electrolyte.A specialized hydrodynamicelectrode used in the study of thekinetics andmechanism ofelectrode reactions and inelectroanalysis for ensuring a known and controllable flow of solution over the electrode. The flow control is achieved by using a flat disc electrode that is rotated in the solution resulting in a definedhydrodynamic boundary layer. Abbreviated as "RDE."A variant of therotating-disk electrode which includes a secondelectrode - a concentric ring electrode - that is placed outside the disk and used toanalyze the species generated on the disk. The ring is electrically insulated from the disk so that theirpotentials can be controlled independently. Abbreviated as "RRDE."Anelectrode made of metal wire (often platinum) rotated about its axis at a known and constant velocity. It is used in the study of thekinetics andmechanism ofelectrode reactions and inelectroanalysis.The ratio between thetrue electrode area and thegeometric electrode area.Stands forrotating-ring-disk electrode.
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Anionically conducting path between separate compartments of anelectrochemical cell. Often theworking andreference (and occasionally even thecounter) electrodes are in completely separate compartments and the required conducting path between them is provided by a tubing filled with highly conductingelectrolyte solution. A common arrangement for a "salt bridge" is an inverted "U" shaped glass tubing with its ends dipped into the solutions of the two cell compartments; however, other materials and shapes are also used. The salt bridge may contain any conducting solution, but very often a highly concentrated potassium chloride solution (often immobilized by some gelling agent) is used.An equation relatingcurrent density,transition time, andconcentration of thereactant in achronopotentiometric experiment, assuming that the current is sufficiently large to immediately result indiffusion limiting conditions. The equation is valid only for planarelectrodes in unstirred solution.The product of the current density and the square root of the transition time divided by the concentration is a constant. The constant is proportional to the square root of thediffusion coefficient of the reactant. Because the equation was derived for an unstirred solution, it ceases to be valid oncenatural convection starts.
Seesolubility.Stands for "saturated calomel electrode." Seecalomel electrode.See maintenance-free battery. Abattery which can be operated without regard to position.Seeelectrode of the second kind.Seerechargeable battery.Acurrent distribution that is controlled by theresistivity of the solution (seeprimary current distribution) and thecharge-transfer resistance of theelectrode reaction occurring on theworking electrode. That is, a current distribution taking into effect theactivation overpotential. A large charge-transfer resistance (that is, a slow reaction), compared to the solution resistance, tends to make the current distribution more uniform. This still ignores the effect of theconcentration overpotential, seetertiary current distribution.Anelectrical potential difference that arises when small suspended particles move through a liquid (e.g., forced by gravity). Also called "Dorn potential. Seeelectrokinetic effects.A slowdischarging of abattery without being connected to an externalload. This is caused partly by impurities and side reactions (reactions other than thecell reaction) and partly by the imperfect separation of the active chemicals in the battery causing a slow "direct" reaction between them. The rate of the self discharge determines theshelf life of anon-rechargeable battery.Aseparator through which certainmolecules can pass but others cannot.Seeworking electrode.A thin structural material (usually a sheet) used to separate theelectrolyte of adividedelectrochemical cell into two or more compartments. A separator is typically either a membrane or a diaphragm. The distinction between these two separators is somewhat blurred. A membrane typically has very small pores that permit onlydiffusional orconductive motion of the solvent or theelectrolyte from one compartment to another. A diaphragm has larger pores so that it permits the flow (seeconvection) of the electrolyte solution from one compartment to another but still restricts the complete intermixing the two solutions.Individualelectrochemical cells can be combined in assemblies by series orparallel coupling (or a combination of the two). In case of "series" coupling, the positiveelectrode of one cell is connected to the negative electrode of the next cell, and so on. The assembly has only two externalterminals. The overall voltage of the assembly is the sum of the individualcell voltages, while thecurrent passing through every cell (and the assembly) is the same. Series coupling can be used in a number of assemblies, such asbattery,cell line, andstack.Stands forstandard hydrogen electrode.The time period anon-rechargeable battery can be stored after manufacturing so that it still can provide a required amount of electricity when connected to aload. The shelf life of modern batteries is many years.A commonly usedreference electrode. Theelectrode assembly consists of a silver metal electrode in contact with solid silver chloride (usually as a coating on the silver metal) immersed in anaqueous chloride salt solutionsaturated with silver chloride. All these are contained in a small vessel, typically made of glass tubing. Theinternal electrolyte of the reference electrode assembly and theexternal electrolyte into which the whole assembly is immersed are in ionic contact through aseparator. A typical separator is a small porous ceramic plug sealed into the end of the glass tubing.The operating principle of this electrode is that of anelectrode of the second kind. Theequilibrium electrode potential is a function of the chlorideconcentration of the internal electrolyte ("filling solution"). The most commonly used electrolyte is 4molar potassium chloride, producing apotential of 0.222 volt against thestandard hydrogen electrode at 25oC. Occasionally, other concentrations of potassium chloride or other chloride salts are used.
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Seebrine electrolysis.Seebrine electrolysis.Seebrine electrolysis.Seebrine electrolysis.The maximum amount of a species that can be dissolved in a given solvent. It is usually expressed as the maximum achievableconcentration. A solution is called "saturated" if it contains the maximum dissolvable amount.Thesolubility of slightly soluble salts is often expressed as the product of the solubilityconcentrations of itsions. E.g., the solubility product of silver chloride is the product of the concentrations of the silver and chloride ions in the saturated solution of this salt. The significance of the solubility product is that its value cannot be exceeded even in the presence of other dissolved salts. Consequently, the solubility of silver chloride is less in a solution containing potassium chloride than in pure water. This is because in the calculation of the solubility product one must use the "total" chloride concentration in the solution, therefore a silver concentration lower than in water is needed to satisfy a constant solubility product.The solubility (the saturated solution concentration) of the salt, in the absence of any other dissolved species in the solution, is the square root of the solubility product for a salt like the silver chloride. Strictly speaking,activities should be used instead of concentrations.
The dissolved species (e.g., a salt) in a solution.Their drop in theelectrolyte solution of athree-electrode cell between theworking and thereference electrodes. This ir drop (which is expressed as apotential) is always included in the measuredpotential of the working electrode. Therefore, it is important to minimize this error, and to place the reference electrode as close as possible to the working electrode (seeLuggin tip). One cancorrect for the ir drop to obtain the real electrode potential, or in some cases one cancompensate for the ir drop during potential control. During the measurement of anelectromotive force (potential measurement without anycurrent flowing), the ir drop is always zero, and the position of the reference electrode is immaterial.Ions in solution are always surrounded by solventmolecules. A few of these molecules will be more or less strongly attached to the ion (mainly because of the attraction of the charged ion and thedipole of the solvent molecule) and this assembly may be considered as a single unit for some purposes. E.g., the solvent molecules will move together with the ion duringdiffusion andelectromigration. The number of solvent molecules so attached to an ion is called the "solvation number." The surface of anelectrode also can, and usually is, solvated. Since the electrodes usually have some excess charge (seeelectrical double layer,) they also attract the solvent dipoles, and the electrode surface is usually covered by a monolayer of strongly oriented solvent molecules.The solvation number is not very exactly defined since its value may depend on the measurement technique.
Seesolvation.Electrochemical phenomena occurring under the influence of soundwaves (typically ultrasound).Seeelectrical source.The quantitative and characteristic measure of theconductivity of a given substance. This characteristic constant is the numerical value of the conductivity between two opposite sides of a unit cube (usually a cube of one centimeter) of the substance. Also called "specific conductivity."Seeenergy density.Seeion-selective electrode.Seepower density.The quantitative and characteristic measure of theresistivity of a given substance. This characteristic constant is the numerical value of the resistivity between two opposite sides of a unit cube (usually a cube of one centimeter) of the substance. Also called "specific resistivity."The simultaneous application of electrochemical and optical spectroscopic techniques to investigate a phenomenon.Aseries-coupled assembly of cells, a term used primarily forfuel cells.Anon-rechargeable cell (battery) whoseemf is accurately known and remains sufficiently constant. It is less and less used nowadays because the availability of electronic voltage standards.Theequilibrium potential of anelectrode when both theoxidized and the reduced species are present in unitconcentration (strictly speaking,activity). (See also theNernst equation.) The standard potentials are always expressed against thestandard hydrogen electrode the potential of which is zero "by definition." Standard potentials are a function of the temperature, they are usually tabulated for 25oC. Also called "normal electrode potential."The standard potential is theelectromotive force of anelectrochemical cell comprised of the electrode in question and the standard hydrogen electrode. Strictly speaking, one must use unitactivities rather thanconcentrations.
The most fundamentalreference electrode in electrochemistry. "By definition" itsequilibrium potential is considered zero at any temperature, because this electrode was chosen as an arbitrary zero point forelectrode potentials. A zero point is needed since the potential of a single electrode cannot be measured, only the difference of two electrode potentials is measurable. All electrode potentials are expressed on this "hydrogen scale." It is ahydrogen electrode with anelectrolyte containing unitconcentration of hydrogenions andsaturated with hydrogen gas at unit atmosphere pressure. This electrode can be somewhat inconvenient to use because of the need to supply hydrogen gas. Therefore, other reference electrodes (e.g.,calomel orsilver/silver chloride) are often used instead, but the measured electrode potentials can be converted to the "hydrogen scale." Also called "normal hydrogen electrode."Strictly speaking, one must use unitactivity rather thanconcentration of hydrogen ions and unitfugacity rather than unit pressure of hydrogen gas.
Therate constant of anelectrode reaction at thestandard electrode potential.For arechargeable battery: the fraction, usually expressed as a percentage, of the totalelectrical energystored in a battery bycharging that is still available fordischarging at a certain point of time. Contrast withdepth of discharge.Seesteady state.A state of a system in which the conditions do not change in time, or at least they do not seem to change with time. That is, the change occurs on a time scale longer than the time scale of the observations. A good example of the creation and slow vanishing of a steady-state condition is theliquid-junction potential.See theGouy-Chapman-Stern model of the double layer.Seeenergy storage.Seebattery.Anelectrical potential difference that arises when liquid is flowing by a solid surface, e.g., when liquid is forced through acapillary tubing or porous solid by a pressure differential. Seeelectrokinetic effects.A group ofelectroanalytical techniques for the determination of trace amounts of substances, consisting of two steps: preconcentration and analysis. The preconcentration involves theelectrodeposition oradsorption of the substance to be determined on the surface of anelectrode. This is followed by the "stripping" analysis of the substance by an electroanalytical technique. For example, traces of metalions can be preconcentrated bycathodic electrodeposition followed byanodic dissolution (stripping). Or traces of halides (e.g., chloride) can be anodically preconcentrated at a mercury electrode as mercury salts, followed by cathodic stripping.Seeelectrochemical capacitor.Anelectrolyte added to the solution for the sole purpose to increase the solutionconductivity, while the electrolyte does not take part in any reactions. Also called "inert electrolyte."
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An early (1905) empirical relation between theoverpotential of theelectrode and thecurrent density passing through the electrode:
"a" and "b" are characteristic constants of the electrode system. A plot ofelectrode potential versus the logarithm of current density is called the "Tafel plot" and the resulting straight line the "Tafel line." "b" is the "Tafel slope" that provides information about themechanism of the reaction, and "a" provides information about therate constant (and theexchange current density) of the reaction.
Rigorously, the equation should be written as:
using the absolute value of the current density and the ± sign foranodic andcathodic overpotentials, respectively. Also, the equation holds only for relatively high overpotentials, approximately 0.1 volt or higher.
SeeTafel equation.SeeTafel equation.SeeTafel equation.Acharging regime delivering moderately high rate ofcurrent when thebattery is at a lowstate of charge and tapering the charging current to lower rates as the battery is charged.The external electrical connection posts of abattery to which either aload (e.g., motor, light bulb) can be connected to use the electrical energy of the battery, or to which acharger can be connected to charge the battery. Every battery has only two terminals (positive and negative) independent whether the battery contains one or more cells internally. This term is also used forcell stacks.Acurrent distribution that is controlled by theresistivity of the solution (seeprimary current distribution), theactivation overpotential (seesecondary current distribution), and theconcentration overpotential. Theconcentration changes occurring at theworking electrode surface affect the rate of theelectrode reaction and can therefore be considered as an additional surfaceresistivity.Agalvanic cell consisting of two identicalhalf cells that are kept at different temperatures.Seeelectrode of the third kind.Anelectrochemical cell containing aworking electrode, acounter electrode, and areference electrode. Acurrent may flow between the working and counter electrodes, while thepotential of the working electrode is measured against the reference electrode. This setup can be used in basic research to investigate thekinetics andmechanism of theelectrode reaction occurring on the working electrode surface, or inelectroanalytical applications.A qualitative term used inelectroplating to describe the ability of the system to produce a uniformly thick deposit on the substrate surface. That is, the "throwing power" is considered "good" when thecurrent distribution is uniform even on an irregularly shaped substrate.The throwing power is a function of both the geometrical arrangement in the electroplating cell and the composition of the plating solution.
Seetransport number.Characteristic time in achronopotentiometric experiment indicating the exhaustion of areactantconcentration at theelectrode surface. Thepotential of the electrode changes sharply upon reaching the transition time.The fraction of the totalcurrent carried in a solution by a givenion. Ions may carry drastically different portions of the total current if theirmobilities are different. E.g., in a solution of sodium chloride, less than half of the current is carried by the positively charged sodiumcations and more than half is carried by the negatively charged chlorideanions because the chloride ions are able to move faster (they have a larger ionic mobility). The transport numbers of the anion and the cation adds up to unity. As a matter of fact, the case when the ions move equally and the transport number of both ions is equal to 0.5 is a rarity. Also called "transference number."For the simplest case of a solution of a single salt of univalent ions, the transport numbers are defined as the mobility of the ion divided by the sum of mobilities of the two ions. If there are more than onesolutes present (e.g., anacidified sodium chloride solution or a mixture of sodium chloride and potassium bromide), every ion will have its own transport number with the sum of them being unity. In these cases, theconcentrations of the ions must also be taken into account in the calculation of the transport numbers, and in the case of polyvalent ions, the charges of the ions must also be accounted for.
A method of maintaining arechargeable battery in a fully charged condition by continuous, long-term, slow-ratecharging, at a level sufficient to balanceself-discharge and occasionaldischarge. See alsofloat charging.Seecurrent density.The surface area of anelectrode taking into consideration the surface roughness. For a perfectly smooth electrode, it is equal to thegeometric electrode area, but it is larger than that for most electrodes. The ratio of the two defines theroughness factor.A classicalelectrochemical cell containing twoelectrodes.
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ultramicroelectrode
uncompensated ir drop
uncompensated (solution) resistance
unpolarizable electrode
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Seeelectrochemical capacitor.A very smallelectrode, with dimensions not larger than few tens of amicrometer, and occasionally with dimensions of a fraction of a micrometer.The part of thesolution ir drop that is not automaticallycompensated for by the electronic control instrumentation.Seeuncompensated ir-drop.Seenon-polarizable electrode.
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Symbol and abbreviation ofvolt.The resistance to flow exhibited by a liquid or gas subjected to deformation.Measurement unit of theelectrical potential. Symbol: "V".A term sometimes used interchangeably withelectrical potential. See alsocell voltage.Seecompliance limits.Seeelectrical source.Seegalvanic cell.The first (1800) laboratorysource of electricity, essentially what we would call today anon-rechargeable battery. Volta assembled (piled up) different metal disks (e.g., copper and zinc) separated by solution soaked pasteboards, repeating the pattern many tens of times. This was essentially abattery containing manyseries-coupledDaniell cells (a cell which was a later improvement) providing rather highcell voltage andcurrent capabilities, but very short life time.An electrochemical measuring technique used forelectrochemical analysis or for the determination of thekinetics andmechanism ofelectrode reactions. "Voltammetry" is a family of techniques with the common characteristics that thepotential of theworking electrode is controlled (typically with apotentiostat) and thecurrent flowing through the electrode is measured. In one of the most common applications of the technique, the potential is scanned linearly in time; this is called the "linear-sweep voltammetry," "LSV," or "LV." "Cyclic voltammetry (CV)" is a linear-sweep voltammetry with the scan continued in the reverse direction at the end of the first scan, this cycle can be repeated a number of times.Graphical representation of the results of avoltammetric measurement.Instrument used for the measurement ofelectrical potential differences.
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Wh
water electrolysis
watt
watt-hour
working electrode
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Symbol and abbreviation ofwatt.Symbol and abbreviation ofwatt-hour.Process for the electrochemical decomposition of water in adividedelectrolytic cell byelectrolysis. Typically, hydrogen gas is produced at the steelcathode (seehydrogen evolution) and oxygen gas is produced at the nickelanode (seeoxygen evolution). The cell is divided to avoid mixing the hydrogen and oxygen gases. Theelectrolyte is typically anaqueous potassium hydroxide solution. Potassium hydroxide is used to provide a largeionic conductivity, even though the potassium is not reacting at any of theelectrodes (seesupporting electrolyte).Measurement unit ofelectrical power. Symbol: "W". Related units are that ofpower density: watt/kilogram (W/kg) and watt/liter (W/l).Measurement unit ofelectrical energy. Symbol: "Wh". Related units are that ofenergy density: watt-hour/kilogram (Wh/kg) and watt-hour/liter (Wh/l).Theelectrode in athree-electrode cell where "the action is." Thekinetics andmechanism of theelectrode reaction may be under investigation, or the reaction occurring on the working electrode may be used to perform anelectrochemical analysis of theelectrolyte solution. It can serve either as ananode or acathode, depending on the applied polarity. One of the electrodes in some "classical two-electrode" cells can also be considered a "working" ("measuring," "indicator," or "sensing") electrode, e.g., in apotentiometric electroanalytical setup where thepotential of the measuring electrode (against areference electrode) is a measure of theconcentration of a species in the solution.
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zinc-carbon cell (battery)
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Alternative name for "electrokinetic potential." Seeelectrokinetic effects.SeeLeclanche cell.
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