(2011-07-20) The slow maturation of a key chemical concept.
The term acid comes from the latin word acetum and the Greek word oxein. Both mean "sour", which betrays the fact that, for many centuries, acidity wasonly measured using our sense of taste (possibly dangerously so) much like pungency today.
In 1777,Antoine Lavoisier (1743-1794)thought that the newly-discovered active component of air (Sheele 1773, Priestley 1774) wasthe long-sought-after "universal acidifying principle". Accordingly, he coined the name oxygen for it (i.e., "generator of acidity",using the aforementioned Greeketymology). As it takes oxygen toturn wine into vinegaror to produce organic acids from other alcohols, Lavoisier guessed that oxygen was essential to acidity. This misguided theory survived long enough for the name oxygen to be universally accepted, in spite of its dubious etymology.
Jacob Berzelius (1779-1848) upheld the same misconception in his theory ofelectrochemicaldualism, as he argued that acids were oxides of non-metals and bases were oxides of metals...
Thereafter,the acids fitting the structure envisioned by Lavoisier were dubbed oxacid (French: oxacide ) . The modern term isoxoacidor oxy-acid. The other acids were dubbed hydracids.
In 1838, Justus von Liebig (1803-1873) defined acids as compounds containing an hydrogen which can be replaced by a metal.
In 1884, SvanteArrhenius (1859-1927) suggested, in his doctoral dissertation,that some substances are ionized in solution. The ideas of Arrhenius were ahead of his time (he only earned a fourth-class degree with that thesis) but he would eventually get a Nobel prize for the work (1903).
In 1887, Arrhenius used the electrolytic theory of dissociation he had introducedin his thesis to formally define an acid as a substance that dissociates in water into anions and hydrogen ions. Conversely, he view a base as a substance dissociating intoa cation and an hydroxide ion (OH-). Although this was a great breakthrough,this definition only applies to aqueous solutions. It fails to recognize the acidity of insoluble substances.
In 1909, using the definition of Arrhenius, Søren Sørensen (1868-1939) introduced the pH scale to quantify the acidity of aqueous solutions.
Green vitriol, kankatum or copperas (used iniron-gall ink): Ferroussulfate (FeSO4, 7 H2O) Melanterite mineral (1850).
Blue vitriol: Copper sulfate (CuSO4, 5 H2O) Chalcanthite mineral (1853).
The corresponding acid (H2SO4) was properly called vitriolic acid in aqueous solution, or oil of vitriol in its viscous concentrated form. The latter was also improperly shortened to vitriol.
H2SO4protonates water moleculesvery easily. In a dilute aqueous solution, sulfuric acid is essentially just a diprotic acid. It's a strong acid withrespect to its first ionization (i.e., un-ionized sufuric acid is virtuallynonexistent in water) and a weak acid with respect to the secondionization:
H2SO4 + H2O H3O+ + HSO4 ( K = 2.4+6 at 25°C ) HSO4 + H2O H3O+ + SO4 ( K = 1.02 at 25°C )
Without water, pure sulfuric acid is an oily substancewhich is a powerful solvent that undergoes an autoprotolysis similar to that of water:
2 H2SO4 H3SO4+ + HSO4 ( K = 2.74 at 25°C )
However, the analogy stops here, sincepure H2SO4 actually contains many other chemical speciesin equilibrium, as listed below. The first entry is the result of subtracting all the other concentrations from 10195.9 mmol/kg, the reciprocal of the nominal molar mass of H2SO4 (98.0785 g/mol).
The above concentrations at chemical equilibrium correspond to the following dimensionless equilibrium constants (cf.mass-action law) if we equate chemical affinities and concentrations (as an approximation).
A + B C + D
[C] [D]/ [A] [B]
H2SO4 + H2SO4 H3SO4+ + HSO4
1.656
H2SO4 + H2SO4 H3O+ + HS2O7
3.427
H2SO4 + H2SO4 H2O + H2S2O7
3.509
H2SO4 + H3SO4+ H3O+ + H2S2O7
2.514
H2SO4 + HSO4 H2O + HS2O7
2.896
H2SO4 + H3O+ H2O + H3SO4+
1.395
H2SO4 + H2O H3O+ + HSO4
1.181
HSO4 + H2O H3O+ + SO4
4.910
The above equilibrium is properly described as 100% sulfuric acid.
H2S2O7 can be viewed assulfuric gas (SO3) dissolved in vitriol oil :
H2SO4 + SO3 H2S2O7
If more trioxide is dissolved than called for by thestoichiometryof pure sulfuric acid,then we effectively have sulfuric acid rated at more than 100%,because adding water to it would yield 100% sulfuric acid! Such stuff is called oleum or fuming sulfuric acid (the vapor is mostly sulfur trioxide).
For better stability in storage, commercial concentrated sulfuric acid is usually rated at 98% (under 1 atm, there's an azeotrope at 98.3%, whichboils at 338°C).
By definition, 20% oleum is 20 kg of SO3 (80.0632 g/mol) dissolved in 80 kg of H2SO4 (98.0785 g/mol).
(2015-09-16) A strong acid and a powerful oxidizing agent.
Commercial concentrated nitric acid is 68% HNO3 which actually forms an azeotrope with water,boiling at 120.5 °C under 1 atm.
One of the earliest extant recipes for the preparation of nitric acid is the distillation ofa mixture of niter (saltpeter; KNO3 ) green vitriol ( FeSO4 ) and alum. This appears in De Inventione Veritatis (part of the Pseudo-Gebercorpus, attributed to the Franciscan alchemistPaul of Tarento,who was active in the second half of the thirteenth century). Related early recipes (heating niter and clay together) are described byAlbertus(1205-1280) and Ramon Llull (c.1232-1316).
(2015-09-21) Stronger than nitric or sulfuric acid...
Commercially available at a 72.5% concentrantion which forms anazeotrope with water at 1 atm, boiling at 203°C (indefinitely stable but hygroscopic).
Perchloric acid was first obtained in 1816, bythe Austrian Count Friedrich von Stadion, who observed the formation of potassium perchlorateby the action of sulfuric acid on potassium chlorate. An aqueous solution of perchloric acid results from the distillation of that mixture.
(2011-07-21) Organic acids which owe their acidity to the -COOH group.
By dehydration, a carboxylic acid RCOOH gives asymmetrical anhydride (RCO)2O. Acetic anhydride (CH3CO)2O is the simplest stable example at room temperature, since formic anhydride decomposes at 24°C.
(2015-08-31) The most acidic phenol (pKa = 0.38) is an explosive soluble in water.
(2011-07-21) Sulfonic acids owe their acidity to the -SO2(OH) group.
Para-toluene-sulfonic acid or PTSA (CAS104-15-4) is also called tosylic acid. It is a non-oxydizing strong acid (pKa = -2.8 in water) which is well-suited to the preparation of standard titration solutions because it's asoluble solid which can be conveniently weighed (172.202 g/mol).
The negative pKa indicates a high dissociation constant in water:
Many other sulfonic acids are even much stronger than that:
Halosulfuric Acids = Halosulfonic Acids :
Being combinations of an halogen atom with a sulfonic group, these are best called halosulfonic acids. The alternate -sulfuric suffix merely evokes the structural similaritywith the molecule of sulfuric acid.
(2015-08-31) A moderately strong acid used in acidimetry (non-hygroscopic crystals).
This is a moderately strong acid: pKa = 1.05
In its solid form, sulfamic acid can be conserved indefinitely at room temperature. Because the crystals are not hygroscopic, they can be weighed accuratelyto prepare standard solutions used in acidimetry. For best accuracy, such solutions must be used promptly, as sulfamic acid degrades (slowly) when dissolved in water.
The standardprocedure is to calibrate a loosely prepared solution of a strong base (e.g. NaOH) by titration of a solution containing a precisely-measuredmass of sulfamic acid crystals. (The volume of water used to dissolve thesulfamic acid is largely irrelevant but the crystals should be as pure as possible.) The basic solution so calibrated cam then be used for the titration of other acidic solutions.
Sulfamic acid is considered a safe acidic cleaning agent for household use,mostly because (unlike stronger acids) it will not produce dangerous chlorine gas when accidentaly mixed withhypochlorite products.
(2011-07-20) A Brønsted-Lowry acid is a proton donor.
(2011-07-20) An acid is an electron-pair acceptor. A base is an electron-pair donor.
Arguing that only hydrogen compounds can be acids would be likearguing that only oxygen compounds can be oxidizing agents. Gilbert N. Lewis (1940, paraphrased)
According to G.N. Lewis, an acid (A) and a base (B:) form an adduct AB, where the bond between A and B is a dative covalent bond (both electrons come from the base).
Lewis was nominated 35 times for the Nobel Prize in Chemistry.
(2011-07-20) Going beyond the pH scale of dilute aqueous solutions.
(2011-07-20) Acids that are stronger than pure sulfuric acid.
(2011-07-20) Great affinities for protons.
In aqueous solution, the hydroxide ion OH has the highest basicity but there are compound that have greaterproton affinity in dry environments.
(2011-07-20) Perchloric acid (HClO4) fluorosulfuric acid (HSO3F) and beyond.
The strongest known superacid is fluoroantimonic acid H2F(SbF6).
(2011-07-21)