This article is about the chemical element. For the nutrient commonly called sodium, seesalt. For the use of sodium as a medication, seeSaline (medicine). For other uses, seesodium (disambiguation).
Sodium is anessential element for all animals and some plants. Sodium ions are the major cation in theextracellular fluid (ECF) and as such are the major contributor to the ECFosmotic pressure.[9] Animal cells actively pump sodium ions out of the cells by means of thesodium–potassium pump, anenzyme complex embedded in thecell membrane, in order to maintain a roughly ten-times higher concentration of sodium ions outside the cell than inside.[10] Innerve cells, the sudden flow of sodium ions into the cell throughvoltage-gated sodium channels enables transmission of a nerve impulse in a process called theaction potential.
Sodium atstandard temperature and pressure is a soft silvery metal that combines with oxygen in the air, formingsodium oxides. Bulk sodium is usually stored in oil or an inert gas. Sodium metal can be easily cut with a knife. It is a good conductor of electricity and heat. Due to having low atomic mass and large atomic radius, sodium is third-least dense of all elemental metals and is one of only three metals that can float on water, the other two being lithium and potassium.[11]
The melting (98 °C) and boiling (883 °C) points of sodium are lower than those of lithium but higher than those of the heavier alkali metals potassium, rubidium, and caesium, following periodic trends down the group.[12] These properties change dramatically at elevated pressures: at 1.5 Mbar, the color changes from silvery metallic to black; at 1.9 Mbar the material becomes transparent with a red color; and at 3 Mbar, sodium is a clear and transparent solid. All of these high-pressureallotropes are insulators andelectrides.[13]
A positiveflame test for sodium has a bright yellow color.
In aflame test, sodium and its compounds glow yellow[14] because the excited3s electrons of sodium emit aphoton when they fall from 3p to 3s; the wavelength of this photon corresponds to theD line at about 589.3 nm.Spin-orbit interactions involving the electron in the 3p orbital split the D line into two, at 589.0 and 589.6 nm;hyperfine structures involving both orbitals cause many more lines.[15]
Twenty isotopes of sodium are known, but only23Na is stable.23Na is created in thecarbon-burning process in stars by fusing twocarbon atoms together; this requires temperatures above 600 megakelvins and a star of at least three solar masses.[16] Tworadioactive,cosmogenic isotopes are the byproduct ofcosmic ray spallation:22Na has ahalf-life of 2.6 years and24Na, a half-life of 15 hours; all other isotopes have a half-life of less than one minute.[17]
Twonuclear isomers have been discovered, the longer-lived one being24mNa with a half-life of around 20.2 milliseconds. Acute neutron radiation, as from a nuclearcriticality accident, converts some of the stable23Na in human blood to24Na; the neutron radiation dosage of a victim can be calculated by measuring the concentration of24Na relative to23Na.[18]
Sodium atoms have 11 electrons, one more than the stable configuration of thenoble gasneon. The first and secondionization energies are 495.8 kJ/mol and 4562 kJ/mol, respectively. As a result, sodium usually formsionic compounds involving the Na+ cation.[19]
Metallic sodium
Metallic sodium is generally less reactive thanpotassium and more reactive thanlithium.[20] Sodium metal is highly reducing, with thestandard reduction potential for the Na+/Na couple being −2.71 volts,[21] though potassium and lithium have even more negative potentials.[22]
The structure ofsodium chloride, showing octahedral coordination around Na+ and Cl− centres. This framework disintegrates when dissolved in water and reassembles when the water evaporates.
Sodium tends to form water-soluble compounds, such ashalides,sulfates,nitrates,carboxylates andcarbonates. The main aqueous species are the aquo complexes [Na(H2O)n]+, wheren = 4–8; withn = 6 indicated from X-ray diffraction data and computer simulations.[27]
Direct precipitation of sodium salts from aqueous solutions is rare because sodium salts typically have a high affinity for water. An exception issodium bismuthate (NaBiO3),[28] which is insoluble in cold water and decomposes in hot water.[29] Because of the high solubility of its compounds, sodium salts are usually isolated as solids by evaporation or by precipitation with an organic antisolvent, such asethanol; for example, only 0.35 g/L of sodium chloride will dissolve in ethanol.[30] Acrown ether such as15-crown-5 may be used as aphase-transfer catalyst.[31]
Like the other alkali metals, sodium dissolves in ammonia and some amines to give deeply colored solutions; evaporation of these solutions leaves a shiny film of metallic sodium. The solutions contain thecoordination complex [Na(NH3)6]+, with the positive charge counterbalanced byelectrons as anions;cryptands permit the isolation of these complexes as crystalline solids. Sodium forms complexes with crown ethers, cryptands and other ligands.[33]
For example,15-crown-5 has a high affinity for sodium because the cavity size of 15-crown-5 is 1.7–2.2 Å, which is enough to fit the sodium ion (1.9 Å).[34][35] Cryptands, like crown ethers and otherionophores, also have a high affinity for the sodium ion; derivatives of thealkalide Na− are obtainable[36] by the addition of cryptands to solutions of sodium in ammonia viadisproportionation.[37]
The structure of the complex of sodium (Na+, shown in yellow) and the antibioticmonensin-A
Many organosodium compounds have been prepared. Because of the high polarity of the C-Na bonds, they behave like sources ofcarbanions (salts with organicanions). Some well-known derivatives includesodium cyclopentadienide (NaC5H5) andtrityl sodium ((C6H5)3CNa).[38]Sodium naphthalene, Na+[C10H8•]−, a strong reducing agent, forms upon mixing Na and naphthalene in ethereal solutions.[39]
Intermetallic compounds
Sodium forms alloys with many metals, such as potassium,calcium,lead, and thegroup 11 and12 elements. Sodium and potassium form KNa2 andNaK. NaK is 40–90% potassium and it is liquid atambient temperature. It is an excellent thermal and electrical conductor. Sodium-calcium alloys are by-products of the electrolytic production of sodium from a binary salt mixture of NaCl-CaCl2 and ternary mixture NaCl-CaCl2-BaCl2. Calcium is only partiallymiscible with sodium, and the 1–2% of it dissolved in the sodium obtained from said mixtures can be precipitated by cooling to 120 °C and filtering.[40]
In a liquid state, sodium is completely miscible with lead. There are several methods to make sodium-lead alloys. One is to melt them together and another is to deposit sodium electrolytically on molten lead cathodes. NaPb3, NaPb, Na9Pb4, Na5Pb2, and Na15Pb4 are some of the known sodium-lead alloys. Sodium also forms alloys withgold (NaAu2) andsilver (NaAg2). Group 12 metals (zinc,cadmium andmercury) are known to make alloys with sodium. NaZn13 and NaCd2 are alloys of zinc and cadmium. Sodium and mercury form NaHg, NaHg4, NaHg2, Na3Hg2, and Na3Hg.[41]
History
Because of its importance in human health, salt has long been an important commodity. In medieval Europe, a compound of sodium with the Latin name ofsodanum was used as aheadache remedy. The name sodium is thought to originate from the Arabicsuda, meaning headache, as the headache-alleviating properties ofsodium carbonate or soda were well known in early times.[42]
Although sodium, sometimes calledsoda, had long been recognized in compounds, the metal itself was not isolated until 1807 bySir Humphry Davy through theelectrolysis ofsodium hydroxide.[43][44] In 1809, the German physicist and chemistLudwig Wilhelm Gilbert proposed the namesNatronium for Humphry Davy's "sodium" andKalium for Davy's "potassium".[45]
The chemical abbreviation for sodium was first published in 1814 byJöns Jakob Berzelius in his system of atomic symbols,[46][47] and is an abbreviation of the element'sNeo-Latin namenatrium, which refers to the Egyptiannatron,[42] a natural mineral salt mainly consisting of hydrated sodium carbonate. Natron historically had several important industrial and household uses, later eclipsed by other sodium compounds.[48]
In a corner of our 60 m3 room farthest away from the apparatus, we exploded 3 mg of sodium chlorate with milk sugar while observing the nonluminous flame before the slit. After a while, it glowed a bright yellow and showed a strong sodium line that disappeared only after 10 minutes. From the weight of the sodium salt and the volume of air in the room, we easily calculate that one part by weight of air could not contain more than 1/20 millionth weight of sodium.
Occurrence
The Earth's crust contains 2.27% sodium, making it thesixth most abundant element on Earth and the fourth most abundant metal, behindaluminium,iron,calcium, andmagnesium and ahead of potassium.[50]Sodium's estimated oceanic abundance is 10.8 grams per liter.[51] Because of its high reactivity, it is never found as a pure element. It is found in many minerals, some very soluble, such ashalite andnatron, others much less soluble, such asamphibole andzeolite. The insolubility of certain sodium minerals such ascryolite andfeldspar arises from their polymeric anions, which in the case of feldspar is a polysilicate. In the universe, sodium is the 15th most abundant element with a 20,000 parts-per-billion abundance,[52] making sodium 0.002% of the total atoms in the universe.
Astronomical observations
Atomic sodium has a very strongspectral line in the yellow-orange part of the spectrum (the same line as is used insodium-vapour street lights). This appears as anabsorption line in many types of stars, including theSun. The line was first studied in 1814 byJoseph von Fraunhofer during his investigation of the lines in the solar spectrum, now known as theFraunhofer lines. Fraunhofer named it the "D" line, although it is now known to actually be a group of closely spaced lines split by afine andhyperfine structure.[53]
The strength of the D line allows its detection in many other astronomical environments. In stars, it is seen in any whose surfaces are cool enough for sodium to exist in atomic form (rather than ionised). This corresponds to stars of roughlyF-type and cooler. Many other stars appear to have a sodium absorption line, but this is actually caused by gas in the foregroundinterstellar medium. The two can be distinguished via high-resolution spectroscopy, because interstellar lines are much narrower than those broadened bystellar rotation.[54]
Employed in rather specialized applications, about 100,000 tonnes of metallic sodium are produced annually.[60] Metallic sodium was first produced commercially in the late nineteenth century[40] bycarbothermal reduction ofsodium carbonate at 1100 °C, as the first step of theDeville process for the production of aluminium:[61][62][63]
Na2CO3 + 2 C → 2 Na + 3 CO
The high demand for aluminium created the need for the production of sodium. The introduction of theHall–Héroult process for the production of aluminium byelectrolysing a molten salt bath ended the need for large quantities of sodium. A related process based on the reduction of sodium hydroxide was developed in 1886.[61]
Though metallic sodium has some important uses, the major applications for sodium use compounds; millions of tons ofsodium chloride,hydroxide, andcarbonate are produced annually. Sodium chloride is extensively used foranti-icing andde-icing and as a preservative; examples of the uses ofsodium bicarbonate include baking, as araising agent, andsodablasting. Along with potassium, many important medicines have sodium added to improve theirbioavailability; though potassium is the better ion in most cases, sodium is chosen for its lower price and atomic weight.[70]Sodium hydride is used as a base for various reactions (such as thealdol reaction) in organic chemistry.
Metallic sodium is used mainly for the production ofsodium borohydride,sodium azide,indigo, andtriphenylphosphine. A once-common use was the making oftetraethyllead and titanium metal; because of the move away from TEL and new titanium production methods, the production of sodium declined after 1970.[60] Sodium is also used as an alloying metal, ananti-scaling agent,[71] and as a reducing agent for metals when other materials are ineffective.
Note the free element is not used as a scaling agent, ions in the water are exchanged for sodium ions.Sodium plasma ("vapor") lamps are often used for street lighting in cities, shedding light that ranges from yellow-orange to peach as the pressure increases.[72] By itself orwith potassium, sodium is adesiccant; it gives an intense blue coloration withbenzophenone when the desiccate is dry.[73]
Sodium-potassium alloy (NaK)phase diagram, showing the melting point of sodium as a function of potassium concentration. NaK with 77% potassium iseutectic and has the lowest melting point of the NaK alloys at −12.6 °C.[78]
Liquid sodium is used as aheat transfer fluid insodium-cooled fast reactors[79] because it has the high thermal conductivity and low neutron absorptioncross section required to achieve a high neutron flux in the reactor.[80] The high boiling point of sodium allows the reactor to operate at ambient (normal) pressure,[80] but drawbacks include its opacity, which hinders visual maintenance, and its strongly reducing properties. Sodium will explode in contact with water, although it will only burn gently in air.[81]
Radioactivesodium-24 may be produced byneutron bombardment during operation, posing a slight radiation hazard; the radioactivity stops within a few days after removal from the reactor.[82] If a reactor needs to be shut down frequently,sodium-potassium alloy (NaK) is used. Because NaK is a liquid at room temperature, the coolant does not solidify in the pipes.[83] Thepyrophoricity of the NaK means extra precautions must be taken to prevent and detect leaks.[84]
Another heat transfer application of sodium is inpoppet valves in high-performance internal combustion engines; the valve stems are partially filled with sodium and work as aheat pipe to cool the valves.[85]
In humans, sodium is an essential mineral that regulatesblood volume, blood pressure,osmotic equilibrium andpH. The minimum physiological requirement for sodium is estimated to range from about 120 milligrams per day in newborns to 500 milligrams per day over the age of 10.[86]
Diet
Sodium chloride, also known as 'edible salt' or 'table salt'[87] (chemical formulaNaCl), is the principal source of sodium (Na) in the diet and is used as seasoning and preservative in such commodities aspickled preserves andjerky. For Americans, most sodium chloride comes fromprocessed foods.[88] Other sources of sodium are its natural occurrence in food and such food additives asmonosodium glutamate (MSG),sodium nitrite, sodium saccharin,baking soda (sodium bicarbonate), andsodium benzoate.[89]
The Committee to Review the Dietary Reference Intakes for Sodium and Potassium, which is part of the National Academies of Sciences, Engineering, and Medicine, has determined that there isn't enough evidence from research studies to establish Estimated Average Requirement (EAR) and Recommended Dietary Allowance (RDA) values for sodium. As a result, the committee has established Adequate Intake (AI) levels instead, as follows. The sodium AI for infants of 0–6 months is established at 110 mg/day, 7–12 months: 370 mg/day; for children 1–3 years: 800 mg/day, 4–8 years: 1,000 mg/day; for adolescents: 9–13 years – 1,200 mg/day, 14–18 years 1,500 mg/day; for adults regardless of their age or sex: 1,500 mg/day.[93]
Sodium chloride (NaCl) contains approximately 39.34% of its total mass as elemental sodium (Na). This means that1 gram of sodium chloride contains approximately393.4 mg of elemental sodium.[94] For example, to find out how much sodium chloride contains 1500 mg of elemental sodium (the value of 1500 mg sodium is the adequate intake (AI) for an adult), we can use the proportion:
393.4 mg Na : 1000 mg NaCl = 1500 mg Na : x mg NaCl
Solving forx gives us the amount of sodium chloride that contains 1500 mg of elemental sodium
x = (1500 mg Na × 1000 mg NaCl) / 393.4 mg Na = 3812.91 mg
This mean that 3812.91 mg of sodium chloride contain 1500 mg of elemental sodium.[94]
There is a strong correlation between higher sodium intake and higher blood pressure.[96] Studies have found that lowering sodium intake by 2 g per day tends to lowersystolic blood pressure by about two to four mm Hg.[97] It has been estimated that such a decrease in sodium intake would lead to 9–17% fewer cases ofhypertension.[97]
Hypertension causes 7.6 million premature deaths worldwide each year.[98] Since edible salt contains about 39.3% sodium[99]—the rest being chlorine and trace chemicals; thus, 2.3 g sodium is about 5.9 g, or 5.3 ml, of salt—about oneUS teaspoon.[100][101]
One scientific review found that people with or without hypertension who excreted less than 3 grams of sodium per day in their urine (and therefore were taking in less than 3 g/d) had ahigher risk of death, stroke, or heart attack than those excreting 4 to 5 grams per day.[102] Levels of 7 g per day or more in people with hypertension were associated with higher mortality and cardiovascular events, but this was not found to be true for people withouthypertension.[102] TheUS FDA states that adults with hypertension and prehypertension should reduce daily sodium intake to 1.5 g.[101]
Physiology
Therenin–angiotensin system regulates the amount of fluid and sodium concentration in the body. Reduction of blood pressure and sodium concentration in the kidney result in the production ofrenin, which in turn producesaldosterone andangiotensin, which stimulates the reabsorption of sodium back into the bloodstream. When the concentration of sodium increases, the production of renin decreases, and the sodium concentration returns to normal.[103] The sodium ion (Na+) is an important electrolyte inneuron function, and in osmoregulation between cells and theextracellular fluid. This is accomplished in all animals byNa+/K+-ATPase, an active transporter pumping ions against the gradient, and sodium/potassium channels.[104] The difference in extracellular and intracellular ion concentration, maintained by the sodium-potassium pump, produce electrical signals in the form ofaction potentials that supports cardiac muscle contraction and promote long distance communication between neurons.[10] Sodium is the most prevalent metallic ion in extracellular fluid.[105]
In humans, unusually low or high sodium levels in the blood is recognized in medicine ashyponatremia andhypernatremia. These conditions may be caused by genetic factors, ageing, or prolonged vomiting or diarrhea.[106]
Biological role in plants
InC4 plants, sodium is amicronutrient that aids metabolism, specifically in regeneration ofphosphoenolpyruvate and synthesis ofchlorophyll.[107] In others, it substitutes forpotassium in several roles, such as maintainingturgor pressure and aiding in the opening and closing ofstomata.[108] Excess sodium in the soil can limit the uptake of water by decreasing thewater potential, which may result in plant wilting; excess concentrations in thecytoplasm can lead to enzyme inhibition, which in turn causes necrosis and chlorosis.[109]
In response, some plants have developed mechanisms to limit sodium uptake in the roots, to store it in cellvacuoles, and restrict salt transport from roots to leaves.[110] Excess sodium may also be stored in old plant tissue, limiting the damage to new growth.Halophytes have adapted to be able to flourish in sodium rich environments.[110]
Sodium forms flammable hydrogen and causticsodium hydroxide on contact with water;[113] ingestion and contact with moisture on skin, eyes ormucous membranes can cause severe burns.[114][115] Sodium spontaneously explodes in the presence of water due to the formation of hydrogen (highly explosive) and sodium hydroxide (which dissolves in the water, liberating more surface). However, sodium exposed to air and ignited or reaching autoignition (reported to occur when a molten pool of sodium reaches about 290 °C, 554 °F)[116] displays a relatively mild fire.
In the case of massive (non-molten) pieces of sodium, the reaction with oxygen eventually becomes slow due to formation of a protective layer.[117]Fire extinguishers based on water accelerate sodium fires. Those based on carbon dioxide andbromochlorodifluoromethane should not be used on sodium fire.[115] Metal fires areClass D, but not all Class D extinguishers are effective when used to extinguish sodium fires. An effective extinguishing agent for sodium fires is Met-L-X.[115] Other effective agents include Lith-X, which hasgraphite powder and anorganophosphateflame retardant, and dry sand.[118]
Sodium fires are prevented in nuclear reactors by isolating sodium from oxygen with surrounding pipes containing inert gas.[119] Pool-type sodium fires are prevented using diverse design measures called catch pan systems. They collect leaking sodium into a leak-recovery tank where it is isolated from oxygen.[119]
Liquid sodium fires are more dangerous to handle than solid sodium fires, particularly if there is insufficient experience with the safe handling of molten sodium. In a technical report for theUnited States Fire Administration,[114] R. J. Gordon writes (emphasis in original)
Molten sodium isextremely dangerous because it is much more reactive than a solid mass. In the liquid form, every sodium atom is free and mobile to instantaneously combine with any available oxygen atom or other oxidizer, and any gaseous by-product will be created as a rapidly expanding gas bubble within the molten mass. Even a minute amount of water can create this type of reaction. Any amount of water introduced into a pool of molten sodium is likely to cause a violent explosion inside the liquid mass, releasing the hydrogen as a rapidly expanding gas and causing the molten sodium to erupt from the container.
When molten sodium is involved in a fire, the combustion occurs at the surface of the liquid. An inert gas, such as nitrogen or argon, can be used to form an inert layer over the pool of burning liquid sodium, but the gas must be applied very gently and contained over the surface. Except for soda ash, most of the powdered agents that are used to extinguish small fires in solid pieces or shallow pools will sink to the bottom of a molten mass of burning sodium – the sodium will float to the top and continue to burn. If the burning sodium is in a container, it may be feasible to extinguish the fire by placing a lid on the container to exclude oxygen.
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