sodium

sodium (Na), chemical element of the alkali metal group (Group 1 [Ia]) of the periodic table. Sodium is a very soft silvery-white metal. Sodium is the most common alkali metal and the sixth most abundant element on Earth, comprising 2.8 percent of Earth’s crust. It occurs abundantly in nature in compounds, especially common salt—sodium chloride (NaCl)—which forms the mineral halite and constitutes about 80 percent of the dissolved constituents of seawater.

Properties and production

Because sodium is extremely reactive, it never occurs in the free state in Earth’s crust. In 1807 Sir Humphry Davy became the first to prepare sodium in its elemental form, applying electrolysis to fused sodium hydroxide (NaOH). Sodium is an important constituent of a number of silicate materials, such as feldspars and micas. There are huge deposits of rock salt in various parts of the world, and sodium nitrate deposits exist in Chile and Peru. The sodium content of the sea is approximately 1.05 percent, corresponding to a concentration of approximately 3 percent of sodium halides. Sodium has been identified in both the atomic and ionic forms in the spectra of stars, including the Sun, and the interstellar medium. Analysis of meteorites indicates that the silicate material present has an average content of approximately 4.6 atoms of sodium for every 100 atoms of silicon.

Lighter than water, sodium can be cut with a knife at room temperature but is brittle at low temperatures. It conducts heat and electricity easily and exhibits the photoelectric effect (emission of electrons when exposed to light) to a marked degree.

Sodium is by far the most commercially important alkali metal. Most processes for the production of sodium involve the electrolysis of molten sodium chloride. Inexpensive and available in tank-car quantities, the element is used to produce gasoline additives, polymers such as nylon and synthetic rubber, pharmaceuticals, and a number of metals such as tantalum, titanium, and silicon. It is also widely used as a heat exchanger and in sodium-vapour lamps. The yellow colour of the sodium-vapour lamp and the sodium flame (the basis of an analytical test for sodium) is identified with two prominent lines in the yellow portion of the light spectrum.

Significant uses

Two of the earliest uses of metallic sodium were in the manufacture of sodium cyanide and sodium peroxide. Significant quantities were used in the manufacture of tetraethyl lead as a gasoline additive, a market that disappeared with the advent of unleaded gasoline. Substantial amounts of sodium are used in the manufacture of sodium alkyl sulfates as the principal ingredient in synthetic detergents.

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Sodium also is used as a starting material in the manufacture of sodium hydride (NaH) and sodium borohydride (NaBH₄). In addition, sodium is employed in the production of dyes and dye intermediates, in the synthesis of perfumes, and in a wide variety of organic reductions. It is used in the purification of hydrocarbons and in the polymerization of unsaturated hydrocarbons. In many organic applications, sodium is used in the form of dispersions in hydrocarbon liquid media.

Molten sodium is an excellent heat-transfer fluid, and, because of this property, it has found use as coolant in liquid-metal fast breeder reactors. Sodium is used extensively in metallurgy as a deoxidant and as a reducing agent for the preparation of calcium, zirconium, titanium, and other transition metals. Commercial production of titanium involves reduction of titanium tetrachloride (TiCl₄) with sodium. The products are metallic Ti and NaCl.

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Principal compounds

Sodium is highly reactive, forming a wide variety of compounds with nearly all inorganic and organic anions (negatively charged ions). It normally has an oxidation state of +1, and its single valence electron is lost with great ease, yielding the colourless sodium cation (Na⁺). Compounds that contain the sodium anion, Na⁻, have also been synthesized. The principal commercial sodium compounds are the chloride, carbonate, and sulfate.

The most important and familiar sodium compound is sodium chloride, or common salt, NaCl. Most other sodium compounds are prepared either directly or indirectly from sodium chloride, which occurs in seawater, in natural brines, and as rock salt. Large quantities of sodium chloride are employed in the production of other heavy (industrial) chemicals as well as being used directly for ice and snow removal, for water conditioning, and in food.

Other major commercial applications of sodium chloride include its use in the manufacture of chlorine and sodium hydroxide by electrolytic decomposition and in the production of sodium carbonate (Na₂CO₃) by the Solvay process. The electrolysis of aqueous sodium chloride produces sodium hypochlorite, NaOCl, a compound of sodium, oxygen, and chlorine used in large quantities in household chlorine bleach. Sodium hypochlorite is also utilized as an industrial bleach for paper pulp and textiles, for chlorination of water, and in certain medicinal preparations as an antiseptic and a fungicide. It is an unstable compound known only in aqueous solution.

The carbonates contain the carbonate ion (CO₃^2–). Sodium bicarbonate, also called sodium hydrogen carbonate, or bicarbonate of soda, NaHCO₃, is a source of carbon dioxide and so is used as an ingredient in baking powders, in effervescent salts and beverages, and as the main constituent of dry-chemical fire extinguishers. Its slight alkalinity makes it useful in treating gastric or urinary hyperacidity and acidosis. It is also employed in certain industrial processes, as in tanning and the preparation of wool. Sodium carbonate, or soda ash, Na₂CO₃, is widely distributed in nature, occurring as constituents of mineral waters and as the solid minerals natron, trona, and thermonatrite. Large quantities of this alkaline salt are used in making glass, detergents, and cleansers. Sodium carbonate is treated with carbon dioxide to produce sodium bicarbonate. The monohydrate form of sodium carbonate, Na₂CO₃·H₂O, is employed extensively in photography as a constituent in developers.

Sodium sulfate, Na₂SO₄, is a white crystalline solid or powder employed in the manufacture of kraft paper, paperboard, glass, and detergents and as a raw material for the production of various chemicals. It is obtained either from deposits of the sodium sulfate minerals mirabilite and thenardite or synthetically by the treatment of sodium chloride with sulfuric acid. The crystallized product is a hydrate, Na₂SO₄·10H₂O, commonly known as Glauber’s salt. Sodium thiosulfate (sodium hyposulfite), Na₂S₂O₃, is used by photographers to fix developed negatives and prints; it acts by dissolving the part of the silver salts coated onto film which remain unchanged by exposure to light.

Sodium hydroxide (NaOH) is a corrosive white crystalline solid that readily absorbs moisture until it dissolves. Commonly called caustic soda, or lye, sodium hydroxide is the most widely used industrial alkali. It is highly corrosive to animal and vegetable tissue. The alkaline solutions it forms when dissolved in water neutralize acids in various commercial processes: in petroleum refining, it removes sulfuric and organic acids; in soapmaking, it reacts with fatty acids. Solutions of NaOH are used in the treatment of cellulose and in the manufacture of many chemicals.

Sodium nitrate, or soda nitre, NaNO₃, is commonly called Chile saltpetre, after its mineral deposits in northern Chile, the principal source. Sodium nitrate is used as a nitrogenous fertilizer and as a component of dynamite.