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Ba, a chemical element in Group II of the periodic table. Atomic number, 56; atomic mass, 137.34. Silvery-white metal. Barium consists of a mixture of seven stable isotopes, with 138Ba predominant (71.66 percent). Nuclear fission of uranium and plutonium produces the radioactive isotope 140Ba, which is used as a radioactive tracer. Barium was discovered by the Swedish chemist K. Scheele (1774) in the form of the oxide BaO, which was named “heavy earth,” or baryta (from the Greek barys, “heavy”). Metallic barium in the form of an amalgam was obtained by the British chemist H. Davy (1808), who electrolyzed the damp hydroxide Ba(OH)2 with a mercury cathode. The barium content of the earth’s crust is 0.05 percent by mass; this element is not found naturally in the free state. Commercially important barium minerals include barite (heavy spar) BaSO4 and the less frequently encountered witherite BaCO3.
Physical and chemical properties
Barium has a cubic, body-centered crystal lattice with the constant a = 5.019 angstroms. Its density is 3.76 g/cm3, its melting point is 710° C, and its boiling point is 1637–1640° C. It is a soft metal (harder than lead but softer than zinc) with a mineralogical hardness of 2. Barium is one of the alkali earth metals and is similar in chemical properties to calcium and strontium, although it is more active. It reacts with most other elements to form compounds in which it is usually divalent. (The outer electron shell of Ba contains two electrons and has a 6$2configuration.) It undergoes rapid oxidation in air, and a film of the oxide (together with the peroxide and the nitride Ba3Na3) forms on its surface. Upon heating, barium ignites readily and burns with a yellow-green flame. It vigorously decomposes water to produce barium hydroxide: Ba + 2H2O = Ba(OH)2 + H2. Because of its chemical activity, barium is stored under kerosene. The oxide BaO consists of colorless crystals; it is readily converted to the carbonate BaCO3 in air and reacts vigorously with water to form Ba(OH)2. Heating of BaO in air to 500° C produces the peroxide BaO2, which decomposes into BaO and O2 at 700° C. Heating of the peroxide with oxygen under high pressure yields the higher peroxide Ba04, a yellow substance that decomposes at 50°-60° C. Barium combines with halogens and sulfur to form halides (for example, BaCl2) and the sulfide BaS; it combines with hydrogen to form the hydride BaH2, which is vigorously decomposed by water and acids. Of the commonly used barium salts, barium chloride BaCl2, and other halides, the nitrate Ba(NO3)2, the sulfide BaS, and the chlorate Ba(CIO3)2 are highly soluble; barium sulfate BaSO4, barium carbonate BaCO3, and barium chromate BaCrO4 are poorly soluble.
Production and use
The basic raw material for production of barium and its compounds is barite, which is reduced with coal in internally fired furnaces: BaSO4 + 4C = BaS + 4CO. The soluble BaS formed is converted to other barium salts. The main industrial method for production of metallic barium is thermal reduction of the oxide with powdered aluminum: 4BaO + 2A1 = 3Ba + BaO · Al2O3.
The mixture is heated in a vacuum (133 millinewtons per sq m, 10-3 mm Hg) at 1100°-1200° C. The barium evaporates and is deposited on the cold portions of the apparatus. The process is conducted in periodic-action electrovacuum equipment, which permits successive reduction, distillation, condensation, and removal of the metal, yielding barium ingots after a single engineering cycle. The metal is purified to an impurity content of less than 1 × 10-4 percent by double vacuum distillation at 900° C.
The practical applications of metallic barium are not great; they are further limited by the difficulty of handling pure barium. The metal is usually placed in a protective casing of some other metal or alloyed with a metal that makes the barium stable. Metallic barium is sometimes produced directly in apparatus by placing in them tablets consisting of a mixture of barium oxide and aluminum which are then subjected to thermal reduction in a vacuum. Barium and its alloys with magnesium and aluminum are used as absorbers (getters) of residual gases in high-vacuum technology. Small amounts of barium are employed in copper and lead metallurgy for deoxidation and removal of sulfur and gases. Very small quantities of barium are added to certain antifriction materials. Thus, the addition of barium to lead markedly increases the hardness of the alloy, which is used for printing type. Barium-nickel alloys are used in the manufacture of spark plugs and vacuum tubes.
Barium compounds are widely used. The peroxide BaO2 is employed to produce hydrogen peroxide, to bleach silk and plant fibers, as a disinfectant, and as one component of ignition mixtures in aluminothermy. The sulfide BaS is used to remove hair from hides. The perchlorate Ba(ClO4)2 is one of the best drying agents. The nitrate Ba(NO3)2 is used in pyrotechnics. The colored salts barium chromate BaCrO4 (yellow) and barium manganate BaMnO4 (green) are good pigments for paint manufacture, and barium platinocyanate Ba[Pt(CN)4] is a screen coating for work with X-ray and radioactive radiation. (Bright yellow-green fluorescence is excited in crystals of this salt under the action of radiation.) Barium titanate BaTiO3) is one of the most important fer-roelectrics. Since barium absorbs X rays and gamma radiation quite well, it is employed as a component of protective materials for X-ray installations and nuclear reactors. Barium compounds are inert carriers in the extraction of radium from uranium ores. Insoluble barium sulfate is nontoxic and is used as a contrast material for fluoroscopic examination of the gastrointestinal tract. Barium carbonate is used as a rodenticide.
REFERENCESDymchishin, D. A. Proizvodstvo barievykh solei. Moscow-Leningrad. 1938.
Beliaev, A. I. Metallurgiia legkikh metallov, 4th ed. Moscow, 1954.
Baranova, M. K. Barii (Obzor literatury). Moscow, 1962.
IU. I. ROMAN’KOV
Barium in the organism
Barium is present in all organs of plants; its content in plant ash depends on the amount of Ba in the soil and ranges from 0.06–0.2 percent to 3 percent (in barite deposits). The barium accumulation constant (barium in ash/barium in soil) is 0.2–6 in herbaceous plants and 1–30 in woody plants. The barium concentration is highest in the roots and branches and lowest in the leaves; it increases with seedling age. Soluble salts of barium are toxic to animals, so that plants containing much barium (up to 2–30 percent in the ash) cause poisoning in herbivores. A barium chloride dosage of 0.2–0.5 g causes acute poisoning in man, and 0.8–0.9 g is lethal. The maximum permissible barium concentration in water used for household purposes is 4.0 g/m3(4.0 mg/l).