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strontium(strŏn`shēəm) [from Strontian, a Scottish town], a metallic chemical element; symbol Sr; at. no. 38; at. wt. 87.62; m.p. 769°C;; b.p. 1,384°C;; sp. gr. 2.6 at 20°C;; valence +2. Strontium is a soft, silver-yellow metal with three allotropic crystalline forms (see allotropyallotropy
[Gr.,=other form]. A chemical element is said to exhibit allotropy when it occurs in two or more forms in the same physical state; the forms are called allotropes.
..... Click the link for more information. ). It is an alkaline-earth metalalkaline-earth metals,
metals constituting Group 2 of the periodic table. Generally, they are softer than most other metals, react readily with water (especially when heated), and are powerful reducing agents, but they are exceeded in each of these properties by the
..... Click the link for more information. ; in its physical and chemical properties it resembles calcium and barium, the elements above and below it in Group 2 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
..... Click the link for more information. . Since strontium reacts vigorously with water and quickly tarnishes in air, it must be stored out of contact with air and water. Among its compounds are the oxide strontia, SrO; peroxide, SrO2 ; hydroxide, Sr(OH)2 ; nitrate, Sr(NO3)2 ; the carbonate strontianite, SrCO3 ; the sulfate celestite, SrSO4 ; carbide, SrC2 ; and halides, SrBr2, SrCl2, SrF2, and SrI2. Celestitecelestite
, mineral appearing in blue-tinged or white orthorhombic crystals or in fibrous masses. The natural sulfate of strontium, SrSO4, it is important as a source of strontium and of certain of its compounds, e.g.
..... Click the link for more information. and strontianite are the chief ores of strontium. The metal may be prepared by electrolysis of fused strontium chloride; small amounts of the metal are used in semiconductor devices. Although strontium has uses similar to those of calcium and barium, it is rarely employed because of its higher cost. Principal uses of strontium compounds are in pyrotechnics (chiefly the nitrate) and in greases (the hydroxide). In fireworks and signal flares strontium compounds add a bright red or crimson color to the flame. Naturally occurring strontium is a mixture of four stable isotopes. Twelve unstable isotopes exist; the most stable of these is the radioactive isotope strontium-90 (half-life 28.1 years), which is the chief immediate hazard in falloutfallout,
minute particles of radioactive material produced by nuclear explosions (see atomic bomb; hydrogen bomb; Chernobyl) or by discharge from nuclear-power or atomic installations and scattered throughout the earth's atmosphere by winds and convection currents.
..... Click the link for more information. . As a result of atmospheric nuclear tests, strontium-90 is dispersed in varying concentrations throughout the earth's atmosphere and soil. Because of its chemical similarity to calcium, it is readily taken up in the tissues of plants and animals; it may enter the human food supply, mainly in milk. It is particularly dangerous for growing children as it is easily deposited in the bones and is believed to induce bone cancer and leukemia. Strontium-90 also has some uses in luminous signs and in nuclear batteries. Strontium was first recognized as distinct from barium in 1790 by A. Crawford in a sample of its carbonate from a mine near Strontian, Scotland; his finding was later confirmed by T. C. Hope, M. H. Klaproth, and others. It was first isolated by electrolysis in 1808 by Humphry Davy.
Sr, a chemical element in Group II of Mendeleev’s periodic system. Atomic number, 38; atomic weight, 87.62. A silver-white metal. Natural strontium is a mixture of the four stable isotopes 84Sr, 86Sr, 87Sr, and 88Sr, with 88Sr the most common (82.56 percent).
Radioactive strontium isotopes have been obtained artificially. These isotopes have mass numbers ranging from 80 to 97, and they include 90Sr, which has a half-life of 27.7 years and is formed in uranium fission. In 1790 the Scottish physician A. Crawford, on studying a mineral found near the village of Strontian in Scotland, discovered that the mineral contained a previously unknown “earth,” which was given the name “strontia.” It was later found that strontia is the oxide of strontium SrO. In 1808, H. Davy subjected a mixture of the moistened hydroxide Sr(OH)2 and mercuric oxide to electrolysis with a mercury cathode and obtained an amalgam of strontium.
Distribution in nature. The average content of strontium in the earth’s crust (clarke) is 3.4 × 10–2 percent by weight. Strontium is an accessory element of calcium in geochemical processes. Approximately 30 minerals of strontium are known, of which celes-tite (SrSO4) and strontianite (SrCO3) are the most important. In magmatic rocks, strontium is found primarily in dispersed form, and it is present as an isomorphous admixture in the crystal lattices of calcium, potassium, and barium minerals. In the biosphere, strontium accumulates in carbonate rocks and, especially, in the sediments of salt lakes and lagoons (celestite deposits).
Physical and chemical properties. At room temperature, the lattice of strontium is face-centered cubic (α-Sr), with spacing a = 6.0848 angstroms (Å). Above 248°C, strontium is converted to the hexagonal modification (β-Sr), with lattice spacings a = 4.32 Å and c = 7.06 Å, and at 614°C it is converted to the body-centered cubic modification (γ-Sr), with a = 4.85 Å. Strontium has an atomic radius of 2.15 Å, and the ionic radius of Sr2+ is 1.20 Å. The density of the α-form is 2.63 g/cm3 (at 20°C). Strontium has a melting point of 770°C, a boiling point of 1383°C, a specific heat of 737.4 kilojoules/kg-°K (0.176 calorie/g-°C), and a resistivity of 22.76 × 10–6 ohm-cm–1. Strontium is paramagnetic, with a magnetic susceptibility of 91.2 × 10–6 at room temperature.
Strontium is a soft ductile metal that is easily cut with a knife. The configuration of the outer electron subshell is 5s2, and in its compounds strontium usually has an oxidation state of + 2. The element is an alkaline-earth metal that is similar in chemical properties to Ca and Ba. Metallic strontium is rapidly oxidized in the air, forming a yellowish surface film containing the oxide SrO, peroxide SrO2, and nitride Sr3N2. Strontium reacts with oxygen under ordinary conditions to form the oxide SrO, a grayish white powder, which in the air is readily converted to the carbonate SrCO3; it reacts vigorously with water, forming the hydroxide Sr(OH)2, which is a stronger base than Ca(OH)2. Strontium is easily ignited upon heating in the air, and powdered strontium ignites spontaneously in the air. Thus, the element is stored in hermetically sealed vessels under a layer of kerosene. Strontium violently decomposes water, with the liberation of hydrogen and the formation of strontium hydroxide. At elevated temperatures, the element reacts with hydrogen (>200°C), nitrogen (>400°C), phosphorus, sulfur, and the halogens. Upon heating, strontium forms intermetallic compounds with metals, for example, SrPb3, SrAg4, SrHg8, and SrHg12. Of the strontium salts, the halides (with the exception of the fluoride), nitrate, acetate, and chlorate dissolve readily in water, while the carbonate, sulfate, oxalate, and phosphate are difficultly soluble. The precipitation of strontium as the oxalate and sulfate is used for the analytical identification of the element. Many salts of strontium form crystal hydrates in which the water of crystallization comprises one to six molecules. Strontium sulfide, SrS, is gradually hydrolyzed by water; strontium nitride, Sr3N2 (black crystals), is easily decomposed by water with the liberation of NH3 and Sr(OH)2. Strontium is readily soluble in liquid ammonia, giving dark blue solutions.
Production and use. The major raw materials for the production of strontium compounds are the concentrates obtained from the dressing of celestite and strontianite. Metallic strontium is obtained by the reduction of strontium oxide using aluminum at 1100o–1150°C;
4SrO + 2A1 = 3Sr + SrO ·Al2O3
The process is carried out in batchwise fashion in electrode vacuum apparatus at a pressure of 1 newton/m2(10–2 mm Hg). Strontium vapors are condensed on the cooled surface of the condenser placed inside the apparatus. At the end of the reduction process, the apparatus is filled with argon, and the condensate, after being melted, flows into a mold. Strontium is also produced by the electrolysis of a melt containing 85 percent SrCI2 and 15 percent KC1, although the yield in terms of the current consumed in this process is low, and the strontium metal obtained contains impurities in the form of strontium nitride and oxide, and the salts of strontium. In industry, alloys of strontium, for example, those with tin, are produced by electrolysis using a liquid cathode.
Metallic strontium has few practical uses. It serves in the deoxi-dation of copper and bronze. Strontium 90 is a source of beta radiation in atomic batteries. Strontium is used in making luminophors and photocells, as well as strongly pyrophoric alloys. Strontium oxide is a constituent of certain optical glasses and oxide-coated cathodes in electron tubes. Compounds of strontium are used to impart a vivid cherry-red color to flames and thus have found use in pyrotechnics. Strontianite is introduced into slag for the removal of sulfur and phosphorus from high-grade steels, and strontium carbonate is used in nonvolatile getters and added to the lacquers and enamels resistant to atmospheric effects that are used for coating porcelain, steel, and heat resistant alloys. Strontium chromate, SrCrO4, an extremely fast pigment, is used in preparing artists” paints, and strontium titanate, SrTiO3, is used as a ferroelectric and constituent of piezoelectric ceramics. Strontium salts of fatty acids (“strontium soaps”) are used for the production of special lubricant greases.
Salts and compounds of strontium have low toxicity, and the safety precautions, which are standard for handling salts of alkali and alkaline-earth metals, should be observed when working with them.
M. E. ERLYKINA
Strontium in organisms. Strontium is a component of microorganisms, plants, and animals. The skeletons of marine radiolari-ans (Acanthria) consist of strontium sulfate (celestite). Marine algae contain 26–140 mg of strontium per 100 g of dry matter, while terrestrial plants contain 2.6 mg; marine animals contain 2–50 mg, and terrestrial animals 1.4 mg. Bacteria contain 0.27–30 mg of strontium. The accumulation of strontium in various organisms depends not only on the species and special features of the organism but also on the ratio of strontium to other elements in the environment, mainly Ca and P, and on the adaptation of the organism to a given geochemical environment.
Animals obtain strontium from water and food. The element is absorbed by the small intestine and eliminated mainly by the large intestine. A number of substances (polysaccharides of algae, cation exchange resins) inhibit the assimilation of strontium. Strontium is stored mainly in bone tissue, with the ash of bone tissue containing approximately 0.02 percent strontium. In other tissues, the content is approximately 0.0005 percent. An excess of strontium salts in the diet of rats causes “strontium” rickets. An increased content of strontium in the organism is observed in animals living on soils with a significant amount of celestite, a content that can lead to bone brittleness, rickets, and other diseases. The disease sarcoidosis sometimes appears in biogeochemical provinces rich in strontium, such as certain regions in Central Asia, Eastern Asia, and northern Europe.
G. G. POLIKARPOV
Strontium 90. Among the artificial isotopes of strontium, the long-lived radionuclide strontium 90 figures prominently in the radioactive pollution of the biosphere. Once in the environment, 90Sr demonstrates an ability to participate (mainly, with Ca) in metabolic processes in plants, animals, and humans. Thus, in evaluating the pollution of the biosphere with 90Sr, the 90Sr/Ca ratio is calculated in strontium units (1 SU = 10–12 curie of 90Sr per gram of Ca). The discrimination of strontium that occurs in the movement of 90Sr and Ca along biological and food chains is expressed quantitatively by the discrimination coefficient—the relationship between the 90Sr/Ca ratio in a given link of the biological or food chain and the ratio in the preceding link. In the final link of a food chain, the concentration of “Sr, as a rule, is significantly less than in the starting link.
Strontium 90 may enter plants directly by either contaminating the leaves or entering through the roots from the soil. In the latter case, the type of soil, the soil moisture, the pH, and the content of Ca and organic substances have a great effect. Legumes and root crops have relatively high accumulations of strontium 90, while grasses, including grains, and flax have lower contents. Significantly less strontium 90 is accumulated in the seeds and fruits of plants than in other organs; for example, there is ten times as much 90Sr in the leaves and stems of wheat than in the grain. In animals, which obtain 90Sr primarily from plant foods, and humans, which obtain it mainly from cow’s milk and fish, the isotope accumulates largely in the bones. The buildup of 90Sr in the organism of animals and humans depends on such factors as the age of the organism, the amount of the radionuclide ingested, and the rate of growth of new bone tissue. Strontium 90 presents a great danger for children, who obtain the isotope from milk and accumulate it in rapidly growing bone tissue.
The biological effect of 90Sr is related to the distribution of the isotope in the body (accumulation in the skeleton). The effect also depends on the dose of beta radiation produced by 90Sr and the isotope’s daughter radioisotope, 90Y. With the prolonged intake of 90Sr, even in relatively small amounts, leukemia and osteosarcoma may develop as a result of the continuous irradiation of the bone tissue. Significant changes in bone tissue are observed with a content of 90Sr in the diet of approximately 1 microcurie per gram of Ca. The Nuclear Test-ban Treaty (1963), which prohibits the testing of nuclear weapons in the atmosphere and outer space and underwater, has led to an almost total elimination of strontium 90 from the atmosphere and to a reduction in the isotope’s mobile forms in the soil.
V. A. KAL’CHENKO
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