STRONTIUM [Symbol Sr, atomic weight 87·62 (O=16)], a metallic chemical element belonging to the alkaline earth group. It is found in small quantities very widely distributed in various rocks and soils, and in mineral waters; its chief sources are the minerals strontianite, celestine and barytocelestine. The metal was detected in the mineral strontianite, found at Strontian in Argyllshire, by Cruikshank in 1787, and by Crawford in 1790; and the discovery was confirmed by Hope in 1792 and by Klaproth in 1793. The metal was isolated in 1807 by Sir H. Davy by electrolysing the moist hydroxide or chloride, and has been obtained by A. Guntz and Roederer (Comptes rendus, 1906, 142, p. 400) by heating the hydride in a vacuum to 1000°. By electrolysing an aqueous solution of the chloride with a mercury cathode, a liquid and a solid amalgam, SrHg11, are obtained; the latter on heating gives a mixture of Sr2Hg5 and SrHg6, and on distillation an amalgam passes over, and not the metal. It is a silver-white ductile metal (of specific gravity 2.54) which melts at 800°. It oxidizes rapidly when exposed to air, and burns when heated in air, oxygen, chlorine, bromine or sulphur vapour. With dry ammonia at 60° the metal forms strontium ammonium, which slowly decomposes in a vacuum at 20° giving Sr(NH3)2; with carbon monoxide it gives Sr(CO)2; with oxygen it forms the monoxide and peroxide, and with nitric oxide it gives the hyponitrite (Roederer, Bull. soc. chim., 1906 [iii.], 35, p. 715).
The hydride, SrH2, was obtained by Guntz on heating strontium amalgam in a current of hydrogen. It is a white solid, which readily decomposes water in the cold and behaves as a strong reducing agent. It dissociates when heated to a high temperature and is not affected by oxygen. The monoxide or strontia, SrO, is formed by strongly heating the nitrate, or commercially by heating the sulphide or carbonate in superheated steam (at about 500–600º C). It is a white amorphous powder which resembles lime in its general character. By heating the amorphous form in the electric furnace H. Moissan succeeded in obtaiuing a crystalline variety. The amorphous form readily slakes with water, and the aqueous solution yields a crystalline hydrated hydroxide approximating in composition to Sr(OH)2·8H2O or Sr(OH)2·9H2O, which on standing in vacuo loses some of its water of crystallization, leaving the monohydrated hydroxide, Sr(OH)2·H2O. The ordinary hydrated variety forms quadratic crystals and behaves as a strong base. It is used in the extraction of sugar from molasses, since it combines with the sugar to form a soluble saccharate, which is removed and then decomposed by carbon dioxide. A hydrated dioxide, approximating in composition to SrO2·8H2O, is formed as a crystalline precipitate when hydrogen peroxide is added to an aqueous solution of strontium hydroxide.
Strontium fluoride, SrF2, is obtained by the action of hydrofluoric acid on the carbonate, or by the addition of potassium fluoride to strontium chloride solution. It may be obtained crystalline by fusing the anhydrous chloride with a large excess of potassium hydrogen fluoride or by heating the amorphous variety to redness with an excess of an alkaline chloride. Strontium chloride, SrCl2·6H2O, is obtained by dissolving the carbonate in hydrochloric acid, or by fusing the carbonate with calcium chloride and extracting the melt with water. It crystallizes in small colorless needles and is easily soluble in water; the concentrated aqueous solution dissolves bromine and iodine readily. By concentrating the aqueous solution between 90–130° C., or by passing hydrochloric acid gas into a saturated aqueous solution, a second hydrated form of composition, SrCl2·2H2O, is obtained. The anhydrous chloride is formed by heating strontium or its monoxide in chlorine, or by heating the hydrated chloride in a current of hydrochloric acid gas. It is a white solid, which combines with gaseous ammonia to form SrCl2·8NH3, and when heated in superheated steam it decomposes with evolution of hydrochloric acid.
Strontium sulphide, SrS, is formed v/hen the carbonate is heated to redness in a stream of sulphuretted hydrogen. It phosphoresces very slightly when pure. Strontium sulphate, SrSO4, found in the mineral kingdom as celestine, is formed when sulphuric acid or a soluble sulphate is added to a solution of a strontium salt. It is a colourless, amorphous solid, which is almost insoluble in water, its solubility diminishing with increasing temperature; it is appreciably soluble in concentrated sulphuric acid. When boiled with alkaline carbonates it is converted into strontium carbonate.
Strontium nitride, Sr3N2, is formed when strontium amalgam is heated to redness in a stream of nitrogen or by igniting the oxide with magnesium (H. R. Ellis, Chem. News, 1909, 99, p. 4). It is readily decomposed by water, with liberation of ammonia. Strontium nitrate, Sr(NO3)2, is obtained by dissolving the carbonate in dilute nitric acid. It crystallizes from water (in which it is very soluble) in monoclinic prisms which approximate in composition to Sr(NO3)2·4H2O or Sr(NO3)2·5H2O. When heated it fuses in its own water of crystallization and becomes anhydrous at 110° C. It is used in pyrotechny for the manufacture of red-fire. A strontium boride, SrB6, was obtained as a black crystalline powder by H. Moissan and P. Williams (Comptes rendus, 1897, 123, p. 633) by reducing the borate with aluminium in the electric furnace.
Strontium carbide, SrC2, is obtained by heating strontium carbonate with carbon in the electric furnace. It resembles calcium carbide, decomposing rapidly with water, giving acetylene. Strontium carbonate, SrCO3, found in the mineral kingdom as strontianite, is formed when a solution of a carbonate is added to one of a strontium salt. It is an amorphous solid, insoluble in water, but its solubility is increased in the presence of ammonium nitrate. It loses carbon dioxide when heated to high temperature.
Strontium salts may be recognized by the characteristic crimson colour they impart to the flame of the Bunsen burner and by the precipitation of the insoluble sulphate. On the preparation of pure strontium salts, see Adrian and Bougarel, Journ. pharm. chem., 1892 (5), p. 345; and S. P. L. Soerenoen, Zeit. anorg. chem., 1895, 11, p. 305. Recent determinations of the atomic weight of strontium are due to T. W. Richards (Zeit. anorg. Chem., 1905, 47, p. 145), who, by estimating the ratios of strontium bromide and chloride to silver, obtained the values 87.663 and 87.661.