Strontium Sulphate, SrSO₄

Strontium Sulphate, SrSO₄, occurs naturally as the mineral celestine, or associated with barium as barytocelestine. Celestine forms rhombic crystals, sometimes of a blue colour, hence its name. Certain waters contain strontium sulphate in solution. It may be obtained as a microcrystalline precipitate by the action of sulphuric acid on a strontium salt, or in larger crystals by the fusion of an excess of strontium chloride with potassium sulphate, by heating the precipitated sulphate with dilute hydrochloric acid in a sealed tube at 150° C., by precipitating very slowly from extremely dilute solutions, or, finally, by evaporating the sulphuric acid solution of strontium sulphate to dryness. The density of the precipitated sulphate is 3.7 – 3.8, of natural celestine 3.92 – 3.96, and of artificial 3.927. The specific heat of celestine is 0.135.

Strontium sulphate melts at 1605° C., but becomes basic through loss of sulphur trioxide. There is a transition point at 1152° C. to α-SrSO₄, isomorphous with α-CaSO₄ and α-BaSO₄, with both of which it is miscible in all proportions. Below this transition point strontium sulphate is completely miscible with barium sulphate, but only partially with calcium sulphate.

Carbon, carbon monoxide, and other reducing agents reduce it at red heat, forming the sulphide. On boiling with sodium carbonate solution it is transformed into strontium carbonate.

Under the influence of the cathode rays, strontium sulphate shows a pale lilac fluorescence, which is not materially affected by the presence of manganese, but is changed to a brilliant orange by bismuth.

Strontium sulphate is only slightly soluble in water, 1 litre of the solution at 18° C. containing 1.245 milli-equivalents, or 114.3 mgm. The heat of solution is -0.3 Cal. The solubility varies very little with temperature, but is notably increased by the presence of acids.

The solubility is diminished by dilute sulphuric acid. It is increased by alkali chlorides and nitrates, by calcium salts, and even by strontium salts, probably owing to the formation of a complex ion. The solubility is decreased by sulphates and alcohol.

A colloidal precipitate of strontium sulphate is obtained by precipitation from an alcoholic solution of strontium thiocyanate.