Calcium Thiosulphate, CaS₂O₃

Calcium Thiosulphate, CaS₂O₃, is prepared by boiling lime-water with sulphur, saturating the liquid at the same time with sulphur dioxide, and evaporating below 60° C. It may also be obtained by the reduction of calcium sulphate by sulphur, by the action of sulphur on calcium sulphite and water between 30° and 40° C., or by mixing concentrated solutions of sodium thiosulphate and calcium chloride, removing by filtration the sodium chloride first formed, concentrating at about 50° C., and allowing to crystallise.

Calcium thiosulphate is also formed when lime which has been used for coal-gas purification is exposed to the air, and it is a by-product of the Le Blanc process.

It forms colourless, six-sided, triclinic crystals of composition CaS₂O₃.6H₂O, which effloresce in vacuo or in dry air above 40° C. The density is 1.872. The solubility at 9° C. is 29.4 grm. of anhydrous salt in 100 grm. of water, and at 25° C. 34.7 grm. In each case the solution is dissociated electrolytically to the extent of about 0.6 per cent.

On keeping, calcium thiosulphate gradually decomposes into sulphur, sulphide, and sulphate.

A triple salt with sodium nitrate and sodium thiosulphate, of composition NaNO₃.CaS₂O₃.Na₂S₂O₃.11H₂O, has been obtained.