Calcium Thioarsenates

Calcium Orthothioarsenate, Ca₃(AsS₄)₂.20H₂O, may be prepared by the methods described for the corresponding barium salt. It yields pale yellow rhombic crystals which are soluble in water. Unlike the barium salt, it decomposes when heated.

Calcium Pyrothioarsenate, Ca₂As₂S₇.nH₂O, is obtained by addition of sodium pyrothioarsenate to a solution of a calcium salt and evaporation of the solution. The residue is yellow and opaque, soluble in water or alcohol. It loses water at 60° C. but absorbs it from the air again on cooling. It loses sulphur on heating to form thioarsenite. When arsenic pentasulphide is dissolved in warm aqueous calcium hydrosulphide and the solution concentrated in vacuo, crystals of the salt, Ca₅As₄S₁₅.12H₂O (or 5CaS.2As₂S₅.12H₂O), are obtained.