Magnesium Thioarsenates

Magnesium Orthothioarsenate, Mg₃(AsS₄)₂.nH₂O, is obtained by heating the pyro-salt, or by treating a hot solution of the latter with magnesium hydrosulphide until hydrogen sulphide is no longer liberated. On cooling or evaporation in vacuo, colourless hygroscopic crystals are formed. Magnesium pyrothioar senate, Mg₂As₂S₇.nH₂O, is obtained by the action of sodium pyro- thioarsenate on a solution of a magnesium salt. It is an amorphous, lemon-yellow mass, soluble in water and in aqueous alcohol. A thioarsenate of composition Mg₅As₄S₁₅.3H₂O remains as a mass of crystals when a solution of magnesium hydrosulphide saturated with arsenic pentasulphide is evaporated in a vacuum. Like the corresponding calcium salt, it is readily soluble in water and is decomposed by hot hydrochloric acid.

A solution containing magnesium and ammonium orthothioarsenates yields, on the addition of alcohol, needle-shaped crystals of the double salt Mg(NH₄)AsS₄.nH₂O. The salt is unstable in air, hydrogen sulphide and ammonia being given off.