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. |