Ammonium orthothioarsenate

Ammonium Thioarsenates. — Ammonium Orthothioarsenate, (NH4)3AsS4, separates as colourless prisms when alcohol is added to a hot solution containing arsenic pentasulphide and excess of ammonium hydrosulphide.5 The crystals gradually turn yellow in air and, when heated, melt and decompose, yielding ammonium sulphide, arsenious sulphide and sulphur. 

A solution containing magnesium and ammonium orthothioarsenates yields, on the addition of alcohol, needle-shaped crystals of the double salt Mg(NH4)AsS4.nH20. The salt is unstable in air, hydrogen sulphide and ammonia being given off. 

Copper Thioarsenates.—Cupric Orthothioarsenate, Cu3(AsS4)2, is formed as a dark brown precipitate when sodium orthothioarsenate is added to a solution of a cupric salt.2 The reaction, however, is complex, sulphides of copper and arsenic also being formed.3 A similar precipitate is formed when ammonium hydrosulphide or hydrogen sulphide is added to a solution of arsenic acid and a copper salt,4 and the proportion of sulphide and thio-salt in the precipitate varies with the concentration of the reactants. Copper hydroxide reacts with alkali thioarsenates to form copper sulphide and alkali arsenate, but some copper orthothioarsenate is formed and remains in solution in excess of alkali thioarsenate.5... 

The double salt, Na3(NH4)3(AsS4)2, is obtained w hen alcohol is added to a mixture of the two thioarsenates in molecular proportions, or when a solution containing ammonium chloride and sodium orthothioarsenate is allowed to evaporate it crystallises as pale yellow hexagonal prisms. A corresponding sodium-potassium salt is obtained by evaporation of a solution of the mixed orthothioarsenates.1... 

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