Ammonium Thioarsenates

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

Ammonium Pyrothioarsenate, (NH₄)₄As₂S₇, has been described as a reddish-yellow mass obtained by evaporation of an ammoniacal solution of arsenic pentasulphide. It is stable at the ordinary temperature, but decomposes on heating. If the ammoniacal solution is filtered and evaporated in vacuo, the composition of the yellow residue is (NH₄)₅As₃S₁₀. According to McCay and others, the solution of arsenic pentasulphide in ammonia contains the orthothioarsenate and thioxyarsenates. The yellow solution on keeping becomes almost colourless and slightly turbid, owing to separation of sulphur produced by decomposition of the thioxyarsenates.