Thioarsenates

Arsenic pentasulphide acts as an acid thioanhydride and with metallic sulphides yields salts known as thioarsenates. These may be regarded as derivatives of the following hypothetical acids:

H₃AsS₄, Orthothioarsenic acid
H₄As₂S₇, Pyrothioarsenic acid
HAsS₃, Metathioarsenic acid
and of two more complex acids of empirical composition H₅As₃S₁₀ and H₁₀As₄S₁₅.

The alkali salts are generally formed when arsenic pentasulphide is dissolved in an aqueous solution of the hydrosulphide and the mixture evaporated or precipitated with alcohol, or when a solution of an arsenate is treated with hydrogen sulphide under suitable conditions. The thioarsenates of the heavy metals (except silver) may be formed by the action of an excess of an alkali thioarsenate on a salt of the heavy metal; if the latter is in excess the sulphide is formed.

The thioarsenates vary in colour, being yellow, red or brown and, in Nature, black. They possess an unpleasant bitter taste. The ortho-salts are generally well crystallised, but the other thioarsenates are not. The ortho-salts of the alkali metals and barium may be heated out of contact with air to a white heat without decomposition and, on cooling, a completely soluble yellow mass is obtained. Mercury orthothio-arsenate sublimes without decomposition. The ortho-salts of other metals, and ammonium, decompose on heating. Meta- and pyro-thioarsenates lose sulphur and form thioarsenites when heated.

The salts of the alkali and alkaline earth metals, as well as of gold, magnesium, beryllium and yttrium, are soluble in water, the solutions being colourless or pale yellow. They gradually decompose, however, when kept, with deposition of sulphur, arsenic and arsenic pentasulphide. The following thioarsenates have been described.