Mercury Thioarsenates

When an alkali orthothioarsenate is added to an aqueous solution of a mercurous salt, a black precipitate of mercurous sulphide separates and a thioarsenate is formed in solution. Berzelius thought this to be the pyro-salt which, when obtained by evaporation and heated, lost mercury and formed the mercuric pyro-salt. Heubach, however, stated that mercurous orthothioarsenate was formed and that this was decomposed by excess of mercurous salt to mercurous sulphide and arsenic acid. The mercuric salts are more stable.

Mercuric Orthothioarsenate, Hg₃(AsS₄)₂, is formed as an orange- coloured precipitate by treating mercuric chloride solution with sodium orthothioarsenate. It may be dried at 100° C. and sublimed without decomposition. Mercuric pyrothioarsenate is obtained as a dark yellow precipitate when mercuric chloride solution is treated with sodium meta- or pyro-thioarsenate. The salt decomposes on heating with loss of arsenic and sulphur.