Mercury Arsenites

Mercurous Orthoarsenite, Hg₃NaO₃, may be obtained by treating a solution of mercurous nitrate with one of sodium orthoarsenite or with a solution of arsenious oxide in 50 per cent, alcohol; in the latter case the mercurous nitrate solution should be acidified with nitric acid and sufficient alcohol added to produce a slight turbidity. The precipitate is pale yellow, but rapidly turns brown on exposure to air. It is slightly soluble in water, being slowly decomposed with separation of mercury. It is also decomposed by hydroxides and carbonates of alkali metals and of barium, and by ammonia. It dissolves in acids, but when these are dilute, basic salts gradually separate.

Complex salts of composition 2Hg₃NaO₃.Hg₂SO₄ and 2Hg₃NaO₃.HgNO₃ have been obtained.

Mercurous Metarsenite, HgAsO₂, separates as a yellowish precipitate when potassium tetrarsenite is gradually added to mercurous nitrate solution. The salt is dried at 125° C. It is unstable and is decomposed by excess of the alkali arsenite, by alkali hydroxides and by aqueous ammonia.

Mercuric Orthoarsenite, Hg₃(NaO₃)₂, is precipitated in an impure state when an aqueous solution of mercuric chloride is treated with a solution of arsenious oxide in 50 per cent, alcohol. The salt is unstable and loses arsenious oxide at 150° C. It is slowly reduced to mercury and mercurous hydroxide by water, in which it is sparingly soluble, and by aqueous alkalis and ammonia. It dissolves in acids, but with sulphuric acid a basic sulphate is formed.

Mercuric Pyroarsenite, Hg₂As₂O₅, was described as a yellowish-white mass, decomposing in light, obtained by adding a solution of potassium tetrarsenite to one of mercuric chloride. According to Dessner, however, the precipitate so obtained is a mixture of arsenite and arsenate. A basic arsenite of composition Hg₅As₂O₈ was described by Reichard also as a white mass, turning yellow and decomposing in daylight; this was obtained when sodium metarsenite was added to an aqueous solution of mercuric chloride.