Mercury Arsenides

Arsenic does not dissolve in mercury even at the boiling point, nor has an amalgam been obtained by the action of sodium amalgam on moist arsenious oxide or by electrolysis of a solution of arsenic trichloride using a mercury cathode; in the latter case arsenic is deposited on the surface of the mercury but does not dissolve. Nevertheless, two arsenides of mercury, which appear to be definite compounds, have been prepared.

Mercuric Arsenide, Hg₃As₂, may be obtained by passing arsine, largely diluted with hydrogen, into an alcoholic solution of mercuric chloride; the yellow chloromercurarsine, AsH₂.HgCl, is first formed, followed by the orange di- and brown tri-chloromercurarsines AsH(HgCl)₂ and As(HgCl)₃, and finally the arsenide is obtained as a black microcrystalline powder. It may also be prepared as follows: 10 g. of arsenious oxide are dissolved in 100 g. of concentrated hydrochloric acid and the solution treated with 40.65 g. of mercuric chloride in 700 to 800 g. of dilute hydrochloric acid (1:5). After filtration, 60 g. of sodium hypophosphite are added and the mixture, shaken from time to time, is kept for several hours; on heating to boiling, the black precipitate of mercuric arsenide appears. Vortmann electrolysed a hydrochloric acid solution of mercuric and arsenic chlorides after adding potassium iodide to prevent the formation of basic salts; much arsine was evolved and a brownish-black amalgam was obtained.

Mercuric arsenide forms microscopic mamellated crystals. When dry it oxidises readily in the air to arsenious oxide and mercury. On heating it volatilises without melting, forming a sublimate of arsenic and mercury and a little arsenious oxide. The arsenide is therefore dried under diminished pressure. When heated with an alkyl iodide, a diarsonium mercuriodide of the type As₂R₆I₂.2HgI₂ is formed.

Mercurous Arsenide, Hg₃As, was obtained by Brukl as a black precipitate by dropping an aqueous solution of a mercurous salt into an atmosphere of arsine in complete absence of air. It is readily oxidised, and on keeping in air it yields mercury and arsenious oxide. It dissolves in nitric acid.