Silver Arsenites

Silver Orthoarsenite, Ag₃AsO₃, is obtained as a yellow precipitate when an aqueous solution of silver nitrate is treated with an alkali orthoarsenite, metarsenite or tetrarsenite, or when ammoniacal silver nitrate is added to an aqueous solution of arsenious oxide. According to Santos, by mixing silver nitrate and sodium arsenite in suitable molecular proportions, mono- and di-hydrogen arsenites may also be obtained. The normal arsenite is practically insoluble in water (solubility at 20° C. 0.0115 g. per litre of water) but dissolves in the presence of acids or of alkali nitrates. It is also soluble in an excess of alkali or ammonium arsenite, and if the solution is heated silver is deposited. It dissolves also in aqueous ammonia if freshly precipitated, and in caustic alkali; if the former solution is boiled for some time with continual replenishing of ammonia, an arsenate is formed and silver deposited; in the caustic alkali solution decomposition occurs with deposition of silver and silver arsenide, alkali arsenite and arsenate remaining in the solution. Silver orthoarsenite melts at about 150° C. At red heat arsenious oxide sublimes, leaving a residue of silver and silver arsenate.

Silver Pyroarsenite, Ag₄As₂O₅, is formed when an alkali pyroarsenite is added to an aqueous solution of silver nitrate, but the salt could not be obtained in a pure state by Stavenhagen or Bloxam. An ammino-compound, Ag₄As₂O₅.4NH₃, in the form of prismatic crystals, was obtained by Reichard by adding alcohol to an ammoniacal solution of silver orthoarsenite. The tetrarsenite, Ag₆As₄O₉, has been reported, but could not be obtained by Stavenhagen.

A colloidal solution of silver arsenite has been prepared by the following method. A 10 per cent, solution of sodium lysalbinate was added to aqueous silver nitrate and the precipitate separated, washed, and dissolved in an alkaline solution of sodium dihydrogen arsenite. After dialysis, the solution yielded on evaporation to dryness a pale yellowish-brown mass which could be dispersed in water to form a sol.