Ammonium Arsenites

The solubility of arsenious oxide in aqueous ammonia at 30° C. has been studied, ammonium metarsenile, NH₄AsO₂, has been separated, and evidence of the existence of ammonium dihydrogen arsenite, (NH₄)H₂NaO₃, obtained. The metarsenite has been prepared in the form of acicular crystals by allowing ammonia to react with an aqueous solution of arsenious oxide at 70° to 80° C., and as rhombic prisms by the action of concentrated aqueous ammonia on powdered arsenious oxide, warming the mixture until a clear solution is obtained and allowing to crystallise. The product loses ammonia on exposure to air. It is readily soluble in water, but on evaporation of the solution arsenious oxide is deposited. Precipitation of the oxide may be brought about by adding a little alcohol to the solution; excess of alcohol leaves the solution clear. The metarsenite is only slightly soluble in aqueous ammonia. When concentrated aqueous ammonia is poured on to arsenious oxide, the crystalline mass first formed was described by Pasteur as ammonium pyroarsenile, (NH₄)₄As₂O₅. The crystals, oblique rectangular prisms, were obtained pure by Stein after washing with alcohol and ether and drying between absorbent paper. They are stable only in contact with the ammoniacal solution; in air, or when dissolved in water, decomposition occurs with loss of ammonia. Stavenhagen, who endeavoured to prepare the above arsenites, could obtain only products of uncertain composition.

The variation of the specific conductivity and of the freezing point of a solution of arsenious oxide progressively neutralised by aqueous ammonia has been investigated, but evidence of compound formation is not obtained.