Cobalt Arsenites

Several compounds have been prepared.

Cobalt orthoarsenite, Co₃(NaO₃)₂.4H₂O, is obtained as a pink precipitate when a solution of cobaltous chloride in 50 per cent, alcohol is treated with a solution of potassium orthoarsenite which has been just neutralised with acetic acid. The precipitate is soluble in dilute acids. On heating it turns black. A basic salt, of composition 7CoO.As₂O₃, is obtained when a solution of cobalt nitrate is treated with sodium orthoarsenite, the former being in excess; the amethyst-coloured precipitate first formed contains combined water, which may be completely removed at 150° C.

Cobalt Pyroarsenite, Co₂As₂O₅, is obtained when a dilute solution of cobaltous nitrate is precipitated with sodium metarsenite,

2Co(NO₃)₂ + 2NaAsO₂ + H₂O = Co₂As₂O₅ + 2NaNO₃ + 2HNO₃

or with potassium pyroarsenite. It forms as a violet-blue voluminous deposit, which decomposes at red heat, arsenious oxide subliming and leaving a dark blue residue. It dissolves in dilute acids and in aqueous ammonia.

Cobalt Tetrarsenite, Co₃As₄O₉, is obtained as an amethyst-coloured powder by the interaction of solutions of potassium hydrogen di-arsenite and cobaltous nitrate. It dissolves in dilute hydrochloric and nitric acids; also in caustic potash, in which it forms a blue solution which decomposes on heating. In aqueous ammonia it gives a brown solution, and in aqueous potassium cyanide a yellow one.