Cobalt Arsenites

When a dilute solution of cobalt nitrate is precipitated with sodium metarsenite, a violet-blue voluminous deposit of cobalt pyro-arsenite, Co₂As₂O₅ or 2CoO.As₂O₃, is obtained.

2Co(NO₃)₂ + Na₂As₂O₄ + H₂O = Co₂As₂O₅ + 2NaNO₃ + 2HNO₃.

At red heat arsenious oxide sublimes, leaving a dark blue residue.

An arsenite of composition 3СоО.2As₂O₃ is obtained as an amethyst- coloured powder by the interaction of solutions of potassium arsenite and cobaltous nitrate. It dissolves in caustic potash to a blue solution; ammonia gives a brown colour, and potassium cyanide a yellow.