Hydrated Cobaltic Oxide, Co(OH)₃

Hydrated Cobaltic Oxide, Co(OH)₃ or Co₂O₃.3H₂O, is obtained in a more or less complete state of hydration when an alkaline oxidiser, such as, for example, an alkaline solution of ammonium persulphate, or of sodium hypochlorite, is added to a cobalt salt. The precipitate varies in composition according to the conditions obtaining at the moment. When an alkaline solution of cobalt sulphate is electrolysed the oxide is obtained in at least two stages of hydration, namely, the dihydrate, Co₂O₃.2H₂O, and the trihydrate, Co₂)O₃.3H₂O.

As obtained by the foregoing methods, hydrated cobaltic oxide is a blackish brown precipitate which dissolves in aqueous hydrogen chloride evolving chlorine, cobaltous chloride passing into solution. The oxide thus behaves like a peroxide.

With well-cooled acids, brownish solutions are obtained which are presumed to contain cobaltic salts.

The oxide always contains traces of alkali. At 385° C. it becomes anhydrous, but loses a little oxygen, a small quantity of Co₃O₄ being formed. Hydrogen begins to reduce it at 110° C., carbon at 260° C., and carbon monoxide at -11° C.