Cobaltous Hydroxide, Co(OH)₂

When potassium hydroxide is added to an aqueous solution of a cobaltous salt, carefully freed from air, a blue precipitate is obtained which, on heating, is converted into the rose-coloured hydroxide - Cobaltous Hydroxide, Co(OH)₂.

The blue precipitate was at one time regarded as a basic salt, but Hantzsch showed that any basic salt can be washed out without impairing the colour of the mass. It may be completely dehydrated at 170° C., whereas the pink hydroxide retains some water even after prolonged exposure to 300° C. in an atmosphere of nitrogen. The formula suggested for the blue compound is, accordingly, CoO.H₂O, and for the pink one, Co(OH)₂.

Cobalt hydroxide dissolves in ordinary distilled water to the extent of 3.18 mgs. of Co(OH)₂ per litre at 20° C.

Like its ferrous analogue, cobalt hydroxide readily absorbs oxygen, yielding a brown mass. When dissolved in acids stable cobaltous salts are obtained, but when dissolved in hot, concentrated aqueous potassium hydroxide and allowed to cool, cobaltous oxide gradually crystallises in microscopic prisms which are deposited as a violet powder. The crystals are pleochroic, of density 3.597 at 15° C., permanent in air and insoluble in ammonium hydroxide. They are soluble in acetic acid, in sodium hydroxide, and in hot solutions of ammonium chloride.