Nickel Dihydroxide, Ni(OH)₂

Nickelous Hydroxide, Nickel Dihydroxide, Ni(OH)₂, is obtained as an apple-green precipitate upon warming aqueous alkali hydroxide with a solution of a nickel salt. The precipitate is not perfectly pure, but contains traces of alkali and of the original acid. It is very slightly soluble in water, but readily soluble in ammonium hydroxide, yielding a deep blue solution from which the nickel hydroxide is deposited in crystalline form on boiling.

Nickel hydroxide may also be obtained in the crystalline form by allowing a solution of sodium chloride to stand over mercury with a nickel wire connecting both liquids. The reaction is extremely slow, however.

Nickel hydroxide is formed when a fifth normal solution of nickel nitrate is exposed to hydrogen under a pressure of 100 atmospheres, and when a similar concentration of nickel acetate is exposed to hydrogen under the same pressure at 120° C.

Nickel hydroxide dissolves in ordinary distilled water to the extent of 12.7 mgs. of Ni(OH)₂ per litre at 20° C. It is soluble in acids, yielding nickel salts; when heated, water is evolved, leaving a residue of nickel monoxide. It is quite insoluble even in concentrated solutions of potassium or sodium hydroxide, and is less readily oxidised in air than the corresponding cobalt derivative. Its solution in ammonia is a solvent for silk, but not for cotton.

Nickel hydroxide has been obtained in colloidal form by treating a solution of nickel sulphate with one of sodium protalbinate or lysalbinate, and dissolving the precipitate in dilute sodium hydroxide.