Nickel Dioxide, NiO₂

Presumably the Nickel Dioxide, NiO₂ described as nickel sesquioxide and obtained when nickel nitrate or chlorate is gently heated, or when potassium chlorate is fused with nickel chloride, really consists of the dioxide in intimate association with more or less nickel monoxide, according to circumstances (vide supra).

The existence of nickel dioxide was first indicated by Dufau, who prepared barium nickelite, BaO.2NiO₂, by igniting nickel oxide and barium carbonate in an electric arc. The nickelite resulted as dark-coloured crystals of density 4.8 at 20° C. and attacked by water.

In the hydrated form, NiO₂.xH₂O, nickel dioxide is obtained by decomposing an aqueous solution of a nickel salt by warming with sodium or potassium hypochlorite, and by passing chlorine through a suspension of nickelous hydroxide in water. It also results when a neutral solution of nickel sulphate is electrolysed. In the literature it is frequently referred to and described as the hydrated sesqui-oxide.

The hydrated oxide readily dissolves in acids, yielding nickelous salts. It decomposes hydrochloric acid evolving chlorine, and hydrogen peroxide evolving oxygen, being itself reduced to nickel monoxide. With sulphurous acid the sulphite and dithionate are produced. With sulphuric acid oxygen is liberated, nickel sulphate resulting; with aqueous ammonia nitrogen is evolved. Acetic acid gives at first a brownish yellow colour which rapidly turns green.

The oxide is reduced by hydrogen, carbon monoxide, and by carbon. The reduction in an atmosphere of hydrogen begins at about 190° C. The oxide burns in fluorine, and when heated in hydrogen sulphide yields water and the monosulphide, NiS. Pellini (vide infra) ascribes the following graphical formula to the dioxide:



A compound of similar composition has been obtained by the action of hydrogen peroxide upon free nickel hydroxide or upon cooled nickel chloride solution to which potassium hydroxide in alcoholic solution is added. It is a greyish green substance giving all the reactions of hydrogen peroxide, and is quite different from the foregoing isomeride. Pellini suggests that it is a true peroxide, namely:



Numerous other substances described as oxides of nickel have been placed on record, and formulae suggested such as Ni₄O₇, Ni₅O₇, Ni₃O₅, NiO, and Ni₃O₂.H₂O. These are probably not separate chemical entities, but mixtures of nickel, nickelous oxide, and nickel dioxide according to circumstances.

When a dilute solution of a nickel salt is electrolysed at 70° C. in the presence of chromic acid and an alkali pyrophosphate, the tetroxide, NiO₄ is obtained.