Hydroxide
Hydroxide is a diatomic anion with chemical formula OH. It consists of an oxygen and hydrogen atom held together by a covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. A hydroxide attached to a strongly electropositive center may itself ionize, liberating a hydrogen cation (H), making the parent compound an acid.
Nickel compounds
Nickel hydroxides are used in nickel–cadmium and Nickel–metal hydride batteries.
Nickel(II) hydroxide Ni(OH), the main hydroxide of nickel is coloured apple green. It is known as the mineral theophrastite.
β-NiO(OH) is a black powder with nickel in the +3 oxidation state. It can be made by oxidising nickel nitrate in a cold alkaline solution with bromine. A mixed oxidation state hydroxide NiO(OH) is made if oxidation happens in a hot alkaline solution. A Ni hydroxide: nickel peroxide hydrate NiO, can be made by oxidising with alkaline peroxide. It is black, and unstable and oxidises water.