Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a catalyst . The hydroxide ion form 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 group attached to a strongly electropositive center may itself dissociate, liberating a hydrogen cation (H+), making the parent compound an acid.
In organic chemistry, the hydroxide ion can act as a catalyst or as a nucleophilic reagent. An OH group, known as an hydroxyl group, is present in alcohols, phenols, carboxylic acids and related functional groups.