What is the formula of the hydroxide ion?

The formula of the hydroxide ion is OH−. It contains one oxygen atom covalently bonded to one hydrogen atom and carries a single negative electric charge.

Hydroxide is a key participant in acid–base chemistry. As a Brønsted–Lowry base, it accepts a proton to become water: OH− + H+ → H2O. In Arrhenius chemistry, a base is commonly described as a substance that increases the concentration of hydroxide ions in aqueous solution. Sodium hydroxide, for example, dissociates into Na+ and OH−.

Hydroxide is not the same as the hydroxyl group, written –OH, which is a neutral part of a molecule. It is also different from the hydroxyl radical, OH•, which has an unpaired electron and is highly reactive. These similar-looking symbols are a frequent source of confusion.

Water itself produces tiny amounts of hydroxide through self-ionization. At about 25 °C, pure water contains approximately 1.0 × 10−7 moles of OH− per liter, balanced by hydronium ions. Adding a base increases hydroxide concentration and generally raises the solution’s pH.