# Hydroxide ion

Shorthand ( constitutional formula ) of the hydroxide ion. The minus sign next to the O underlines the position of the negative charge in the molecule.

The hydroxide ion (outdated hydroxyl ion , also hydroxide ion ) is a negatively charged ion that is created when bases react with water . Its chemical formula is OH - . In this ion, the hydrogen atom is bound to the oxygen atom and the oxygen atom has one more electron than it has in the ground state (atomic number 8, oxidation number −2), hence the negative charge. The salts formed together with a positively charged counterion are called hydroxides .

## Inorganic chemistry

A general base B reacts with water according to the following scheme. The double arrows indicate a chemical equilibrium .

${\ displaystyle \ mathrm {B + H_ {2} O \ \ rightleftharpoons \ HB ^ {+} + OH ^ {-}}}$

The pH of the resulting solution can be determined from the concentration of the hydroxide ions . To do this, the so-called pOH value is first calculated

${\ displaystyle \ mathrm {pOH} = - \ log \ c (\ mathrm {OH ^ {-}})}$

and from this the pH value:

${\ displaystyle \ mathrm {pH} = \ mathrm {p} K _ {\ mathrm {w}} - \ mathrm {pOH} \}$

There is a p K w value for each temperature . At 22 ° C pK w = 14.

Hydroxide ions are also found in pure water at 22 ° C in a concentration of 10 −7 mol · l −1 . This is related to the autoprotolysis of water according to the following reaction equation:

${\ displaystyle \ mathrm {H_ {2} O + H_ {2} O \ \ rightleftharpoons \ H_ {3} O ^ {+} + OH ^ {-}}}$

The H 3 O + - and the hydroxide ion ensure a low electrical conductivity even with distilled water .

## Organic chemistry

Hydroxide ions are often used as nucleophiles in nucleophilic substitution for the synthesis of alcohols . Alkanols are formed from haloalkanes .