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Hydroxide is a diatomic anion with chemical formula OH^−. It consists of an oxygen and hydrogen atom held together by a single 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.
The corresponding electrically neutral compound HO^• is the hydroxyl radical. The corresponding covalently bound group â€“OH of atoms is the hydroxy group.
Both the hydroxide ion and hydroxy group are nucleophiles and can act as catalysts in organic chemistry.

Many inorganic substances which bear the word ''hydroxide'' in their names are not ionic compounds of the hydroxide ion, but covalent compounds which contain hydroxy groups.

Hydroxide ion

The hydroxide ion is a natural part of water because of the self-ionization reaction in which its complement, hydronium, is passed hydrogen:
: H₃O⁺ + OH^− 2H₂O
The equilibrium constant for this reaction, defined as
:''K''_w = ⁺OH^−] ⁺denotes the concentration of hydrogen cations and H^−the concentration of hydroxide ions
has a value close to 10^−14 at 25 Â°C, so the concentration of hydroxide ions in pure water is close to 10^−7 mol∙dm^−3, in order to satisfy the equal charge constraint. The pH of a solution is equal to the decimal cologarithm of the hydrogen cation concentration;Strictly speaking pH is the cologarithm of the hydrogen cation activity the pH of pure water is close to 7 at ambient temperatures. The concentration of hydroxide ions can be expressed in terms of pOH, which is close to (14 âˆ’ pH),pOH signifies the minus the logarithm to base 10 of H^− alternatively the logarithm of so the pOH of pure water is also close to 7. Addition of a base to water will reduce the hydrogen cation concentration and therefore increase the hydroxide ion concentration (increase pH, decrease pOH) even if the base does not itself contain hydroxide. For example, ammonia solutions have a pH greater than 7 due to the reaction NH₃ + H⁺ , which decreases the hydrogen cation concentration, which increases the hydroxide ion concentration. pOH can be kept at a nearly constant value with various buffer solutions.

In aqueous solution the hydroxide ion is a base in the Brønsted–Lowry sense as it can accept a protonIn this context proton is the term used for a solvated hydrogen cation from a Brønsted–Lowry acid to form a water molecule. It can also act as a Lewis base by donating a pair of electrons to a Lewis acid. In aqueous solution both hydrogen and hydroxide ions are strongly solvated, with hydrogen bonds between oxygen and hydrogen atoms. Indeed, the bihydroxide ion has been characterized in the solid state. This compound is centrosymmetric and has a very short hydrogen bond (114.5  pm) that is similar to the length in the bifluoride ion (114 pm). In aqueous solution the hydroxide ion forms strong hydrogen bonds with water molecules. A consequence of this is that concentrated solutions of sodium hydroxide have high viscosity due to the formation of an extended network of hydrogen bonds as in hydrogen fluoride solutions.

In solution, exposed to air, the hydroxide ion reacts rapidly with atmospheric carbon dioxide, acting as an acid, to form, initially, the bicarbonate ion.
:OH^− + CO₂
The equilibrium constant for this reaction can be specified either as a reaction with dissolved carbon dioxide or as a reaction with carbon dioxide gas (see Carbonic acid for values and details). At neutral or acid pH, the reaction is slow, but is catalyzed by the enzyme carbonic anhydrase, which effectively creates hydroxide ions at the active site.

Solutions containing the hydroxide ion attack glass. In this case, the silicates in glass are acting as acids. Basic hydroxides, whether solids or in solution, are stored in airtight plastic containers.

The hydroxide ion can function as a typical electron-pair donor ligand, forming such complexes as tetrahydroxoaluminate/tetrahydroxido aluminate l(OH)₄sup>−. It is also often found in mixed-ligand complexes of the type L_''x''(OH)_''y''sup>''z''+, where L is a ligand. The hydroxide ion often serves as a bridging ligand, donating one pair of electrons to each of the atoms being bridged. As illustrated by b₂(OH)sup>3+, metal hydroxides are often written in a simplified format. It can even act as a 3-electron-pair donor, as in the tetramer tMe₃(OH)sub>4.

When bound to a strongly electron-withdrawing metal centre, hydroxide ligands tend to ionise into oxide ligands. For example, the bichromate ion CrO₄sup>− dissociates according to
: ₃CrO–Hsup>− rO₄sup>2− + H⁺
with a p''K''_a of about 5.9.IUPAC SC-Database
A comprehensive database of published data on equilibrium constants of metal complexes and ligands

Vibrational spectra

The infrared spectra of compounds containing the OH functional group have strong absorption bands in the region centered around 3500 cm^−1. The high frequency of molecular vibration is a consequence of the small mass of the hydrogen atom as compared to the mass of the oxygen atom, and this makes detection of hydroxyl groups by infrared spectroscopy relatively easy. A band due to an OH group tends to be sharp. However, the band width increases when the OH group is involved in hydrogen bonding. A water molecule has an HOH bending mode at about 1600 cm^−1, so the absence of this band can be used to distinguish an OH group from a water molecule.

When the OH group is bound to a metal ion in a coordination complex, an M−OH bending mode can be observed. For example, in n(OH)₆sup>2− it occurs at 1065 cm^−1. The bending mode for a bridging hydroxide tends to be at a lower frequency as in bipyridine)Cu(OH)₂Cu(bipyridine">bipyridine.html" ;"title="bipyridine">bipyridine)Cu(OH)₂Cu(bipyridine)]²⁺ (955 cm^−1). M−OH stretching vibrations occur below about 600 cm^−1. For example, the tetrahedron, tetrahedral ion [Zn(OH)₄]^2− has bands at 470 cm^−1 (Raman-active, polarized) and 420 cm^−1 (infrared). The same ion has a (HO)–Zn–(OH) bending vibration at 300 cm^−1.

Applications

Sodium hydroxide solutions, also known as lye and caustic soda, are used in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents, and as a drain cleaner. Worldwide production in 2004 was approximately 60 million tonnes. The principal method of manufacture is the chloralkali process.

Solutions containing the hydroxide ion are generated when a salt of a weak acid is dissolved in water. Sodium carbonate is used as an alkali, for example, by virtue of the hydrolysis reaction
: + H₂O + OH^− ( p''K''_a2= 10.33 at 25 °C and zero ionic strength)
Although the base strength of sodium carbonate solutions is lower than a concentrated sodium hydroxide solution, it has the advantage of being a solid. It is also manufactured on a vast scale (42 million tonnes in 2005) by the Solvay process.Kostick, Dennis (2006)
"Soda Ash"
chapter in ''2005 Minerals Yearbook,'' United States Geological Survey. An example of the use of sodium carbonate as an alkali is when washing soda (another name for sodium carbonate) acts on insoluble esters, such as triglycerides, commonly known as fats, to hydrolyze them and make them soluble.

Bauxite, a basic hydroxide of aluminium, is the principal ore from which the metal is manufactured. Similarly, goethite (α-FeO(OH)) and lepidocrocite (γ-FeO(OH)), basic hydroxides of iron, are among the principal ores used for the manufacture of metallic iron.

Inorganic hydroxides

Alkali metals

Aside from NaOH and KOH, which enjoy very large scale applications, the hydroxides of the other alkali metals also are useful. Lithium hydroxide is a strong base, with a p''K''_b of −0.36. Lithium hydroxide is used in breathing gas purification systems for spacecraft, submarines, and rebreathers to remove carbon dioxide from exhaled gas.
:2 LiOH + CO₂ → Li₂CO₃ + H₂O
The hydroxide of lithium is preferred to that of sodium because of its lower mass. Sodium hydroxide, potassium hydroxide, and the hydroxides of the other alkali metals are also strong bases.

Alkaline earth metals

Beryllium hydroxide Be(OH)₂ is amphoteric.Thomas R. Dulsk
A manual for the chemical analysis of metals
ASTM International, 1996, p. 100 The hydroxide itself is insoluble in water, with a solubility product log ''K''*_sp of −11.7. Addition of acid gives soluble hydrolysis products, including the trimeric ion e₃(OH)₃(H₂O)₆sup>3+, which has OH groups bridging between pairs of beryllium ions making a 6-membered ring. At very low pH the aqua ion e(H₂O)₄sup>2+ is formed. Addition of hydroxide to Be(OH)₂ gives the soluble tetrahydroxoberyllate/tetrahydroxidoberyllate