Hydroxide

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 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 naturally produced from water by the self-ionization reaction:
The equilibrium constant for this reaction, defined as
has a value close to 10⁻¹⁴ at 25 °C, so the concentration of hydroxide ions in pure water is close to 10⁻⁷ mol∙dm⁻³, to satisfy the equal charge constraint. The pH of a solution is equal to the decimal cologarithm of the hydrogen cation concentration; 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, 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 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 an aqueous solution the hydroxide ion is a base in the Brønsted–Lowry sense as it can accept a proton 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 ions 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 that is similar to the length in the bifluoride ion . 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, which acts as a lewis acid, to form, initially, the bicarbonate ion.
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. 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/tetrahydroxidoaluminate ⁻. It is also often found in mixed-ligand complexes of the type ^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 ³⁺, metal hydroxides are often written in a simplified format. It can even act as a 3-electron-pair donor, as in the tetramer ₄.
When bound to a strongly electron-withdrawing metal centre, hydroxide ligands tend to ionise into oxide ligands. For example, the bichromate ion ⁻ dissociates according to
with a pK_a of about 5.9.

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 ²⁻ it occurs at 1065 cm⁻¹. The bending mode for a bridging hydroxide tends to be at a lower frequency as in ²⁺. M−OH stretching vibrations occur below about 600 cm⁻¹. For example, the tetrahedral ion ²⁻ has bands at 470 cm⁻¹ and 420 cm⁻¹. The same ion has a –Zn– bending vibration at 300 cm⁻¹.

Applications

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
An example of the use of sodium carbonate as an alkali is when washing soda 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 and lepidocrocite, 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 used in breathing gas purification systems for spacecraft, submarines, and rebreathers to remove carbon dioxide from exhaled gas.
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

Be₂ is amphoteric. 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 ³⁺, which has OH groups bridging between pairs of beryllium ions making a 6-membered ring. At very low pH the aqua ion ²⁺ is formed. Addition of hydroxide to Be₂ gives the soluble tetrahydroxoberyllate or tetrahydroxidoberyllate anion, ²⁻.
The solubility in water of the other hydroxides in this group increases with increasing atomic number. Magnesium hydroxide Mg₂ is a strong base, as are the hydroxides of the heavier alkaline earths: calcium hydroxide, strontium hydroxide, and barium hydroxide. A solution or suspension of calcium hydroxide is known as limewater and can be used to test for the weak acid carbon dioxide. The reaction Ca₂ + CO₂ Ca²⁺ + + OH⁻ illustrates the basicity of calcium hydroxide. Soda lime, which is a mixture of the strong bases NaOH and KOH with Ca₂, is used as a CO₂ absorbent.

Boron group elements

The simplest hydroxide of boron B₃, known as boric acid, is an acid. Unlike the hydroxides of the alkali and alkaline earth hydroxides, it does not dissociate in aqueous solution. Instead, it reacts with water molecules acting as a Lewis acid, releasing protons.
A variety of oxyanions of boron are known, which, in the protonated form, contain hydroxide groups.
Aluminium hydroxide Al₃ is amphoteric and dissolves in alkaline solution.
In the Bayer process for the production of pure aluminium oxide from bauxite minerals this equilibrium is manipulated by careful control of temperature and alkali concentration. In the first phase, aluminium dissolves in hot alkaline solution as, but other hydroxides usually present in the mineral, such as iron hydroxides, do not dissolve because they are not amphoteric. After removal of the insolubles, the so-called red mud, pure aluminium hydroxide is made to precipitate by reducing the temperature and adding water to the extract, which, by diluting the alkali, lowers the pH of the solution. Basic aluminium hydroxide AlO, which may be present in bauxite, is also amphoteric.
In mildly acidic solutions, the hydroxo/hydroxido complexes formed by aluminium are somewhat different from those of boron, reflecting the greater size of Al vs. B. The concentration of the species ⁷⁺ is very dependent on the total aluminium concentration. Various other hydroxo complexes are found in crystalline compounds. Perhaps the most important is the basic hydroxide AlO, a polymeric material known by the names of the mineral forms boehmite or diaspore, depending on crystal structure. Gallium hydroxide, indium hydroxide, and thallium hydroxide are also amphoteric. Thallium hydroxide is a strong base.