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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 A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
, 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 In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions.
Sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...
is a multi-million-ton per annum
commodity chemical Commodity chemicals (or bulk commodities or bulk chemicals) are a group of chemicals that are made on a very large scale to satisfy global markets. The average prices of commodity chemicals are regularly published in the chemical trade magazines an ...
. The corresponding
electrically neutral Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
compound HO• is the hydroxyl radical. The corresponding covalently bound group â€“OH of atoms is the
hydroxy group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
. Both the hydroxide ion and hydroxy group are
nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
s and can act as catalysts in organic chemistry. Many inorganic substances which bear the word ''hydroxide'' in their names are not
ionic compound In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged i ...
s of the hydroxide ion, but covalent compounds which contain
hydroxy group In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
s.


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 Poh ( pa, ਪੋਹ, ) is the tenth month of the Nanakshahi calendar, which governs the activities within Sikhism. This month coincides with Pausha in the Hindu calendar and the Indian national calendar, and December and January in the Gregorian ...
, 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 An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
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 A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
by donating a pair of electrons to a Lewis acid. In aqueous solution both hydrogen and hydroxide ions are strongly solvated, with
hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
s 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 Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...
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 The carbonic anhydrases (or carbonate dehydratases) () form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid (i.e. bicarbonate and hydrogen ions). The active site ...
, which effectively creates hydroxide ions at the active site. Solutions containing the hydroxide ion attack glass. In this case, the
silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...
s in glass are acting as acids. Basic hydroxides, whether solids or in solution, are stored in
airtight A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers, but as technology advanced it applied to a larger categor ...
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 In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually r ...
, 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 Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...
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 Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional ...
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 A molecular vibration is a periodic motion of the atoms of a molecule relative to each other, such that the center of mass of the molecule remains unchanged. The typical vibrational frequencies range from less than 1013 Hz to approximately 1014 Hz ...
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 Raman may refer to: People * Raman (name) *C. V. Raman (1888–1970), Indian Nobel Prize-winning physicist Places * Raman, Punjab (India) * Raman, Rawalpindi, Pakistan * Raman District, Yala Province, Thailand ** Raman Railway Station * Ra ...
-active, polarized) and 420 cm−1 (infrared). The same ion has a (HO)–Zn–(OH) bending vibration at 300 cm−1.


Applications

Sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...
solutions, also known as lye and caustic soda, are used in the manufacture of pulp and paper, textiles, drinking water, soaps and
detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are more ...
s, 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 Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...
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 The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as ...
) 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,
submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...
s, 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 Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...
, potassium hydroxide, and the hydroxides of the other
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s are also
strong base In chemistry, there are three definitions in common use of the word base, known as Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. Rou ...
s.


Alkaline earth metals

Beryllium hydroxide Beryllium hydroxide, Be(OH)2, is an amphoteric hydroxide, dissolving in both acids and alkalis. Industrially, it is produced as a by-product in the extraction of beryllium metal from the ores beryl and bertrandite. The natural pure beryllium hydro ...
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 A metal ion in aqueous solution or aqua ion is a cation, dissolved in water, of chemical formula (H2O)nsup>z+. The solvation number, ''n'', determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for most elements in periods 3 ...
e(H2O)4sup>2+ is formed. Addition of hydroxide to Be(OH)2 gives the soluble tetrahydroxoberyllate/tetrahydroxido
beryllate Beryllium compounds {{Short pages monitor