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| Section3 = }} Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a 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. A hydroxide attached to a strongly electropositive center may itself ionize, liberating a hydrogen cation (H+), making the parent compound an acid. The corresponding electrically neutral compound •HO is the hydroxyl radical. The corresponding covalently-bound group -OH of atoms is the hydroxyl group. Hydroxide ion and hydroxyl group are nucleophiles and can act as a catalyst 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 hydroxyl groups. ==Hydroxide ion== The hydroxide ion is a natural part of water, because of the self-ionization reaction: : H+ + OH− H2O The equilibrium constant for this reaction, defined as :Kw = ()()〔() denotes the concentration of hydrogen cations and () the concentration of hydroxide ions〕 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, 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 − pH,〔p(OH) signifies the minus the logarithm to base 10 of , alternatively the logarithm of 1/〕 so 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+ NH4+, which results in a decrease in hydrogen cation concentration and an increase in 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 proton〔In 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 H3O2− 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 HF2− (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 HCO3− 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 air-tight plastic containers. The hydroxide ion can function as a typical electron-pair donor ligand, forming such complexes as ()−. 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).〔Greenwood, p. 1168〕 When bound to a strongly electron-withdrawing metal centre, hydroxide ligands tend to ionises into oxide ligands. For example, the bichromate ion ()− dissociates according to :()− ()2− + H+ with a pKa of about 5.9.〔(IUPAC SC-Database ) A comprehensive database of published data on equilibrium constants of metal complexes and ligands〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「hydroxide」の詳細全文を読む スポンサード リンク
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