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Hydroxide is a diatomic anion with
chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and ...
OH. It consists of an
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
and
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

atom
held together by a single
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
, and carries a negative
electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respectively). Like c ...
. It is an important but usually minor constituent of water. It functions as a
base Base or BASE may refer to: Brands and enterprises * Base (mobile telephony provider), a Belgian mobile telecommunications operator *Base CRM Base CRM (originally Future Simple or PipeJump) is an enterprise software company based in Mountain Vie ...
, a
ligand In coordination chemistry A coordination complex consists of a central atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...
, a
nucleophile In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

nucleophile
, and a
catalyst Catalysis () is the process of increasing the rate of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In cla ...

catalyst
. The hydroxide ion forms
salts In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
, some of which
dissociate Dissociation in chemistry and biochemistry is a general process in which molecules (or ionic compounds such as salt (chemistry), salts, or coordination complex, complexes) separate or split into smaller particles such as atoms, ions, or radical (c ...
in aqueous solution, liberating solvated hydroxide ions.
Sodium hydroxide Sodium hydroxide, also known as lye A lye is a metal hydroxide traditionally obtained by leaching wood ashes, or a strong alkali In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chem ...

Sodium hydroxide
is a multi-million-ton per annum commodity chemical. The corresponding
electrically neutral Electric charge is the physical property A physical property is any property Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or with something, whether as an attribute or as a component of said thi ...
compound HO is the
hydroxyl radical The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals is produced from the d ...

hydroxyl radical
. The corresponding
covalently A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
bound
group A group is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal number, label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with ...
 –OH of atoms is the
hydroxy group A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group ( ...
. Hydroxide ion and hydroxy group are
nucleophile In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

nucleophile
s and can act as catalysts in
organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and th ...
. Many
inorganic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they under ...
substances which bear the word ''hydroxide'' in their names are not
ionic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s of the hydroxide ion, but covalent compounds which contain
hydroxy group A hydroxy or hydroxyl group is a functional group with the chemical formula -OH and composed of one oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group ( ...
s.


Hydroxide ion

The hydroxide ion is a natural part of
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
because of the self-ionization reaction in which its complement,
hydronium In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...

hydronium
, is passed hydrogen: :
H3O+
H<sub>3</sub>O<sup>+</sup>
+ OH 2H2O The
equilibrium constant The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency ...
for this reaction, defined as :''K''w = +OH] +denotes the concentration of hydrogen cations and Hthe concentration of hydroxide ions has a value close to 10−14 at 25 °C, so the
concentration In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in t ...

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
pH
of a solution is equal to the decimal
cologarithmIn mathematics, the base-''b'' cologarithm, sometimes shortened to colog, of a number is the base-''b'' logarithm of the multiplicative inverse, reciprocal of the number. It is equal to the ''negative'' base-''b'' logarithm of the number: : \operato ...
of the
hydrogen cation Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. With a standard atomic weight of , hydrogen is the lightest element in the periodic table. Hydrogen is the abundance of the chemical elements, mos ...
concentration;Strictly speaking pH is the cologarithm of the hydrogen cation
activity Activity may refer to: * Action (philosophy), in general * Human activity: human behavior, in sociology behavior may refer to all basic human actions, economics may study human economic activities and along with cybernetics and psychology may stud ...
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 The Nanakshahi calendar is a tropical solar calendar used in Sikhism. It is based on the "Barah Maha" (Twelve Months), a composition composed by the Sikh gurus reflecting the ...
, which is close to (14 − 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 Ammonia is a compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fort ...

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 solution A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or Base (chemistr ...
s. In
aqueous solution An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute an ...
the hydroxide ion is a
base Base or BASE may refer to: Brands and enterprises * Base (mobile telephony provider), a Belgian mobile telecommunications operator *Base CRM Base CRM (originally Future Simple or PipeJump) is an enterprise software company based in Mountain Vie ...
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 In chemistry Chemistry is the scientific discipline involved with C ...
by donating a pair of electrons to a Lewis acid. In aqueous solution both hydrogen and hydroxide ions are strongly solvated, with
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
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 The bifluoride ion is an inorganic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties ...
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 The viscosity of a is a measure of its to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, has a higher viscosity than . Viscosity can be conceptualized as quantifying the inter ...

viscosity
due to the formation of an extended network of hydrogen bonds as in
hydrogen fluoride Hydrogen fluoride is a chemical 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 in the preparation ...

hydrogen fluoride
solutions. In solution, exposed to air, the hydroxide ion reacts rapidly with atmospheric
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

carbon dioxide
, acting as an acid, to form, initially, the
bicarbonate In inorganic chemistry, bicarbonate (International Union of Pure and Applied Chemistry, IUPAC-recommended nomenclature: hydrogen carbonate) is an intermediate form in the deprotonation of carbonic acid. It is a Polyatomic ion, polyatomic anion w ...

bicarbonate
ion. :OH + CO2 The
equilibrium constant The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency ...
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 In chemistry, carbonic acid is a dibasic acid with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical ...

Carbonic acid
for values and details). At neutral or acid pH, the reaction is slow, but is catalyzed by the
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
carbonic anhydrase The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyst, catalyze the interconversion between carbon dioxide and water and the Dissociation (chemistry), dissociated ions of carbonic acid (i.e. bicarbonate and ...

carbonic anhydrase
, which effectively creates hydroxide ions at the active site. Solutions containing the hydroxide ion attack
glass Glass is a non- crystalline, often transparency and translucency, transparent amorphous solid, that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by ...

glass
. In this case, the
silicate In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, ...
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 category ...
plastic containers. The hydroxide ion can function as a typical electron-pair donor
ligand In coordination chemistry A coordination complex consists of a central atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...
, forming such complexes as tetrahydroxoaluminate/tetrahydroxido
aluminate In chemistry, an aluminate is a compound containing an oxyanionAn oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodi ...
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 res ...

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
infrared spectra
of compounds containing the OH
functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions re ...
have strong
absorption bands A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency range, c ...
in the region centered around 3500 cm−1. The high frequency of
molecular vibration A molecular vibration is a periodic motion Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of Mechanical equilibrium, equilibrium) or between two or more different states. The term ...
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 A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ''ligands'' or complexing ...
, 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.html" ;"title="bipyridine.html" ;"title="bipyridine">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 Sodium hydroxide, also known as lye A lye is a metal hydroxide traditionally obtained by leaching wood ashes, or a strong alkali In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chem ...

Sodium hydroxide
solutions, also known as
lye A lye is a metal hydroxideMetal hydroxides are hydroxides of metals. They are often strong base (chemistry), bases. They consist of hydroxide anions and metallic cations. Some metal hydroxides, such as alkali metal hydroxides, ionize completel ...
and caustic soda, are used in the manufacture of pulp and
paper Paper is a thin sheet material Material is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composition ...

paper
,
textile A textile is a flexible material made by creating an interlocking bundle of yarn Yarn is a long continuous length of interlocked fibres, suitable for use in the production of textiles, sewing, crocheting, knitting, weaving, embroidery, ...

textile
s,
drinking water Drinking water, also known as potable water, is water that is safe to drinking, drink or use for food preparation. The amount of drinking water required to maintain good health varies, and depends on physical activity level, age, health-related ...

drinking water
,
soap Soap is a salt (chemistry), salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, ...

soap
s and
detergent A detergent is a surfactant Surfactants are compounds that lower the (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as s, agents, , s, or s. The word "surfact ...
s, and as a
drain cleaner A drain cleaner is a chemical product that unblocks sewer pipes or clogged wastewater drains. The term may also refer to a mechanical device such as a plumber's snake A man clears a sanitary sewer pipe with a motorized snake. A plumber's snake o ...
. Worldwide production in 2004 was approximately 60 million
tonne The tonne ( or ; symbol: t) is a metric unit of mass equal to 1,000 kilogram The kilogram (also kilogramme) is the base unit of mass Mass is the physical quantity, quantity of ''matter'' in a physical body. It is also a meas ...
s. The principal method of manufacture is the
chloralkali process The chloralkali process (also chlor-alkali and chlor alkali In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, s ...
. Solutions containing the hydroxide ion are generated when a salt of a
weak acid Acid strength is the tendency of an acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special ca ...

weak acid
is dissolved in water.
Sodium carbonate Sodium carbonate, ·10, (also known as Natrium Carbonate, 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 modera ...

Sodium carbonate
is used as an alkali, for example, by virtue of the
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord fo ...

hydrolysis
reaction : + H2O + OH ( p''K''a2= 10.33 at 25 °C and zero
ionic strength The ionic strength of a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and th ...
) 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 The Solvay process or ammonia-soda process is the major industrial process for the production of (soda ash, Na2CO3). The ammonia-soda process was developed into its modern form by the Belgian chemist during the 1860s. The ingredients for this are ...

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 Washing is a method of cleaning Cleaning is the process of removing unwanted substances, such as dirt, infectious agents, and other impurities, from an object or environment. Cleaning occurs in many different contexts, and uses many different m ...
(another name for sodium carbonate) acts on insoluble esters, such as
triglyceride A triglyceride (TG, triacylglycerol, TAG, or triacylglyceride) is an ester An ester is a derived from an (organic or inorganic) in which at least one –OH group is replaced by an –O– () group, as in the substitution reaction of a an ...

triglyceride
s, commonly known as fats, to hydrolyze them and make them soluble.
Bauxite Bauxite is a sedimentary rock Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation ( ...

Bauxite
, a basic hydroxide of
aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

aluminium
, is the principal ore from which the metal is manufactured. Similarly,
goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" Polymorphism (materials science), polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite ha ...

goethite
(α-FeO(OH)) and
lepidocrocite Lepidocrocite (γ-FeO(OH)), also called esmeraldite or hydrohematite, is an iron oxide-hydroxide mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chem ...
(γ-FeO(OH)), basic hydroxides of
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

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 Lithium hydroxide is an inorganic compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, pro ...

Lithium hydroxide
is a strong base, with a base dissociation constant, p''K''b of −0.36. Lithium hydroxide is used in breathing gas purification systems for spacecraft, submarines, and rebreathers to remove
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

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 A lye is a metal hydroxide traditionally obtained by leaching wood ashes, or a strong alkali In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chem ...

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 Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord fo ...

hydrolysis
products, including the trimeric ion [Be3(OH)3(H2O)6]3+, which has OH groups bridging between pairs of beryllium ions making a 6-membered ring. At very low pH the Metal ions in aqueous solution, aqua ion [Be(H2O)4]2+ is formed. Addition of hydroxide to Be(OH)2 gives the soluble tetrahydroxoberyllate/tetrahydroxidoberyllate anion, [Be(OH)4]2−. The solubility in water of the other hydroxides in this group increases with increasing atomic number. Magnesium hydroxide Mg(OH)2 is a strong base (up to the limit of its solubility, which is very low in pure water), 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 Acid strength is the tendency of an acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special ca ...

weak acid
carbon dioxide. The reaction Ca(OH)2 + CO2 Ca2+ + + OH illustrates the basicity of calcium hydroxide. Soda lime, which is a mixture of the strong bases NaOH and KOH with Ca(OH)2, is used as a CO2 absorbent.


Boron group elements

The simplest hydroxide of boron B(OH)3, 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. :B(OH)3 + H2O tetrahydroxyborate, + H+ A variety of oxyanions of boron are known, which, in the protonated form, contain hydroxide groups. Aluminium hydroxide Al(OH)3 is amphoteric and dissolves in alkaline solution. :Al(OH)3 (solid) + OH (aq) aluminate,  (aq) 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(OH), 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(III) vs. B(III). The concentration of the species [Al13(OH)32]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(OH), a polymeric material known by the names of the mineral forms boehmite or diaspore, depending on crystal structure. Gallium hydroxide, indium hydroxide, and thallium(III) hydroxide are also amphoteric. Thallium(I) hydroxide is a strong base.


Carbon group elements

Carbon forms no simple hydroxides. The hypothetical compound C(OH)4 (orthocarbonic acid or methanetetrol) is unstable in aqueous solution: :C(OH)4 → + H3O+ : + H+ H2CO3 Carbon dioxide is also known as carbonic anhydride, meaning that it forms by dehydration of carbonic acid H2CO3 (OC(OH)2). Silicic acid is the name given to a variety of compounds with a generic formula [SiO''x''(OH)4−2''x'']''n''. ''Orthosilicic acid'' has been identified in very dilute aqueous solution. It is a weak acid with p''K''a1 = 9.84, p''K''a2 = 13.2 at 25 °C. It is usually written as H4SiO4, but the formula Si(OH)4 is generally accepted. Other silicic acids such as ''metasilicic acid'' (H2SiO3), ''disilicic acid'' (H2Si2O5), and ''pyrosilicic acid'' (H6Si2O7) have been characterized. These acids also have hydroxide groups attached to the silicon; the formulas suggest that these acids are protonated forms of polyoxyanions. Few hydroxo complexes of germanium have been characterized. Tin(II) hydroxide Sn(OH)2 was prepared in anhydrous media. When tin(II) oxide is treated with alkali the pyramidal hydroxo complex is formed. When solutions containing this ion are acidified, the ion [Sn3(OH)4]2+ is formed together with some basic hydroxo complexes. The structure of [Sn3(OH)4]2+ has a triangle of tin atoms connected by bridging hydroxide groups. Tin(IV) hydroxide is unknown but can be regarded as the hypothetical acid from which stannates, with a formula n(OH)6sup>2−, are derived by reaction with the (Lewis) basic hydroxide ion. Hydrolysis of Pb2+ in aqueous solution is accompanied by the formation of various hydroxo-containing complexes, some of which are insoluble. The basic hydroxo complex [Pb6O(OH)6]4+ is a cluster of six lead centres with metal–metal bonds surrounding a central oxide ion. The six hydroxide groups lie on the faces of the two external Pb4 tetrahedra. In strongly alkaline solutions soluble plumbate ions are formed, including [Pb(OH)6]2−.


Other main-group elements

In the higher oxidation states of the pnictogens, chalcogens, halogens, and noble gases there are oxoacids in which the central atom is attached to oxide ions and hydroxide ions. Examples include phosphoric acid H3PO4, and sulfuric acid H2SO4. In these compounds one or more hydroxide groups can
dissociate Dissociation in chemistry and biochemistry is a general process in which molecules (or ionic compounds such as salt (chemistry), salts, or coordination complex, complexes) separate or split into smaller particles such as atoms, ions, or radical (c ...
with the liberation of hydrogen cations as in a standard Brønsted–Lowry acid. Many oxoacids of sulfur are known and all feature OH groups that can dissociate. Telluric acid is often written with the formula H2TeO4·2H2O but is better described structurally as Te(OH)6. ''Ortho''-periodic acidThe name is not derived from "period", but from "iodine": per-iodic acid (compare iodic acid, perchloric acid), and it is thus pronounced per-iodic , and not as . can lose all its protons, eventually forming the periodate ion [IO4]. It can also be protonated in strongly acidic conditions to give the octahedral ion [I(OH)6]+, completing the isoelectronic series, [E(OH)6]''z'', E = Sn, Sb, Te, I; ''z'' = −2, −1, 0, +1. Other acids of iodine(VII) that contain hydroxide groups are known, in particular in salts such as the ''meso''periodate ion that occurs in K4[I2O8(OH)2]·8H2O. As is common outside of the alkali metals, hydroxides of the elements in lower oxidation states are complicated. For example, phosphorous acid H3PO3 predominantly has the structure OP(H)(OH)2, in equilibrium with a small amount of P(OH)3. The oxoacids of chlorine, bromine, and iodine have the formula OA(OH), where ''n'' is the oxidation number: +1, +3, +5, or +7, and A = Cl, Br, or I. The only oxoacid of fluorine is F(OH), hypofluorous acid. When these acids are neutralized the hydrogen atom is removed from the hydroxide group.


Transition and post-transition metals

The hydroxides of the transition metals and post-transition metals usually have the metal in the +2 (M = Mn, Fe, Co, Ni, Cu, Zn) or +3 (M = Fe, Ru, Rh, Ir) oxidation state. None are soluble in water, and many are poorly defined. One complicating feature of the hydroxides is their tendency to undergo further condensation to the oxides, a process called olation. Hydroxides of metals in the +1 oxidation state are also poorly defined or unstable. For example, silver hydroxide Ag(OH) decomposes spontaneously to the oxide (Ag2O). Copper(I) and gold(I) hydroxides are also unstable, although stable adducts of CuOH and AuOH are known. The polymeric compounds M(OH)2 and M(OH)3 are in general prepared by increasing the pH of an aqueous solutions of the corresponding metal cations until the hydroxide precipitates out of solution. On the converse, the hydroxides dissolve in acidic solution. Zinc hydroxide Zn(OH)2 is amphoteric, forming the tetrahydroxidozincate ion in strongly alkaline solution. Numerous mixed ligand complexes of these metals with the hydroxide ion exist. In fact these are in general better defined than the simpler derivatives. Many can be made by deprotonation of the corresponding metal aquo complex. :L''n''M(OH2) + B L''n''M(OH) + BH+ (L = ligand, B = base) Vanadic acid H3VO4 acid dissociation constant#Polyprotic acids, shows similarities with phosphoric acid H3PO4 though it has a much more complex vanadate oxoanion chemistry. Chromic acid H2CrO4, has similarities with sulfuric acid H2SO4; for example, both form acid salts A+[HMO4]. Some metals, e.g. V, Cr, Nb, Ta, Mo, W, tend to exist in high oxidation states. Rather than forming hydroxides in aqueous solution, they convert to oxo clusters by the process of olation, forming polyoxometalates.


Basic salts containing hydroxide

In some cases the products of partial hydrolysis of metal ion, described above, can be found in crystalline compounds. A striking example is found with zirconium(IV). Because of the high oxidation state, salts of Zr4+ are extensively hydrolyzed in water even at low pH. The compound originally formulated as ZrOCl2·8H2O was found to be the chloride salt of a tetrameric cation [Zr4(OH)8(H2O)16]8+ in which there is a square of Zr4+ ions with two hydroxide groups bridging between Zr atoms on each side of the square and with four water molecules attached to each Zr atom.Wells, p. 561 The mineral malachite is a typical example of a basic carbonate. The formula, Cu2CO3(OH)2 shows that it is halfway between basic copper carbonate, copper carbonate and copper hydroxide. Indeed, in the past the formula was written as CuCO3·Cu(OH)2. The crystal structure is made up of copper, carbonate and hydroxide ions. The mineral atacamite is an example of a basic chloride. It has the formula, Cu2Cl(OH)3. In this case the composition is nearer to that of the hydroxide than that of the chloride CuCl2·3Cu(OH)2. Copper forms hydroxyphosphate (libethenite), arsenate (olivenite), sulfate (brochantite), and nitrate compounds. White lead is a basic lead carbonate, (PbCO3)2·Pb(OH)2, which has been used as a white pigment because of its opaque quality, though its use is now restricted because it can be a source for lead poisoning.


Structural chemistry

The hydroxide ion appears to rotate freely in crystals of the heavier alkali metal hydroxides at higher temperatures so as to present itself as a spherical ion, with an effective ionic radius of about 153 pm. Thus, the high-temperature forms of KOH and NaOH have the sodium chloride#Crystal structure, sodium chloride structure, which gradually freezes in a monoclinically distorted sodium chloride structure at temperatures below about 300 °C. The OH groups still rotate even at room temperature around their symmetry axes and, therefore, cannot be detected by X-ray diffraction. The room-temperature form of NaOH has the thallium(I) iodide, thallium iodide structure. LiOH, however, has a layered structure, made up of tetrahedral Li(OH)4 and (OH)Li4 units.Wells, p. 548 This is consistent with the weakly basic character of LiOH in solution, indicating that the Li–OH bond has much covalent character. The hydroxide ion displays cylindrical symmetry in hydroxides of divalent metals Ca, Cd, Mn, Fe, and Co. For example, magnesium hydroxide Mg(OH)2 (brucite) crystallizes with the cadmium iodide layer structure, with a kind of close-packing of magnesium and hydroxide ions. The amphoterism, amphoteric hydroxide Al(OH)3 has four major crystalline forms: gibbsite (most stable), bayerite, nordstrandite, and doyleite.Crystal structures are illustrated at Web mineral
GibbsiteBayeriteNorstrandite
an
Doyleite
/ref> All these Polymorphism (materials science), polymorphs are built up of double layers of hydroxide ions – the aluminium atoms on two-thirds of the octahedral holes between the two layers – and differ only in the stacking sequence of the layers. The structures are similar to the brucite structure. However, whereas the brucite structure can be described as a close-packed structure in gibbsite the OH groups on the underside of one layer rest on the groups of the layer below. This arrangement led to the suggestion that there are directional bonds between OH groups in adjacent layers. This is an unusual form of
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
ing since the two hydroxide ion involved would be expected to point away from each other. The hydrogen atoms have been located by neutron diffraction experiments on α-AlO(OH) (diaspore). The O–H–O distance is very short, at 265 pm; the hydrogen is not equidistant between the oxygen atoms and the short OH bond makes an angle of 12° with the O–O line. A similar type of hydrogen bond has been proposed for other amphoteric hydroxides, including Be(OH)2, Zn(OH)2, and Fe(OH)3. A number of mixed hydroxides are known with stoichiometry A3MIII(OH)6, A2MIV(OH)6, and AMV(OH)6. As the formula suggests these substances contain M(OH)6 octahedral structural units. Layered double hydroxides may be represented by the formula . Most commonly, ''z'' = 2, and M2+ = Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, or Zn2+; hence ''q'' = ''x''.


In organic reactions

Potassium hydroxide and sodium hydroxide are two well-known reagents in
organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and th ...
.


Base catalysis

The hydroxide ion may act as a base catalyst. The base abstracts a proton from a weak acid to give an intermediate that goes on to react with another reagent. Common substrates for proton abstraction are alcohols, phenols, amines, and carbon acids. The acid dissociation constant, p''K''a value for dissociation of a C–H bond is extremely high, but the pKa alpha hydrogens of a carbonyl compound are about 3 log units lower. Typical p''K''a values are 16.7 for acetaldehyde and 19 for acetone. Dissociation can occur in the presence of a suitable base. :RC(O)CH2R' + B RC(O)CHR' + BH+ The base should have a p''K''a value not less than about 4 log units smaller, or the equilibrium will lie almost completely to the left. The hydroxide ion by itself is not a strong enough base, but it can be converted in one by adding sodium hydroxide to ethanol :OH + EtOH EtO + H2O to produce the alkoxide, ethoxide ion. The acid dissociation constant, pKa for self-dissociation of ethanol is about 16, so the alkoxide ion is a strong enough base. The addition of an alcohol to an aldehyde to form a hemiacetal is an example of a reaction that can be catalyzed by the presence of hydroxide. Hydroxide can also act as a Lewis-base catalyst.


As a nucleophilic reagent

The hydroxide ion is intermediate in nucleophilicity between the fluoride ion F, and the amide ion .pdf
The hydrolysis of an ester :R1C(O)OR2 + H2O R1C(O)OH + HOR2 also known as saponification is an example of a nucleophilic acyl substitution with the hydroxide ion acting as a nucleophile. In this case the leaving group is an alkoxide ion, which immediately removes a proton from a water molecule to form an alcohol. In the manufacture of soap, sodium chloride is added to salting out, salt out the sodium salt of the carboxylic acid; this is an example of the application of the common ion effect. Other cases where hydroxide can act as a nucleophilic reagent are amide hydrolysis, the Cannizzaro reaction, nucleophilic aliphatic substitution, nucleophilic aromatic substitution, and in elimination reactions. The reaction medium for KOH and NaOH is usually water but with a phase-transfer catalyst the hydroxide anion can be shuttled into an organic solvent as well, for example in the generation of the reactive intermediate dichlorocarbene.


Notes


References


Bibliography

* * * * * {{Authority control Hydroxides, Oxyanions Water chemistry