{{Short description|Boron-oxygen anion or functional group}} {{distinguish|Borat}} A '''borate''' is any of several [[boron]] [[oxyanion]]s, [[anion|negative ion]]s consisting of boron and [[oxygen]], such as [[orthoborate]] {{chem2|[BO3](3-)}}, [[metaborate]] {{chem2|[BO2]-}}, or [[tetraborate]] {{chem2|[B4O7](2-)}}; or any [[salt (chemistry)|salt]] with such anions, such as [[sodium metaborate]], {{chem2|[Na+][BO2-]}} and [[disodium tetraborate]] {{chem2|[Na+]2[B2O4(2-)]}}. The name also refers to certain [[functional group]]s in [[molecule]]s consisting of boron and oxygen, and [[ester]]s with such groups, such as [[triethyl orthoborate]] {{chem2|B(OC2H5)3}}. ==Natural occurrence== Borate ions occur, alone or with other anions, in many [[borate mineral|borate]] and [[borosilicate]] [[mineral]]s such as [[borax]], [[boracite]], [[ulexite]] (boronatrocalcite) and [[colemanite]]. Borates also occur in seawater, where they make an important contribution to the absorption of low frequency sound in seawater. Borates also occur in plants, including almost all fruits. ==Anions== [[File:Tetrahydroxyborate-2D-dimensions.png|thumb|150px|The structure of the tetrahydroxyborate anion.]] The main borate anions are: * [[tetrahydroxyborate]] {{chem2|[B(OH)4]-}}, found in [[sodium tetrahydroxyborate]] {{chem2|NaB(OH)4}}. * [[orthoborate]] {{chem2|[BO3](3-)}}, found in [[sodium orthoborate]] {{chem2|Na3BO3}} * [[perborate]] {{chem2|[B2O4(OH)4](2-)}}, as in [[sodium perborate]] {{chem2|Na2H4B2O8}} * [[metaborate]] {{chem2|[BO2]-}} or {{chem2|[B3O6](3-)}}, found in [[sodium metaborate]] {{chem2|Na3B3O6}} * [[diborate]] {{chem2|[B2O5](4-)}}, found in [[magnesium diborate]] {{chem2|Mg2B2O5}} ([[suanite]]), * [[triborate]] {{chem2|[B3O7](5-)}}, found in [[calcium aluminum triborate]] {{chem2|CaAlB3O7}} ([[johachidolite]]), * [[tetraborate]] {{chem2|[B4O7](2-)}}, found in anhydrous [[borax]] {{chem2|Na2B4O7}} * [[tetraborate|tetrahydroxy tetraborate]] {{chem2|[B4O5(OH)4](2-)}}, found in borax "decahydrate" {{chem2|Na2B4O5(OH)4*8H2O}} * [[tetraborate(6-)]] {{chem2|[B4O9](6-)}} found in [[lithium tetraborate(6-)]] {{chem2|Li6B4O9}} * [[octaborate]] {{chem2|[B8O13](2-)}} found in [[disodium octaborate]] {{chem2|Na2B8O13}} ==Preparation== In 1905, Burgess and Holt observed that fusing mixtures of [[boric oxide]] {{chem2|B2O3}} and [[sodium carbonate]] {{chem2|Na2CO3}} yielded on cooling two crystalline compounds with definite compositions, consistent with anhydrous borax {{Chem2|Na2B4O7}} = {{chem2|Na2O*2B2O3}} and sodium octaborate {{chem2|Na2B8O13}} = {{chem2|Na2O*4B2O3}}. ==Structures== Borate anions or functional groups consist of [[trigonal planar molecular geometry|trigonal planar]] BO3 or [[tetrahedral molecular geometry|tetrahedral]] BO4 structural units, joined together via shared oxygen atoms and may be cyclic or linear in structure. Polymeric borate anions may have linear chains of 2, 3 or 4 trigonal BO3 structural units, each sharing oxygen atoms with adjacent unit(s). as in [[lithium metaborate|LiBO2]], contain chains of trigonal BO3 structural units. Other anons contain contay cycles, as in [[Sodium metaborate|NaBO2]] and KBO2 contain the cyclic [B3O6]2− ion. ==Reactions== ===Aquous solution=== In aqueous solution, [[boric acid]] {{chem2|B(OH)3}} can act as a weak [[Brønsted acid]], that is, a [[proton]] donor, with [[acid dissociation constant|p''K''a ~ 9]]. However, it more often acts as a [[Lewis acid]], accepting an [[electron pair]] from a [[hydroxyl ion]] produced by the water [[autoprotolysis]]: : {{chem2|B(OH)3}} + 2 {{H2O}} {{Eqm}} {{chem2|[B(OH)4]-}} + {{H3O+}} (p''K''a = 8.98) Plymeric boron oxoanions are formed in aqueous solution of boric acid at [[pH]] 7–10 if the boron concentration is higher than about 0.025 mol/L. The best known of these is the [[tetraborate]] ion {{chem2|[B4O7](2-)}}, found in the mineral borax: :4 {{chem2|[B(OH)4]-}} + 2 {{H+}} {{eqm}} {{chem2|[B4O5(OH)4](2-)}} + 7 {{H2O}} Other such anions are triborate(1−) and pentaborate(1−). These ions, similarly to the complexed borates mentioned above, are more acidic than boric acid itself. As a result of this, the pH of a concentrated polyborate solution will increase more than expected when diluted with water. The condensation reaction for the formation of tetraborate(2−) is as follows: ===Borate salts=== A number of metal borates are known can be obtained by treating boric acid or boron oxides with metal oxides. ==Borosilicates== [[Borosilicate|Borosilicate glass]], also known as [[pyrex]], can be viewed as a [[silicate]] in which some [SiO4]4− units are replaced by [BO4]5− centers, together with additional cations to compensate for the difference in valence states of Si(IV) and B(III). Because this substitution leads to imperfections, the material is slow to crystallise and forms a glass with low [[coefficient of thermal expansion]], thus resistant to cracking when heated, unlike [[soda glass]]. ==Uses== [[File:Borax crystals.jpg|thumb|left|150px|Borax crystals]] Common borate salts include [[sodium metaborate]] (NaBO2) and borax. Borax is soluble in water, so mineral deposits only occur in places with very low rainfall. Extensive deposits were found in [[Death Valley]] and shipped with [[twenty-mule team]]s from 1883 to 1889. In 1925, deposits were found at [[Boron, California|Boron]], [[California]] on the edge of the [[Mojave Desert]]. The [[Atacama Desert]] in [[Chile]] also contains mineable borate concentrations. [[Lithium metaborate]], lithium tetraborate, or a mixture of both, can be used in borate fusion sample preparation of various samples for analysis by [[X-ray fluorescence|XRF]], [[atomic absorption spectroscopy|AAS]], [[ICP-OES]] and [[ICP-MS]]. Borate fusion and energy dispersive X-ray fluorescence spectrometry with polarized excitation have been used in the analysis of contaminated soils. [[Disodium octaborate tetrahydrate]] (commonly abbreviated DOT) is used as a [[Wood preservation#Borate preservatives|wood preservative]] or fungicide. [[Zinc borate]] is used as a [[flame retardant]]. == Borate esters== [[Borate ester]]s are [[organic compound]]s, which are conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols. ==Mixed anion compounds== Some chemicals contain another anion in addition to borate. These include [[borate chloride]]s, [[borate carbonate]]s, [[borate nitrate]]s, [[borate sulfate]]s, [[borate phosphate]]s. More complex anions can be formed by condensing borate triangles or tetrahedra with other [[oxyanion]]s to yield materials such as [[borosulfates]], [[boroselenates]], [[borotellurates]], [[boroantimonates]], [[borophosphates]], or [[boroselenites]]. == Thin films == Metal borate thin films have been grown by a variety of techniques, including liquid-phase [[epitaxy]] (e.g. FeBO3, β‐BaB2O4), [[Electron-beam physical vapor deposition|electron-beam evaporation]] (e.g. CrBO3, β‐BaB2O4), [[pulsed laser deposition]] (e.g. β‐BaB2O4,  Eu(BO2)3), and [[atomic layer deposition]] (ALD). Growth by ALD was achieved using [[Precursor (chemistry)|precursors]] composed of the [[Trispyrazolylborate|tris(pyrazolyl)borate]] [[ligand]] and either ozone or water as the [[Oxidizing agent|oxidant]] to deposit CaB2O4, SrB2O4, BaB2O4, Mn3(BO3)2, and CoB2O4 films. ==See also== {{Portal|Earth sciences}} * [[Nanochannel glass materials]] * [[Porous glass]] * [[Vycor|Vycor glass]] * [[Silly Putty]] * [[Slime (toy)]] * [[Tris(2,2,2-trifluoroethyl) borate]] ==References== {{cite web |title= Underlying Physics and Mechanisms for the Absorption of Sound in Seawater |url= http://resource.npl.co.uk/acoustics/techguides/seaabsorption/physics.html |publisher= National Physical Laboratory |access-date= 2008-04-21 }} {{cite journal |last1= Allen |first1=A. H. |last2=Tankard |first2=A. 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