Borate

[[File:Borate-3D-balls.png|thumb|The orthoborate anion, a simple borate anion]] A '''borate''' is any of a range of boron oxyanions, anions containing boron and oxygen, such as orthoborate {{chem2|BO3(3−)}}, metaborate {{chem2|BO2−}}, or tetraborate {{chem2|B4O7(2−)}}; or any salt of such anions, such as sodium metaborate, {{chem2|Na+[BO2]−}} and borax {{chem2|(Na+)2[B4O7](2−)}}. The name also refers to esters of such anions, such as trimethyl borate {{chem2|B(OCH3)3}}.

== Natural occurrence == Borate ions occur, alone or with other anions, in many borate and borosilicate minerals such as borax, boracite, ulexite (boronatrocalcite) and colemanite. Borates also occur in seawater, contributing to the absorption of low-frequency sound in seawater.<ref name=NPL/>

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 teams from 1883 to 1889. In 1925, deposits were found at Boron, California on the edge of the Mojave Desert. The Atacama Desert in Chile also contains mineable borate concentrations.

Borates also occur in plants, including almost all fruits.<ref name=allen1904/>

== Anions == The main borate anions are: * tetrahydroxyborate {{chem2|[B(OH)4]-}}, found in sodium tetrahydroxyborate {{chem2|Na[B(OH)4]}}. * orthoborate {{chem2|[BO3](3-)}}, found in trisodium orthoborate {{chem2|Na3[BO3]}} * {{chem|[BO|4|]|5-}}, found in the calcium yttrium borosilicate oxyapatite {{chem|Ca|3|Y|7|BO|4|(|SiO|4|)|5|O}} * perborate {{chem2|[B2O4(OH)4](2-)}}, as in sodium perborate {{chem2|Na2[H4B2O8]}} * metaborate {{chem2|[BO2]-}} or its cyclic trimer {{chem2|[B3O6](3-)}}, found in sodium metaborate {{chem2|Na3[B3O6]}} * diborate {{chem2|[B2O5](4-)}}, found in magnesium diborate (suanite) {{chem2|Mg2[B2O5]}} , * triborate {{chem2|[B3O7](5-)}}, found in calcium aluminium triborate (johachidolite) {{chem2|Ca[AlB3O7]}} , * tetraborate {{chem2|[B4O7](2-)}}, found in anhydrous borax {{chem2|Na2[B4O7]}} * tetrahydroxytetraborate {{chem2|[B4O5(OH)4](2-)}}, found in borax "decahydrate" {{chem2|Na2[B4O5(OH)4]*8H2O}} * tetraborate(6-) {{chem2|[B4O9](6-)}}, found in lithium tetraborate(6-) {{chem2|Li6[B4O9]}} * pentaborate {{chem2|[B5O8]-}} or {{chem2|[B10O16](2-)}}, found in sodium pentaborate {{chem2|Na2[B10O16]*10H2O}} * octaborate {{chem2|[B8O13](2-)}}, found in disodium octaborate {{chem2|Na2[B8O13]}}

{{Gallery | title = Borate ions | height = 175 | align = center | File:Tetrahydroxyborate-2D-dimensions.png | The structure of the tetrahydroxyborate ion ({{chem2|[B(OH)4]−}}). This anion has a tetrahedral molecular geometry at the boron atom. | alt1 = The structure of the tetrahydroxyborate ion | File:Orthoborate ion.png | The structure of the orthoborate ion ({{chem2|[BO3](3−)}}). This anion has a trigonal planar molecular geometry. | alt2 = The structure of the orthoborate ion | File:Metaborate ion trimer.png | The structure of the trimer of the metaborate ion ({{chem2|[B3O6](3−)}}). This anion is a cyclic molecule and has a trigonal planar molecular geometry at the boron atoms. All nine atoms of this anion lie on the same plane. | alt3 = The structure of the trimer of the metaborate ion | File:Tetrahydroxytetraborate ion.png | The structure of the tetrahydroxytetraborate ion ({{chem2|[B4O5(OH)4](2-)}}). This anion is a bridged bicyclic molecule, contains oxygen atoms bridging the boron atoms, which are linked to four hydroxyl groups ({{chem2|\sOH}}), one per each boron atom. The anion has a tetrahedral molecular geometry at the two tetracoordinated boron atoms. It has a trigonal planar molecular geometry at the two tricoordinated boron atoms. | alt4 = The structure of the tetrahydroxytetraborate ion | File:Tetraborate ion.png | The structure of the tetraborate ion ({{chem2|[B4O7](2-)}}). This anion has the same topology as the tetrahydroxytetraborate ion, but without the hydroxyl groups and all boron atoms have a trigonal planar molecular geometry.<ref name=pubchem>{{cite web | url=https://pubchem.ncbi.nlm.nih.gov/compound/Tetraborate | title=Tetraborate }}</ref> | alt5 = The structure of the tetrahydroxyborate ion | File:Perborate ion.png | The structure of the perborate ion ({{chem2|[B2O4(OH)4](2-)}}). This anion is a cyclic molecule with a tetrahedral molecular geometry at the boron atoms. It contains two bridging peroxide groups ({{chem2|\sO\sO\s}}) and four hydroxyl groups ({{chem2|\sOH}}) attached to boron atoms, two per each boron. The ring has a chair conformation.<ref name="carrondo1978"/> | alt6 = The structure of the perborate ion | File:Octaborate ion.png | The structure of the repeating unit of the octaborate ion ({{chem2|[B8O13](2-)}}) in the alpha form of disodium octaborate (α-{{chem2|Na2[B8O13]}}).<ref name="bubnova2002"/> This anion is cyclic and polymeric. It has a tetrahedral molecular geometry at the negatively charged boron atoms and a trigonal planar molecular geometry at the neutral boron atoms. | alt7 = The structure of the repeating unit of the octaborate ion in the alpha form of disodium octaborate }}

== 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}} (which can be written {{chem2|Na2O*2B2O3}}) and sodium octaborate {{chem2|Na2B8O13}} (which can be written {{chem2|Na2O*4B2O3}}).<ref name=burg1905/>

== Structures == Borate anions (and functional groups) consist of trigonal planar {{chem2|BO3}} and/or tetrahedral {{chem2|BO4}} structural units, joined together via shared oxygen atoms (corners) or atom pairs (edges) into larger clusters so as to construct various ions such as {{chem2|[B2O5](4-)}}, {{chem2|[B3O8](7-)}}, {{chem2|[B4O12](12-)}}, {{chem2|[B5O6(OH)5](2-)}}, {{chem2|[B6O13](8-)}}, etc. These anions may be cyclic or linear in structure, and can further polymerize into infinite chains, layers, and tridimensional frameworks.<ref name=wibe2001/><ref name=muta2016/> The terminal (unshared) oxygen atoms in the borate anions may be capped with hydrogen atoms ({{chem2|\sOH}}) or may carry a negative charge ({{chem2|\sO−}}).

The planar {{chem2|BO3}} units may be stacked in the crystal lattice to have π-conjugated molecular orbitals, which often results in useful optical properties such as strong harmonics generation, birefringence, and UV transmission.<ref name=muta2016/>

Polymeric borate anions, such as {{chem2|LiBO2}}, may have linear chains of 2, 3 or 4 trigonal {{chem2|BO3}} structural units, each sharing oxygen atoms with adjacent unit(s).<ref name=wibe2001/> Other anions contain cycles; for instance, {{chem2|NaBO2}} and {{chem2|KBO2}} contain the cyclic {{chem2|[B3O6](3−)}} ion,<ref name=GandE.P205/> consisting of a six-membered ring of alternating boron and oxygen atoms with one extra oxygen atom attached to each boron atom.

The thermal expansion of crystalline borates is dominated by the fact that {{chem2|BO3}} and {{chem2|BO4}} polyhedra and rigid groups consisting of these polyhedra practically do not change their configuration and size upon heating, but sometimes rotate like hinges, which results in greatly anisotropic thermal expansion including linear negative expansion. <ref name=bubn2008/>

== Reactions == === Aqueous solution === In aqueous solution, boric acid {{chem2|B(OH)3}} can act as a weak Brønsted acid, that is, a proton donor, with p''K''a ~ 9. However, it more often acts as a Lewis acid, accepting an electron pair from a hydroxide ion produced by the water autoprotolysis:<ref name=atki2010/>

: {{chem2|B(OH)3}} + 2 {{H2O}} {{Eqm}} {{chem2|[B(OH)4]-}} + {{H3O+}} {{Spaces|21}} (p''K'' = 8.98)<ref name=ingri1962/>

This reaction is very fast, with a characteristic time less than 10 μs.<ref name=momi1967/> Polymeric 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 anions observed in solution are triborate(1−) and pentaborate(1−), in equilibrium with boric acid and tetrahydroxyborate according to the following overall reactions:<ref name=momi1967/>

: 2 {{chem2|B(OH)3}} + {{chem2|[B(OH)4]-}} {{Eqm}} {{chem2|[B3O3(OH)4](-)}} + 3 {{chem2|H2O}} {{Spaces|5}} (fast, p''K'' = −1.92)

: 4 {{chem2|B(OH)3}} + {{chem2|[B(OH)4]-}} {{Eqm}} {{chem2|[B5O6(OH)4](-)}} + 6 {{chem2|H2O}} {{Spaces|5}} (slow, p''K'' = −2.05)

In the pH range 6.8 to 8.0, any alkali salts of "boric oxide" anions with general formula {{chem2|[B_{''x''}O_{''y''}(OH)_{''z''}]((''q''-)}} where 3''x'' + ''q'' = 2''y'' + ''z'' will eventually equilibrate in solution to a mixture of {{chem2|B(OH)3}}, {{chem2|[B(OH)4](-)}}, {{chem2|[B3O3(OH)4](-)}}, and {{chem2|[B5O6(OH)4](-)}}.<ref name=momi1967/>

Like the complexed borates mentioned above, these ions are more acidic than boric acid. As a result, the pH of a concentrated polyborate solution will increase more than expected when diluted with water.

=== Borate salts === Several metal borates are known. They can be obtained by treating boric acid or boron oxides with metal oxides.{{cn|date=September 2023}}

=== Mixed anion salts === Some chemicals contain another anion in addition to borate. These include borate chlorides, borate carbonates, borate nitrates, borate sulfates, borate phosphates.

=== Complex oxyanions containing boron === More complex anions can be formed by condensing borate triangles or tetrahedra with other oxyanions to yield materials such as borosulfates, boroselenates, borotellurates, boroantimonates, borophosphates, or boroselenites.

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. It forms a glass with a low coefficient of thermal expansion, thus resistant to cracking when heated, unlike soda glass.

== Uses == thumb|left|150px|Borax crystals

Lithium metaborate, lithium tetraborate, or a mixture of the two, can be used to prepare borate fusion samples for analysis by XRF, AAS, ICP-OES and ICP-MS. Borate fusion and energy dispersive X-ray fluorescence spectrometry with polarized excitation have been used to analyse contaminated soils.<ref name=hett2004/>

Disodium octaborate tetrahydrate {{chem2|Na2B8O13*4H2O}} (commonly abbreviated DOT) is used as a wood preservative and a fungicide. Zinc borate is used as a flame retardant.

Some borates with large anions and multiple cations, such as {{chem2|K2Al2B2O7}} and {{chem2|Cs3Zn6B9O21}}, have been considered for applications in nonlinear optics.<ref name=muta2016/>

== Borate esters== Borate esters are organic compounds which can be conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols (or their chalcogen analogs).<ref name=goldbook.iupac>{{cite book | chapter-url=https://goldbook.iupac.org/terms/view/E02219 | doi=10.1351/goldbook.E02219 | chapter=Esters | title=The IUPAC Compendium of Chemical Terminology | year=2014 }}</ref>

== Thin films == Metal borate thin films have been grown by a variety of techniques, including liquid-phase epitaxy (e.g. FeBO3,<ref name=yagu2016/> β-BaB2O4<ref name=liuj2006/>), electron-beam evaporation (e.g. CrBO3,<ref name=jham2011/> β-BaB2O4<ref name=maia2004/>), pulsed laser deposition (e.g. β-BaB2O4,<ref name=xiao1995/>  Eu(BO2)3<ref name=alek2006/>), and atomic layer deposition (ALD). Growth by ALD was achieved using precursors composed of the tris(pyrazolyl)borate ligand and either ozone or water as the oxidant to deposit CaB2O4,<ref name=saly2010/> SrB2O4,<ref name=saly2011/> BaB2O4,<ref name=saly2009/> Mn3(BO3)2,<ref name=kles2016/> and CoB2O4<ref name=kles2016 /> films.

== Physiology == Borate anions are found largely as the undissociated acid in aqueous solution at physiological pH. No further metabolism occurs in either animals or plants. In animals, boric acid/borate salts are completely absorbed following oral ingestion. Absorption occurs via inhalation, although quantitative data are unavailable. Limited data indicate that boric acid/salts are not absorbed through intact skin to any significant extent, although absorption occurs through severely abraded skin. It is distributed throughout the body, is not retained in tissues except for bone, and is rapidly excreted in the urine.<ref name=EPA2005/>

== See also == {{Portal|Earth sciences}} * Nanochannel glass materials * Porous glass * Vycor glass * Silly Putty * Slime (toy) * Tris(2,2,2-trifluoroethyl) borate * Triethanolamine borate

== References == <references>

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<ref name=muta2016>Miriding Mutailipu, Min Zhang, Xiaoyu Dong, Yanna Chen, and Shilie Pan (2016): "Effects of the Orientation of [B5O11]7– Fundamental Building Blocks on Layered Structures Based on the Pentaborates". ''Inorganic Chemistry'', volume 55, issue 20, pages 10608–10616. {{doi|10.1021/acs.inorgch}}</ref>

<ref name=EPA2005>U.S. Environmental Protection Agency (2005), "Boric Acid/Sodium Borate Salts". HED Chapter of the Tolerance Reassessment Eligibility Decision Document (TRED), EPA-HQ-OPP-2005-0062-0004, p.11 (January 2006). As cited by PubChem.</ref>

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<ref name=burg1905>Charles Hutchens Burgess and Alfred Holt (1905): "Some physical characters of the sodium borates, with a new and rapid method for the determination of melting points." ''Proceedings of the Royal Society of London'', volume 74, pages 285–295. {{doi|10.1098/rspl.1904.0112}}</ref>

<ref name=wibe2001>Wiberg E. and Holleman A.F. (2001) ''Inorganic Chemistry'', Elsevier {{ISBN|0-12-352651-5}}</ref>

<ref name=GandE.P205>{{Greenwood&Earnshaw2nd|page=205}}</ref>

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</references>

==External links== {{Commons category|Borates}} * [https://webmineral.com/data/Suanite.shtml Suanite at webmineral] * [https://webmineral.com/data/Johachidolite.shtml Johachidolite at webmineral] * [http://npic.orst.edu/hottopic/AltCCA.pdf Non-CCA Wood Preservatives: Guide to Selected Resources - National Pesticide Information Center] {{Webarchive|url=https://web.archive.org/web/20071031032706/http://npic.orst.edu/hottopic/AltCCA.pdf |date=2007-10-31 }}

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