Editing Borate

{{Short description|Boron-oxygen anion or functional group}}{{Not to be confused with|Borat}}

A '''borate''' is any of a range of [[boron]] [[oxyanion]]s, [[anion]]s containing [[boron]] and [[oxygen]], such as [[orthoborate]] {{chem2|BO3(3−)}}, [[metaborate]] {{chem2|BO2−}}, or [[tetraborate]] {{chem2|B4O7(2−)}}; or any [[salt (chemistry)|salt]] of such anions, such as [[sodium metaborate]], {{chem2|Na+[BO2]−}} and [[borax]] {{chem2|(Na+)2[B4O7](2−)}}. The name also refers to [[ester]]s of such anions, such as [[trimethyl borate]] {{chem2|B(OCH3)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, contributing to the absorption of low-frequency sound in seawater.<ref name=NPL/>

Common borate salts include [[sodium metaborate]] (NaBO<sub>2</sub>) 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.

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 [[Suanite|magnesium diborate (suanite)]] {{chem2|Mg2[B2O5]}} ,
* [[triborate]] {{chem2|[B3O7](5-)}}, found in [[calcium aluminium triborate|calcium aluminium triborate (johachidolite)]] {{chem2|Ca[AlB3O7]}} ,
* [[tetraborate]] {{chem2|[B4O7](2-)}}, found in anhydrous [[borax]] {{chem2|Na2[B4O7]}}
* [[tetraborate|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 compound|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 [[Bicyclic molecule|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 [[Coordination geometry#Table of coordination geometries|tetracoordinated]] boron atoms. It has a trigonal planar molecular geometry at the two [[Coordination geometry#Table of coordination geometries|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 compound|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 molecular geometry|trigonal planar]] {{chem2|BO3}} and/or [[tetrahedral molecular geometry|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 [[pi bond|π-conjugated]] [[molecular orbital]]s, which often results in useful optical properties such as strong [[harmonic]]s generation, [[birefringence]], and [[ultraviolet light|UV transmission]].<ref name=muta2016/>

Polymeric borate anions 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/> as in [[lithium metaborate|{{chem2|LiBO2}}]], contain chains of trigonal {{chem2|BO3}} structural units. Other anions contain cycles; for instance, [[Sodium metaborate|{{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 [[acid dissociation constant|p''K''<sub>a</sub> ~&nbsp;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 [[microsecond|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&nbsp;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 chloride]]s, [[borate carbonate]]s, [[borate nitrate]]s, [[borate sulfate]]s, [[borate phosphate]]s.

=== Complex oxyanions containing boron ===
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]].

[[Borosilicate|Borosilicate glass]], also known as [[pyrex]], can be viewed as a [[silicate]] in which some [SiO<sub>4</sub>]<sup>4−</sup> units are replaced by [BO<sub>4</sub>]<sup>5−</sup> 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 ==
[[File:Borax crystals.jpg|thumb|left|150px|Borax crystals]]

[[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 to analyse contaminated soils.<ref name=hett2004/>

[[Disodium octaborate]] tetrahydrate {{chem2|Na2B8O13*4H2O}} (commonly abbreviated DOT) is used as a [[Wood preservation#Borate preservatives|wood preservative]] or fungicide. [[Zinc borate]] is used as a [[flame retardant]].

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

== Borate esters==
[[Borate ester]]s are [[organic compound]]s, which are conveniently prepared by the [[Stoichiometry|stoichiometric]] condensation reaction of boric acid with [[Alcohol (chemistry)|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. FeBO<sub>3</sub>,<ref name=yagu2016/> β-BaB<sub>2</sub>O<sub>4</sub><ref name=liuj2006/>), [[Electron-beam physical vapor deposition|electron-beam evaporation]] (e.g. CrBO<sub>3</sub>,<ref name=jham2011/> β-BaB<sub>2</sub>O<sub>4</sub><ref name=maia2004/>), [[pulsed laser deposition]] (e.g. β-BaB<sub>2</sub>O<sub>4</sub>,<ref name=xiao1995/> &nbsp;Eu(BO<sub>2</sub>)<sub>3</sub><ref name=alek2006/>), 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 CaB<sub>2</sub>O<sub>4</sub>,<ref name=saly2010/> SrB<sub>2</sub>O<sub>4</sub>,<ref name=saly2011/> BaB<sub>2</sub>O<sub>4</sub>,<ref name=saly2009/> Mn<sub>3</sub>(BO<sub>3</sub>)<sub>2</sub>,<ref name=kles2016/> and CoB<sub>2</sub>O<sub>4</sub><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|Vycor glass]]
* [[Silly Putty]]
* [[Slime (toy)]]
* [[Tris(2,2,2-trifluoroethyl) borate]]

== References ==
<references>

<ref name=momi1967>Robert K. Momii and Norman H. Nachtrieb (1967): "Nuclear Magnetic Resonance Study of Borate-Polyborate Equilibria in Aqueous Solution". ''Inorganic Chemistry'', volume 6, issue 6, pages 1189-1192. {{doi|10.1021/ic50052a026}}</ref>

<ref name=muta2016>Miriding Mutailipu, Min Zhang, Xiaoyu Dong, Yanna Chen, and Shilie Pan (2016): "Effects of the Orientation of [B<sub>5</sub>O<sub>11</sub>]<sup>7–</sup> 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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<ref name=bubn2008>Rimma S. Bubnova and Stanislav K. Filatov (2008): "Strong anisotropic thermal expansion in borates". ''Basic Solid State Physics'', volume 245, issue 11, pages 2469-2476. {{doi|10.1002/pssb.200880253}}</ref>

<ref name="carrondo1978">{{cite journal |author1=Carrondo, M. A. A. F. de C. T. |author2=Skapski, A. C. |journal= [[Acta Crystallogr B]]|volume=34 |pages= 3551 |title= Refinement of the X-ray crystal structure of the industrial bleaching agent disodium tetrahydroxo-di-μ-peroxo-diborate hexahydrate, Na<sub>2</sub>[B<sub>2</sub>(O<sub>2</sub>)<sub>2</sub>(OH)<sub>4</sub>]·6H<sub>2</sub>O |year= 1978 |doi= 10.1107/S0567740878011565 }}</ref>

<ref name="bubnova2002">{{cite journal | url=https://www.degruyter.com/document/doi/10.1524/zkri.217.9.444.22881/html | doi=10.1524/zkri.217.9.444.22881 | title=Thermal behaviour of the rigid boron-oxygen groups in the α-Na<sub>2</sub>B<sub>8</sub>O<sub>13</sub> crystal structure | year=2002 | last1=Bubnova | first1=R. S. | last2=Shepelev | first2=Ju. F. | last3=Sennova | first3=N. A. | last4=Filatov | first4=S. K. | journal=Zeitschrift für Kristallographie – Crystalline Materials | volume=217 | issue=9 | pages=444–450 | bibcode=2002ZK....217..444B | s2cid=95388918 }}</ref>

</references>

==External links==
{{Commons category|Borates}}
* [http://webmineral.com/data/Suanite.shtml Suanite at webmineral]
* [http://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 }}

{{Borates}}

[[Category:Borate minerals]]
[[Category:Borates| ]]
[[Category:Industrial minerals]]
[[Category:Inorganic compounds]]
[[Category:Neutron poisons]]
[[Category:Boron oxyanions]]
[[Category:Pesticides]]
[[Category:Preservatives]]
[[Category:Rheology]]
[[Category:Testicular toxicants]]