Borate

A borate is any of a range of boron oxyanions, anions containing boron and oxygen, such as orthoborate Template:Chem2, metaborate Template:Chem2, or tetraborate Template:Chem2; or any salt of such anions, such as sodium metaborate, Template:Chem2 and borax Template:Chem2. The name also refers to esters of such anions, such as trimethyl borate Template:Chem2.

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 Template:Chem2, found in sodium tetrahydroxyborate Template:Chem2.
• orthoborate Template:Chem2, found in trisodium orthoborate Template:Chem2
• Template:Chem, found in the calcium yttrium borosilicate oxyapatite Template:Chem
• perborate Template:Chem2, as in sodium perborate Template:Chem2
• metaborate Template:Chem2 or its cyclic trimer Template:Chem2, found in sodium metaborate Template:Chem2
• diborate Template:Chem2, found in magnesium diborate (suanite) Template:Chem2 ,
• triborate Template:Chem2, found in calcium aluminium triborate (johachidolite) Template:Chem2 ,
• tetraborate Template:Chem2, found in anhydrous borax Template:Chem2
• tetrahydroxytetraborate Template:Chem2, found in borax "decahydrate" Template:Chem2
• tetraborate(6-) Template:Chem2, found in lithium tetraborate(6-) Template:Chem2
• pentaborate Template:Chem2 or Template:Chem2, found in sodium pentaborate Template:Chem2
• octaborate Template:Chem2, found in disodium octaborate Template:Chem2

{{#invoke:Gallery|gallery}}

Preparation

In 1905, Burgess and Holt observed that fusing mixtures of boric oxide Template:Chem2 and sodium carbonate Template:Chem2 yielded on cooling two crystalline compounds with definite compositions, consistent with anhydrous borax Template:Chem2 (which can be written Template:Chem2) and sodium octaborate Template:Chem2 (which can be written Template:Chem2).<ref name=burg1905/>

Structures

Borate anions (and functional groups) consist of trigonal planar Template:Chem2 and/or tetrahedral Template:Chem2 structural units, joined together via shared oxygen atoms (corners) or atom pairs (edges) into larger clusters so as to construct various ions such as Template:Chem2, Template:Chem2, Template:Chem2, Template:Chem2, Template:Chem2, 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 (Template:Chem2) or may carry a negative charge (Template:Chem2).

The planar Template:Chem2 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 may have linear chains of 2, 3 or 4 trigonal Template:Chem2 structural units, each sharing oxygen atoms with adjacent unit(s).<ref name=wibe2001/> as in [[lithium metaborate|Template:Chem2]], contain chains of trigonal Template:Chem2 structural units. Other anions contain cycles; for instance, [[Sodium metaborate|Template:Chem2]] and Template:Chem2 contain the cyclic Template:Chem2 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 Template:Chem2 and Template:Chem2 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 Template:Chem2 can act as a weak Brønsted acid, that is, a proton donor, with pK_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/>

Template:Chem2 + 2 Template:H2O Template:Eqm Template:Chem2 + Template:H3O+ Template:Spaces (pK = 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 Template:Chem2, found in the mineral borax:

4 Template:Chem2 + 2 Template:H+ Template:Eqm Template:Chem2 + 7 Template: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 Template:Chem2 + Template:Chem2 Template:Eqm Template:Chem2 + 3 Template:Chem2 Template:Spaces (fast, pK = −1.92)

4 Template:Chem2 + Template:Chem2 Template:Eqm Template:Chem2 + 6 Template:Chem2 Template:Spaces (slow, pK = −2.05)

In the pH range 6.8 to 8.0, any alkali salts of "boric oxide" anions with general formula Template:Chem2 where 3x + q = 2y + z will eventually equilibrate in solution to a mixture of Template:Chem2, Template:Chem2, Template:Chem2, and Template:Chem2.<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.Template:Cn

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

File:Borax crystals.jpg

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 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 Template:Chem2 (commonly abbreviated DOT) is used as a wood preservative or fungicide. Zinc borate is used as a flame retardant.

Some borates with large anions and multiple cations, like Template:Chem2 and Template:Chem2 have been considered for applications in nonlinear optics.<ref name=muta2016/>

Borate esters

Borate esters are organic compounds, which are conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols (or their chalcogen analogs<ref name=goldbook.iupac>Template:Cite book</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

• Nanochannel glass materials
• Porous glass
• 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. {{#invoke:doi|main}}</ref>

<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. {{#invoke:doi|main}}</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>

<ref name=NPL>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

<ref name=allen1904>Template:Cite journal</ref>

<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. {{#invoke:doi|main}}</ref>

<ref name=wibe2001>Wiberg E. and Holleman A.F. (2001) Inorganic Chemistry, Elsevier Template:ISBN</ref>

<ref name=GandE.P205>Template:Greenwood&Earnshaw2nd</ref>

<ref name=atki2010>Template:Cite book</ref>

<ref name="ingri1962">Template:Cite journal</ref>

<ref name=hett2004>Template:Cite journal</ref>

<ref name=yagu2016>Template:Cite journal</ref>

<ref name=liuj2006>Template:Cite journal</ref>

<ref name=jham2011>Template:Cite journal</ref>

<ref name=maia2004>Template:Cite journal</ref>

<ref name=xiao1995>Template:Cite journal</ref>

<ref name=alek2006>Template:Cite journal</ref>

<ref name=saly2010>Template:Cite journal</ref>

<ref name=saly2011>Template:Cite journal</ref>

<ref name=saly2009>Template:Cite journal</ref>

<ref name=kles2016>Template:Cite journal</ref>

<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. {{#invoke:doi|main}}</ref>

<ref name="carrondo1978">Template:Cite journal</ref>

<ref name="bubnova2002">Template:Cite journal</ref>

</references>

External links

• Suanite at webmineral
• Johachidolite at webmineral
• Non-CCA Wood Preservatives: Guide to Selected Resources - National Pesticide Information Center Template:Webarchive