Borate

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

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.

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.

Anions

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

Preparation

In 1905, Burgess and Holt observed that fusing mixtures of boric oxide and sodium carbonate yielded on cooling two crystalline compounds with definite compositions, consistent with anhydrous borax (which can be written ) and sodium octaborate (which can be written ).

Structures

Borate anions (and functional groups) consist of trigonal planar and/or tetrahedral structural units, joined together via shared oxygen atoms (corners) or atom pairs (edges) into larger clusters so as to construct various ions such as , , , , , etc. These anions may be cyclic or linear in structure, and can further polymerize into infinite chains, layers, and tridimensional frameworks. The terminal (unshared) oxygen atoms in the borate anions may be capped with hydrogen atoms () or may carry a negative charge ().

The planar 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.

Polymeric borate anions may have linear chains of 2, 3 or 4 trigonal structural units, each sharing oxygen atoms with adjacent unit(s). as in , contain chains of trigonal structural units. Other anions contain cycles; for instance, and contain the cyclic ion, 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 and 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.

Reactions

Aqueous solution

In aqueous solution, boric acid 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:

: + 2 + (p''K'' = 8.98)

This reaction is very fast, with a characteristic time less than 10 μs. 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 , found in the mineral borax:

: 4 + 2 + 7

Other anions observed in solution are triborate(1−) and pentaborate(1−), in equilibrium with boric acid and tetrahydroxyborate according to the following overall reactions:

: 2 + + 3 (fast, p''K'' = −1.92)

: 4 + + 6 (slow, p''K'' = −2.05)

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

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.

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 iO₄sup>4− units are replaced by O₄sup>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

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.

Disodium octaborate tetrahydrate (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 and have been considered for applications in nonlinear optics.

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).

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 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 precursors composed of the tris(pyrazolyl)borate ligand and either ozone or water as the oxidant to deposit CaB₂O₄, SrB₂O₄, BaB₂O₄, Mn₃(BO₃)₂, and CoB₂O₄ 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.

See also

* Nanochannel glass materials
* Porous glass
* Vycor glass
* Silly Putty
* Slime (toy)
* Tris(2,2,2-trifluoroethyl) borate

References

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.

Miriding Mutailipu, Min Zhang, Xiaoyu Dong, Yanna Chen, and Shilie Pan (2016): "Effects of the Orientation of ₅O₁₁sup>7– Fundamental Building Blocks on Layered Structures Based on the Pentaborates". ''Inorganic Chemistry'', volume 55, issue 20, pages 10608–10616.

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.

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.

Wiberg E. and Holleman A.F. (2001) ''Inorganic Chemistry'', Elsevier

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.

External links

Suanite at webmineral

Johachidolite at webmineral

Non-CCA Wood Preservatives: Guide to Selected Resources - National Pesticide Information Center

{{Borates

Borate minerals

Industrial minerals
Inorganic compounds
Neutron poisons
Boron oxyanions
Pesticides
Preservatives
Rheology
Testicular toxicants