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A borate is any of a range of boron , 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 of such anions, such as trimethyl borate .
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 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.
The planar units may be stacked in the crystal lattice to have pi bond molecular orbitals, which often results in useful optical properties such as strong 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.
This reaction is very fast, with a characteristic time less than 10 microsecond. 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:
Other anions observed in solution are triborate(1−) and pentaborate(1−), in equilibrium with boric acid and tetrahydroxyborate according to the following overall reactions:
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.
Borosilicate, also known as pyrex, can be viewed as a silicate in which some SiO44− units are replaced by BO45− 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.
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.
Wiberg E. and Holleman A.F. (2001) Inorganic Chemistry, Elsevier
Anions
The main borate anions are:
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 ().
Reactions
Aqueous solution
In aqueous solution, boric acid can act as a weak Brønsted acid, that is, a proton donor, with p Ka ~ 9. However, it more often acts as a Lewis acid, accepting an electron pair from a hydroxide ion produced by the water autoprotolysis:
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 , , , , .
Complex oxyanions containing boron
More complex anions can be formed by condensing borate triangles or tetrahedra with other to yield materials such as borosulfates, boroselenates, borotellurates, boroantimonates, borophosphates, or boroselenites.
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.
Borate esters
are , which are conveniently prepared by the Stoichiometry 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 Oxidizing agent 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
External links
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