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Borates in chemistry are chemical compounds containing boron bonded to three oxygen atoms written as B(O)3. In B(O)4− anions, this number increases to four.
The borate ion is BO33−. It forms salts with metallic elements. Boron found in nature is commonly as a borate mineral. Boron is also found combined with silicate to form complex borosilicate minerals such as the tourmalines.
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Overview
Borate exists in many forms. In acid and near-neutral conditions, it is boric acid, commonly written as H3BO3 but more correctly B(OH)3. The pKa of boric acid is 9.14 at 25C. Boric acid does not dissociate in aqueous solution, but is acidic due to its interaction with water molecules, forming tetrahydroxyborate:
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- B(OH)3 + H2O ⇌ B(OH)4− + H+
- Ka = 5.8x10−10 mol/l; pKa = 9.24.
- B(OH)3 + H2O ⇌ B(OH)4− + H+
Polyborate anions are formed 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:
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- 4B(OH)4− + 2H+ ⇌ B4O72− + 9H2O
Even though boric acid adds hydroxide to form B(OH)4−, you may find for pKa values and other calculations, the fictitious ions are easier to use. Thus for a typical polyprotic acid, the deprotonation series dihydrogen borate [H2BO3−;, hydrogen borate [HBO32− and borate [BO33− may be written as pH increases.
Polymeric ions
Boric acid can be used to form many polymeric ions. The tetraborate ion, B4O72−, is very common; the hydrogen tetraborate ion, HB4O7−, as well as triborate and pentaborate, are also seen. The various metaborate ions have an empirical formula of BO2−, and form metaborate compounds. Formation of these complicated, potentially infinite network structures is extensive, perhaps surpassed only by the silicates.
Common borate salts
Common borate salts include sodium metaborate, NaBO2, and sodium tetraborate, Na2B4O7. The latter also occurs naturally as the hydrous mineral borax, Na2B4O7·10H2O. Boron, California contains large borax deposits and is used extensively for borate mining. The Atacama Desert in Chile also contains mineable borate concentrations. Gerry, New York has been described by Agapito Associates Inc. as a "very nice place to mine for [borate]" although it is uncertain how much damage that might do to the surrounding area.
Various forms of borate are used as wood preservatives or fungicides such as disodium octaborate tetrahydrate.
Borate esters
Borate esters are organic compounds of the type B(OR)3 where R is an organic residue (for example alkyl or aryl). Borate esters include trimethyl borate, B(OCH3)3, which is used as a precursor to boronic esters for Suzuki couplings.
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- This page was last modified on 20 November 2008, at 21:32.
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