Borate
From Free net encyclopedia
The borate ion is BO33−. A borate is a compound of the borate ion 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.
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. For a typical polyprotic acid, the deprotonation series dihydrogen borate [H2BO3−], hydrogen borate [HBO32−] and borate [BO33−] would be expected as pH increases. Boric acid, however, adds hydroxide, forming B(OH)4−, and then forms more complicated ions. The fictitious ions are easier to use for pKa values and other calculations.
Borate can 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 include sodium metaborate, NaBO2, and sodium tetraborate, Na2B4O7. The latter also occurs naturally as the hydrous mineral borax, Na2B4O7·10H2O. Death Valley, California contains large borax deposits and is used extensively for borate mining. The Atacama Desert in Chile also contains mineable borate concentrations.
Various forms of borate are used as wood preservatives.
Borate esters include trimethyl borate, B(OCH3)3, which is used as a precursor to boronic esters for Suzuki couplings.