are chemical compounds
bonded to three oxygen
atoms written as B(OR)3
. In B(OR)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.
Borate exists in many forms. In acid and near-neutral conditions, it is boric acid
, commonly written as H3
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
- B(OH)3 + H2O ⇌ B(OH)4− + H+
- Ka = 5.8x10−10 mol/l; pKa = 9.24.
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:
- 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.
Boric acid can be used to form many polymeric ions. The tetraborate ion, B4
, is very common; the hydrogen tetraborate ion, HB4
, 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, Na2
. The latter also occurs naturally as the hydrous mineral borax
contains large borax deposits and is used extensively for borate mining. The Atacama Desert
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.
are organic compounds
of the type B(OR)3
where R is an organic residue (for example alkyl
). Borate esters include trimethyl borate
, which is used as a precursor to boronic esters
for Suzuki couplings.