Borate

Borates in chemistry are chemical compounds containing boron bonded to three oxygen atoms written as B(OR)₃. In B(OR)₄⁻ anions, this number increases to four.

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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 H₃BO₃ but more correctly B(OH)₃. The pK_a 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⁺

K_a = 5.8x10⁻¹⁰ mol/l; pK_a = 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, 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. Boron, 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 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.