Borate

Borates are chemical compounds containing boron oxoanions, with boron in oxidation state +3. The simplest borate ion is the trigonal planar, BO₃³⁻, although many others are known. The anion BO₃³⁻ can be found in salts with metallic elements. Many borates are readily hydrated to include structural hydroxide groups, and they should formally be considered hydroxoborates.

In nature, boron is commonly found as a borate mineral. It is also found combined with silicate to form complex borosilicate minerals such as the tourmalines.

Natural occurrence

Boron, California, contains large borax deposits that are extensively mined for borate. 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. Critics, however, are concerned about how mining might potentially damage the surrounding area.

Aqueous chemistry

In aqueous solution, borate exists in many forms. In acidic 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.

Polymeric anions containing structural OH units (polyhydroxoborates) 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 ion, found in the mineral borax:

4B(OH)₄⁻ + 2H⁺ ⇌ B₄O₅(OH)₄ ²⁻ + 7H₂O

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

A number of polymeric borate ions are known in anhydrous compounds, which are made by reacting B(OH)₃ or B₂O₃ with metal oxides, for example:-

• diborate B₂O₅⁴⁻ e.g. in Mg₂B₂O₅ (suanite)
• triborate B₃O₇⁵ in CaAlB₃O₇ (johachidolite)
• tetraborate B₄O₉⁶⁻ in e.g. Li₆B₄O₉
• metaborates containing the linear [BO₂⁻]_n with three coordinate boron e.g. in LiBO₂, CaB₂O₄
• metaborates containing 3 and four coordinate boron, often these are high pressure modifications.

Common borate salts

Common borate salts include sodium metaborate, NaBO₂, and sodium tetraborate, Na₂B₄O₇, which is usually encountered as borax the so-called decahydrate, and actually contains the hydroxoborate ion, B₄O₅(OH)₄²⁻ and is formulated Na₂[B₄O₅(OH)₄]·8H₂O.

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.

Uses

Various forms of borate are used as wood preservatives or fungicides such as disodium octaborate tetrahydrate.

See also

• Borax
• Boron

References

ISBN links support NWE through referral fees

• Chang, Raymond. 2006. Chemistry, 9th ed. New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031
• Cotton, F. Albert, and Geoffrey Wilkinson. 1980. Advanced Inorganic Chemistry, 4th ed. New York: Wiley. ISBN 0471027758
• Greenwood, N.N., and A. Earnshaw. 1998. Chemistry of the Elements, 2nd ed. Oxford, UK: Burlington, MA: Butterworth-Heinemann, Elsevier Science. ISBN 0750633654
• Holleman, Arnold Frederick, Egon Wiberg, and Nils Wiberg. 2001. Inorganic Chemistry. San Diego, CA: Academic Press. ISBN 0123526515

External links

• Borates. IMA Europe. Retrieved January 11, 2009.

• Non-CCA Wood Preservatives: Borates. National Pesticide Information Center. Retrieved January 11, 2009.

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