borate

Borates in chemistry 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.^[1] BO₃³⁻ 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. Many borates are readily hydrated and contain structural hydroxide groups and should formally be considered as hydroxoborates.

Aqueous chemistry

In aqueous solution 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.

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:

4 B(OH)−4 + 2 H⁺ B₄O₅(OH)2−4 + 7 H₂O

Even though boric acid adds hydroxide to form B(OH)−4, the fictitious ions are easier to use for pKa values and other calculations. Thus for a typical polyprotic acid, the deprotonation series dihydrogen borate [H₂BO−3], hydrogen borate [HBO2−3], and borate [BO3−3] 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^[1]:-

• diborate B₂O₅⁴⁻ e.g. in Mg₂B₂O₅ (suanite)
• triborate B₃O₇^5- 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)₄ ²⁻ and is formulated Na₂[B₄O₅(OH)₄]·8H₂O.
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.

External links

• Non-CCA Wood Preservatives: Guide to Selected Resources - National Pesticide Information Center

References