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. 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.
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 pKa of boric acid is 9.14 at 25°C. Boric acid does not dissociate in aqueous solution, but is acidic due to its interaction with water molecules, forming tetrahydroxyborate:
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:
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.
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.
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