Tetraborate
# Tetraborate

In chemistry, tetraborate or pyroborate is an anion (negative ion) with formula $B_4O2−7;$ or a salt containing that anion, such as sodium tetraborate, $Na_2B_4O_7.$ It is one of the boron oxoacids, that is, a borate.

The name is also applied to the hydrated ion $[B_4O_5(OH)_4]^{2−}$ as present in borax

The ion occurs in boric acid solutions at neutral pH, being formed by condensation of orthoborate and tetrahydroxyborate anions:

    2 $B(OH)_3$ + 2 $[B(OH)_4]^−$ ⇌ $[B_4O_5(OH)_4]−2$ + 5 $H_2O$

The tetraborate anion (tetramer) includes two tetrahedral and two trigonal boron atoms symmetrically assembled in a fused bicyclic structure. The two tetrahedral boron atoms are linked together by a common oxygen atom, and each also bears a negative net charge brought by the supplementary $OH^−$ groups laterally attached to them. This intricate molecular anion also exhibits three rings: two fused distorted hexagonal (boroxole) rings and one distorted octagonal ring. Each ring is made of a succession of alternate boron and oxygen atoms. Boroxole rings are a very common structural motif in polyborate ions.

The hydrated tetraborate anion occurs in the mineral borax (sodium tetraborate octahydrate) with the formula $Na_2[B_4O_5(OH)_4]·8H_2O.$ The borax chemical formula is also commonly written in a more compact notation as $Na_2B_4O_7·10H_2O.$ Sodium borate can be obtained in high purity and so can be used to make a standard solution in titrimetric analysis.

