Chemistry:Tetraborate

Structure of the hydrated tetraborate (borax) ion, [B
4O
5(OH)
4]²⁻.

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 H₂O

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 Na₂[B₄O₅(OH)₄]·8H₂O. The borax chemical formula is also commonly written in a more compact notation as Na₂B₄O₇·10H₂O. Sodium borate can be obtained in high purity and so can be used to make a standard solution in titrimetric analysis.