Tetraborate

The name is also applied to the hydrated ion [B4O5(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: The tetraborate anion (tetramer) includes two tetrahedral and two trigonal boron atoms symmetrically assembled in a fused bicyclic structure.

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 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.