Tetrahydroxyborate (systematically named tetrahydroxyboranuide and tetrahydroxidoborate(1−)) is an inorganic anion with the chemical formula (also written as or ). It contributes no colour to tetrahydroxyborate salts. It is found in the mineral hexahydroborite, Ca(B(OH)₄)₂ (originally formulated CaB₂O₄). It is classified as a weak base.


Chemical properties




Basicity

Tetrahydroxyborate can assimilate a proton into the anion by recombination: Because of this capture of a proton (), tetrahydroxyborate has Arrhenius-basic character. Its protonation product is boric acid. In aqueous solution, most tetrahydroxyborate ions are undissociated.


Structure

It is a boron oxoanion with a tetrahedral geometry. It is isoelectronic with the hypothetical compound orthocarbonic acid.


Chemical reactions

Tetrahydroxyborate undergoes the typical chemical reactions of a hydroxyborate. Upon treatment with a standard acid, it converts to boric acid and a metal salt. Oxidation of tetrahydroxyborate gives perborate. When heated to a high temperature, tetrahydroxyborate salts decompose to produce metaborate salts and water, or to produce boric acid and a metal hydroxide: :''n'' (OH)₄sup>− → (BO)_''n'' + 2''n''  :(OH)₄sup>− → B(OH)₃ + HO⁻


Production

Tetrahydroxyborate is produced by boric acid alkalinisation. In this process boric acid and hydroxide anions react to produce tetrahydroxyborate according to the following reaction: :B(OH)₃ + HO⁻ → (OH)₄sup>− This process also involves aquatrihydroxyboron as an intermediate, and occurs in two steps. No catalyst is needed for alkalinisation (step 2). #B(OH)₃ + → (OH)₃H₂O#(OH)₃H₂O+ HO⁻ → (OH)₄sup>− + Catalytic amounts of water are used for the process. By altering the process conditions, other borate anions may also be produced on the same production site.


Uses

Tetrahydroxyborate can be used as a cross-link in polymers.


Occurrence

The tetrahydroxyborate anion is found in Na(OH)₄ Na₂(OH)₄l and Cu^II(OH)₄l. File:Sodium-tetrahydroxyborate-xtal-3D-balls.png|ball-and-stick model of the crystal
structure of sodium tetrahydroxyborate File:Sodium-tetrahydroxyborate-xtal-3D-SF.png|space-filling model of the crystal
structure of sodium tetrahydroxyborate


See also

* Borate * Tetrafluoroborate


References

{{Reflist Category:Borates Category:Anions