Sodium metaborate

Sodium metaborate is a colorless solid chemical compound of sodium, boron, and oxygen with formula. The formula can be written also as · to highlight the relation to the main oxides of sodium and boron.


Solid anhydrous sodium metaborate crystallizes in the hexagonal space group. It actually contains the trimeric anion. The six oxygen atoms are evenly divided into two distinct structural sites, with different B–O bond lengths.

Hydrates and solubility

The following hydrates crystallize from solutions of the proper composition in various temperature ranges:
Early reports of a monohydrate · have not been confirmed.
The anhydrous salt can be prepared from the tetraborate by heating to 270 °C in vacuum.


Sodium metaborate is prepared by the fusion of sodium carbonate and boron oxide or borax. Another way to create the compound is by the fusion of borax with sodium hydroxide at 700° C:
The boiling point of sodium metaborate is lower than that of boron oxide and borax In fact, while the metaborate boils without change of composition, borax gives off a vapor
of sodium metaborate with a small excess of sodium oxide.


Electrochemical conversion to borax

of a concentrated solution of 20% ·4 with an anion exchange membrane and inert anode converts the metaborate anion to tetraborate, and the sodium salt of the later precipitates as a white powder.

Reduction to sodium borohydride

Sodium metaborate is also a byproduct of hydrolysis of sodium borohydride, a proposed hydrogen storage material for hydrogen-fueled vehicles that is safer and more efficient on a weight basis than most other alternatives. The reaction is
and requires a catalyst.
To be economical, that approach would require a cheap and efficient method to recycle the metaborate to the borohydride. Several methods have been studied, such as the reaction with various reducing agents at high temperatures and pressure, or with magnesium hydride by ball milling at room temperature, followed by extraction of the with isopropylamine.
Another alternative that has been considered is the electrolytic reduction of a concentrated sodium metaborate solution, namely
However, this method is not efficient since it competes with the reduction of hydroxide, 4 → 2 + +4e-

Conversion to sodium alkoxides

Anhydrous sodium metaborate refluxed with methanol yields the corresponding sodium methoxyborate:
The analogous reaction with ethanol yields the ethoxyborate.


Sodium metaborate is used in the manufacturing of borosilicate glasses. It is also a component of herbicides and antifreeze products.