Boron trioxide

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Boron trioxide
IUPAC name
Boron trioxide
Other names
Boric anhydride
Boric oxide
Boron sesquioxide
Diboron trioxide
Molar mass 69.6182 g/mol
Appearance Hard glassy solid or white powder.
Odor Odorless
Density 2.460 g/cm3 (liquid)
2.55 g/cm3 (trigonal)
3.11–3.146 g/cm3 (monoclinic)
Melting point 450–510 °C (842–950 °F; 723–783 K) (trigonal)
510 °C (950 °F; 783 K) (tetrahedral)
Boiling point 1,860 °C (3,380 °F; 2,130 K)
1.1 g/100 ml (10 °C)
2.77 g/100 ml (20 °C)
3.6 g/100 ml (25 °C)
15.7 100 g/100 ml (100 °C)
Solubility Soluble in ethanol, glycerol, methanol
Vapor pressure ~0 mmHg
Acidity (pKa) ~ 4
80.8 J·mol-1·K-1
-1254 kJ/mol
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
3,163 mg/kg (mouse, oral)
Related compounds
Related compounds
Boric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Boron trioxide is a glass like solid or white powder with the chemical formula B2O3.



Boron trioxide is very unreactive towards most common reagents. While an anhydride oxide, it does not react with water visibly.

It can however be reduced to elemental boron with magnesium powder in a thermite reaction. Magnesium boride is produces as side product. A similar effect occurs when powdered aluminium is used, though aluminium not not give a good yield.


When produced from boric acid, it forms as a non crystalline mass that is very hard and difficult to grind, forming a very fine powder when ground very similar to a solid plane of glass.


It is used in glassmaking, whether as a boron additive for making borosilicate glass or as a fluxing agent so this may be a source, however it is easy to produce from the starting materials boric acid or borax.


Boric acid can be dehydrated above 300 degrees to form boron trioxide. Boron trioxide prepared at up to 800 degrees is a desiccant but it is slow acting if not finely powdered. Made at higher temperature it has an induction period.


  • Producing elemental boron
  • Making the ester trimethyl borate
  • Fluxing agent for glass and enamels
  • Acid catalyst in organic synthesis



Boron trioxide has low toxicity, though it is irritant to skin, mouth, nose and eyes.


No special storage is required, storing it in closed plastic bottles is good enough.


Can be dumped in trash.


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