boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria
Boric acid anhydride
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||69.6182 g/mol|
|Appearance||white, glassy solid|
|Density||2.460 g/cm3, liquid;|
2.55 g/cm3, trigonal;
|Melting point||450 °C (842 °F; 723 K) (trigonal) |
510 °C (tetrahedral)
|Boiling point||1,860 °C (3,380 °F; 2,130 K) , sublimes at 1500 °C|
|1.1 g/100mL (10 °C) |
3.3 g/100mL (20 °C)
15.7 g/100mL (100 °C)
|Solubility||partially soluble in methanol|
|Acidity (pKa)||~ 4|
Heat capacity (C)
|66.9 J/mol K|
|80.8 J/mol K|
Std enthalpy of
Gibbs free energy (ΔfG⦵)
|Occupational safety and health (OHS/OSH):|
|P201, P202, P281, P308+P313, P405, P501|
|NFPA 704 (fire diamond)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|3163 mg/kg (oral, mouse)|
|NIOSH (US health exposure limits):|
|TWA 15 mg/m3|
|TWA 10 mg/m3|
IDLH (Immediate danger)
|Supplementary data page|
|Boron trioxide (data page)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Boron trioxide or diboron trioxide is the oxide of boron with the formula B2O3. It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called boric oxide or boria. It has many important industrial applications, chiefly in ceramics as a flux for glazes and enamels and in the production of glasses.
Boron trioxide has three known forms, one amorphous and two crystalline.
The amorphous form (g-B2O3) is by far the most common. It is thought to be composed of boroxol rings which are six-membered rings composed of alternating 3-coordinate boron and 2-coordinate oxygen.
Because of the difficulty of building disordered models at the correct density with many boroxol rings, this view was initially controversial, but such models have recently been constructed and exhibit properties in excellent agreement with experiment. It is now recognized, from experimental and theoretical studies, that the fraction of boron atoms belonging to boroxol rings in glassy B2O3 is somewhere between 0.73 and 0.83, with 0.75 = 3/4 corresponding to a 1:1 ratio between ring and non-ring units. The number of boroxol rings decays in the liquid state with increasing temperature.
The crystalline form (α-B2O3) is exclusively composed of BO3 triangles. It crystal structure was initially believed to be the enantiomorphic space groups P31(#144) and P32(#145), like γ-glycine; but was later revised to the enantiomorphic space groups P3121(#152) and P3221(#154) in the trigonal crystal system, like α-quartz
Crystallization of α-B2O3 from the molten state at ambient pressure is strongly kinetically disfavored (compare liquid and crystal densities). It can be obtained with prologed annealing of the amorphous solid ~200°C under at least 10 kbar of pressure.
The trigonal network undergoes a coesite-like transformation to monoclinic β-B2O3 at several gigapascals (9.5 GPa).
Boron trioxide is produced by treating borax with sulfuric acid in a fusion furnace. At temperatures above 750°C, the molten boron oxide layer separates out from sodium sulfate. It is then decanted, cooled and obtained in 96–97% purity.
Another method is heating boric acid above ~300°C. Boric acid will initially decompose into steam, (H2O(g)) and metaboric acid (HBO2) at around 170°C, and further heating above 300°C will produce more steam and diboron trioxide. The reactions are:
Boric acid goes to anhydrous microcrystalline B2O3 in a heated fluidized bed. Carefully controlled heating rate avoids gumming as water evolves.
Boron oxide will also form when diborane (B2H6) reacts with oxygen in the air or trace amounts of moisture:
Molten boron oxide attacks silicates. Containers can be passivated internally with a graphitized carbon layer obtained by thermal decomposition of acetylene.
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