Boron trioxide

Boron trioxide
CAS number 1303-86-2 YesY
PubChem 518682
ChemSpider 452485 YesY
ChEBI CHEBI:30163 YesY
RTECS number ED7900000
Jmol-3D images Image 1
Molecular formula B2O3
Molar mass 69.6182 g/mol
Appearance white, glassy solid
Density 2.460 g/cm3, liquid;

2.55 g/cm3, trigonal;
3.11–3.146 g/cm3, monoclinic

Melting point

450 °C (trigonal)
510 °C (tetrahedral)

Boiling point

1860 °C,[1] sublimates at 1500 °C[2]

Solubility in water 22 g/L
Solubility partially soluble in methanol
Acidity (pKa) ~ 4
MSDS External MSDS
EU classification Repr. Cat. 2
NFPA 704
NFPA 704.svg
LD50 3150 mg/kg (oral, rat)
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 YesY trioxide (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Boron trioxide (or diboron trioxide) is one of the oxides of boron. It is a white, glassy solid with the formula B2O3. It is almost always found as the vitreous (amorphic) form; however, it can be crystallized after extensive annealing. It is one of the most difficult compounds known to crystallize.

Glassy boron oxide (g-B2O3) is thought to be composed of boroxol rings which are six-membered rings composed of alternating 3-coordinate boron and 2-coordinate oxygen. This view is controversial, however, because no model has ever been made of glassy boron oxide of the correct density containing a large number of six-membered rings. The rings are thought to make a few BO3 triangles, but mostly link (polymerize) into ribbons and sheets.[3][4] The crystalline form (α-B2O3) see structure in the infobox[5]) is exclusively composed of BO3 triangles. This trigonal, quartz-like network undergoes a coesite-like transformation to monoclinic β-B2O3 at several gigapascals and is 9.5 GPa.[6]



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.[2]

Another method is heating boric acid above ~300 °C. Boric acid will initially decompose into water steam and metaboric acid (HBO2) at around 170 °C, and further heating above 300 °C will produce more steam and boron trioxide. The reactions are:

H3BO3 → HBO2 + H2O
2 HBO2 → B2O3 + H2O

Boric acid goes to anhydrous microcrystalline B2O3 in a heated fluidized bed.[7] Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.[8]

Crystallization of molten α-B2O3 at ambient pressure is strongly kinetically disfavored (compare liquid and crystal densities). Threshold conditions for crystallization of the amorphous solid are 10 kbar and ~200 °C.[9] Its proposed crystal structure in enantiomorphic space groups P31(#144); P32(#145)[10][11] (e.g., γ-glycine) has been revised to enantiomorphic space groups P3121(#152); P3221(#154)[12](e.g., α-quartz).


The bulk modulus of β-B2O3 is rather high (K = 180 GPa). The Vickers hardness of g-B2O3 is 1.5 GPa and of β-B2O3 is 16 GPa.[13]


See also


  1. ^ High temperature corrosion and materials chemistry: proceedings of the Per Kofstad Memorial Symposium. Proceedings of the Electrochemical Society. The Electrochemical Society. 2000. p. 496. ISBN 1566772613. 
  2. ^ a b Patnaik, Pradyot (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill. p. 119. ISBN 0070494398. Retrieved 2009-06-06. 
  3. ^ Eckert, H. (1992). "Structural characterization of noncrystalline solids and glasses using solid state NMR". Prog. NMR Spectrosc. 24 (3): 159. doi:10.1016/0079-6565(92)80001-V. 
  4. ^ S.-J. Hwang, C. Femandez, J.P. Amoureux, J. Cho, S.W. Martin & M. Pruski. (1997). "Quantitative study of the short range order in B,O, and B,S, by MAS and two-dimensional triple-quantum MAS 11B NMR". Solid State Nuclear Magnetic Resonance 8 (2): 109–121. doi:10.1016/S0926-2040(96)01280-5. PMID 9203284. 
  5. ^ G.E. Gurr, P.W. Montgomery, C.D. Knutson, B.T.Gorres (1970). "The Crystal Structure of Trigonal Diboron Trioxide". Acta Cryst. B 26 (7): 906–915. doi:10.1107/S0567740870003369. 
  6. ^ V. V. Brazhkin et al. (2003). "Structural transformations in liquid, crystalline and glassy B2O3 under high pressure". JETPh Lett. 78 (6): 845. doi:10.1134/1.1630134. 
  7. ^ Kocakusak, S; Akcay, K; Ayok, T; Kooroglu, H; Koral, M; Savasci, O; Tolun, R (1996). "Production of anhydrous, crystalline boron oxide in fluidized bed reactor". Chemical Engineering and Processing 35 (4): 311–317. doi:10.1016/0255-2701(95)04142-7. 
  8. ^ C.R. Morelock, General Electric Research Laboratory Report #61-RL-2672M(1961)
  9. ^ "Crystal Growth Kinetics of Boron Oxide Under Pressure". J. Appl. Phys. 57 (6): 2233. 1985. doi:10.1063/1.334368. 
  10. ^ Gurr, G. E.; Montgomery, P. W.; Knutson, C. D.; Gorres, B. T. (1970). "The crystal structure of trigonal diboron trioxide". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry 26 (7): 906–915. doi:10.1107/S0567740870003369. 
  11. ^ Strong, S. L.; Wells, A. F.; Kaplow, R. (1971). "On the crystal structure of B2O3". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry 27 (8): 1662–1663. doi:10.1107/S0567740871004515. 
  12. ^ Effenberger, Herta; Lengauer, Christian L.; Parthé, Erwin (2001). "Trigonal B2O3 with Higher Space-Group Symmetry: Results of a Reevaluation". Monatshefte für Chemie/Chemical Monthly 132 (12): 1515–1517. doi:10.1007/s007060170008. 
  13. ^ V. A. Mukhanov, O. O. Kurakevich, and V. L. Solozhenko (2008). "On the Hardness of Boron (III) Oxide". Journal of Superhard Materials 30: 71. doi:10.3103/S1063457608010097. 

External links

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Look at other dictionaries:

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  • boric oxide — noun A white solid, of empirical formula BO; it is an inorganic polymer used in making glass and enamel and for synthesizing other boron compounds. Syn: boron trioxide …   Wiktionary

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