Boron trioxide

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Boron trioxide
Kristallstruktur Bortrioxid.png
Other names
boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria
Boric acid anhydride
CAS number 1303-86-2 YesY
ChEBI CHEBI:30163 YesY
ChemSpider 452485 YesY
EC number 215-125-8
Jmol-3D images Image
PubChem 518682
RTECS number ED7900000
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 (842 °F; 723 K) (trigonal)
510 °C (tetrahedral)
Boiling point 1,860 °C (3,380 °F; 2,130 K) ,[1] sublimates at 1500 °C[2]
1.1 g/100mL (10 °C)
3.3 g/100mL (20 °C)
15.7 100 g/100mL (100 °C)
Solubility partially soluble in methanol
Acidity (pKa) ~ 4
66.9 J/mol K
80.8 J/mol K
-1254 kJ/mol
-832 kJ/mol
MSDS External MSDS
Main hazards irritant[3]
EU classification Repr. Cat. 2
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point noncombustible
3163 mg/kg (oral, rat)
US health exposure limits (NIOSH):
TWA 15 mg/m3[3]
TWA 10 mg/m3[3]
2000 mg/m3[3]
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Phase behaviour
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY verify (what isYesY/N?)
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 (amorphous) form; however, it can be crystallized after extensive annealing (that is, under prolonged heat). It is known as one of the most difficult compounds to crystallize. [clarification needed]

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.[4][5] The crystalline form (α-B2O3) (see structure in the infobox[6]) is exclusively composed of BO3 triangles. This trigonal, quartz-like network undergoes a coesite-like transformation to monoclinic β-B2O3 at several gigapascals (9.5 GPa).[7]


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.[8] Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.[9] Can be used for rocket fuel and car fuel

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.[10] Its proposed crystal structure in enantiomorphic space groups P31(#144); P32(#145)[11][12] (e.g., γ-glycine) has been revised to enantiomorphic space groups P3121(#152); P3221(#154)[13](e.g., α-quartz).

Boron oxide will also form when Diborane (B2H6) reacts with oxygen in the air or trace amounts of moisture:

2B2H6(g) + 3O2(g) → 2B2O3(s) + 6H2(g)
B2H6(g) + 3H2O(v) → B2O3(s) + 6H2(g)[14]


See also[edit]


  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 1-56677-261-3. 
  2. ^ a b Patnaik, P. (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill. p. 119. ISBN 0-07-049439-8. Retrieved 2009-06-06. 
  3. ^ a b c d "NIOSH Pocket Guide to Chemical Hazards". National Institute for Occupational Safety and Health (NIOSH). id=0060. 
  4. ^ Eckert, H. (1992). "Structural characterization of noncrystalline solids and glasses using solid state NMR". Progress in Nuclear Magnetic Resonance Spectroscopy 24 (3): 159–293. doi:10.1016/0079-6565(92)80001-V. 
  5. ^ Hwang, S.-J.; Fernandez, C.; Amoureux, J. P.; Cho, J.; Martin, S. W.; Pruski, M. (1997). "Quantitative study of the short range order in B2O3 and B2S3 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. 
  6. ^ Gurr, G. E.; Montgomery, P. W.; Knutson, C. D.; Gorres, B. T. (1970). "The Crystal Structure of Trigonal Diboron Trioxide". Acta Crystallographica B 26 (7): 906–915. doi:10.1107/S0567740870003369. 
  7. ^ Brazhkin, V. V.; Katayama, Y.; Inamura, Y.; Kondrin, M. V.; Lyapin, A. G.; Popova, S. V.; Voloshin, R. N. (2003). "Structural transformations in liquid, crystalline and glassy B2O3 under high pressure". JETP Letters 78 (6): 393–397. doi:10.1134/1.1630134. 
  8. ^ Kocakuşak, S.; Akçay, K.; Ayok, T.; Koöroğlu, H. J.; Koral, M.; Savaşçi, Ö. T.; 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. 
  9. ^ Morelock, C. R. (1961). "Research Laboratory Report #61-RL-2672M". General Electric. 
  10. ^ Aziz, M. J.; Nygren, E.; Hays, J. F.; Turnbull, D. (1985). "Crystal Growth Kinetics of Boron Oxide Under Pressure". Journal of Applied Physics 57 (6): 2233. doi:10.1063/1.334368. 
  11. ^ Gurr, G. E.; Montgomery, P. W.; Knutson, C. D.; Gorres, B. T. (1970). "The crystal structure of trigonal diboron trioxide". Acta Crystallographica B 26 (7): 906–915. doi:10.1107/S0567740870003369. 
  12. ^ Strong, S. L.; Wells, A. F.; Kaplow, R. (1971). "On the crystal structure of B2O3". Acta Crystallographica B 27 (8): 1662–1663. doi:10.1107/S0567740871004515. 
  13. ^ Effenberger, H.; Lengauer, C. L.; Parthé, E. (2001). "Trigonal B2O3 with Higher Space-Group Symmetry: Results of a Reevaluation". Monatshefte für Chemie 132 (12): 1515–1517. doi:10.1007/s007060170008. 
  14. ^ AirProducts (2011). "Diborane Storage & Delivery". 

External links[edit]