Boron trioxide Identifiers CAS number PubChem ChemSpider ChEBI RTECS number ED7900000 Jmol-3D images Image 1 Properties 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
450 °C (trigonal)
510 °C (tetrahedral)
Boiling point Solubility in water 22 g/L Solubility partially soluble in methanol Acidity (pKa) ~ 4 Hazards MSDS External MSDS EU classification Repr. Cat. 2 NFPA 704 LD50 3150 mg/kg (oral, rat) Supplementary data page Structure and
n, εr, etc. Thermodynamic
Solid, liquid, gas
Spectral data UV, IR, NMR, MS (what is: /?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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. The crystalline form (α-B2O3) see structure in the infobox) 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.
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 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. Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.
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. Its proposed crystal structure in enantiomorphic space groups P31(#144); P32(#145) (e.g., γ-glycine) has been revised to enantiomorphic space groups P3121(#152); P3221(#154)(e.g., α-quartz).
- Fluxing agent for glass and enamels
- Starting material for synthesizing other boron compounds such as boron carbide
- An additive used in glass fibres (optical fibres)
- It is used in the production of borosilicate glass
- The inert capping layer in the LEC process for the production of gallium arsenide single crystal
- As an acid catalyst in organic synthesis
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- ^ a b Patnaik, Pradyot (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill. p. 119. ISBN 0070494398. http://books.google.com/?id=Xqj-TTzkvTEC&pg=PA119. Retrieved 2009-06-06.
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- ^ 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.
- ^ 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. http://www.jetpletters.ac.ru/ps/47/article_679.shtml.
- ^ 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.
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- ^ 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.
- ^ 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.
- ^ 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.
- National Pollutant Inventory: Boron and compounds
- Australian Government information
- US NIH hazard information. See NIH.
- Material Safety Data Sheet
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