Boron trioxide

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Boron trioxide
Crystal structure of B2O3 [1]
Kristallstruktur Bortrioxid.png
Names
Other names
boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria
Boric acid anhydride
Identifiers
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
Properties
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 (842 °F; 723 K) (trigonal)
510 °C (tetrahedral)
Boiling point 1,860 °C (3,380 °F; 2,130 K) ,[2] sublimates at 1500 °C[3]
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
Thermochemistry
66.9 J/mol K
80.8 J/mol K
-1254 kJ/mol
-832 kJ/mol
Hazards
Main hazards Irritant Xi[4]
Safety data sheet See: data page
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
Lethal dose or concentration (LD, LC):
3163 mg/kg (oral, mouse)[5]
US health exposure limits (NIOSH):
TWA 15 mg/m3[4]
TWA 10 mg/m3[4]
2000 mg/m3[4]
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Thermodynamic
data
Phase behaviour
solid–liquid–gas
UV, IR, NMR, MS
Except where otherwise noted, data are 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).

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. Because of the difficulty of building disordered models at the correct density with a large number of boroxol rings, this view was initially controversial, but such models have recently been constructed and exhibit spectroscopic properties in excellent agreement with experiment.[6] The rings are thought to make a few BO3 triangles, but mostly link (polymerize) into ribbons and sheets.[7][8] The crystalline form (α-B2O3) (see structure in the infobox[1]) 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).[9]

Preparation[edit]

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.[3]

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.[10] Carefully controlled heating rate avoids gumming as water evolves. Molten boron oxide attacks silicates. Internally graphitized tubes via acetylene thermal decomposition are passivated.[11]

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.[12] Its proposed crystal structure in enantiomorphic space groups P31(#144); P32(#145)[13][14] (e.g., γ-glycine) has been revised to enantiomorphic space groups P3121(#152); P3221(#154)[15](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)[16]

Applications[edit]

See also[edit]

References[edit]

  1. ^ a b 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. 
  2. ^ 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. 
  3. ^ a b Patnaik, P. (2003). Handbook of Inorganic Chemical Compounds. McGraw-Hill. p. 119. ISBN 0-07-049439-8. Retrieved 2009-06-06. 
  4. ^ a b c d "NIOSH Pocket Guide to Chemical Hazards #0060". National Institute for Occupational Safety and Health (NIOSH). 
  5. ^ "Boron oxide". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH). 
  6. ^ Ferlat, G.; Charpentier, T.; Seitsonen, A. P.; Takada, A.; Lazzeri, M.; Cormier, L.; Calas, G.; Mauri. F. (2008). "Boroxol Rings in Liquid and Vitreous B2O3 from First Principles". Phys. Rev. Lett. 101: 065504. doi:10.1103/PhysRevLett.101.065504. 
  7. ^ 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. 
  8. ^ 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. 
  9. ^ 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. 
  10. ^ 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. 
  11. ^ Morelock, C. R. (1961). "Research Laboratory Report #61-RL-2672M". General Electric. 
  12. ^ 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. 
  13. ^ 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. 
  14. ^ 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. 
  15. ^ 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. 
  16. ^ AirProducts (2011). "Diborane Storage & Delivery" (PDF). 

External links[edit]