Boron trioxide
![Crystal structure of B2O3 [1]](/info/File:B2O3powder.JPG) |
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| Names | |
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| Other names
boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria
Boric acid anhydride |
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| Identifiers | |
1303-86-2  |
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| ChEBI | CHEBI:30163 |
| ChemSpider | 452485 |
| EC Number | 215-125-8 |
| Jmol 3D model | Interactive image |
| PubChem | 518682 |
| RTECS number | ED7900000 |
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| Properties | |
| B2O3 | |
| 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) ,[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) |
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| Solubility | partially soluble in methanol |
| Acidity (pKa) | ~ 4 |
| Thermochemistry | |
| 66.9 J/mol K | |
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Std molar
entropy (S |
80.8 J/mol K |
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Std enthalpy of
formation (ΔfH |
-1254 kJ/mol |
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Gibbs free energy (ΔfG˚)
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-832 kJ/mol |
| Vapor pressure | {{{value}}} |
| Supplementary data page | |
| Refractive index (n), Dielectric constant (εr), etc. |
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Thermodynamic
data |
Phase behaviour solid–liquid–gas |
| UV, IR, NMR, MS | |
 verify (what is  ?) |
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| 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.[4] The rings are thought to make a few BO3 triangles, but mostly link (polymerize) into ribbons and sheets.[5][6] 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).[7]
Preparation
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.[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]
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(g) → B2O3(s) + 6H2(g)[14]
Applications
- 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 Liquid Encapsulation Czochralski process for the production of gallium arsenide single crystal
- As an acid catalyst in organic synthesis
See also
References
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External links
- National Pollutant Inventory: Boron and compounds
- Australian Government information
- US NIH hazard information. See NIH.
- Material Safety Data Sheet
- CDC - NIOSH Pocket Guide to Chemical Hazards - Boron oxide
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