Boron(III) oxide

Boron trioxide

Names
Other names boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria Boric acid anhydride
Identifiers
CAS Number	1303-86-2 Y
3D model (Jmol)	Interactive image
ChEBI	CHEBI:30163 Y
ChemSpider	452485 Y
ECHA InfoCard	100.013.751
EC Number	215-125-8
PubChem	518682
RTECS number	ED7900000
InChI InChI=1S/B2O3/c3-1-5-2-4 Y Key: JKWMSGQKBLHBQQ-UHFFFAOYSA-N Y InChI=1/B2O3/c3-1-5-2-4 Key: JKWMSGQKBLHBQQ-UHFFFAOYAI
SMILES O=BOB=O
Properties
Chemical 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 (842 °F; 723 K) (trigonal) 510 °C (tetrahedral)
Boiling point	1,860 °C (3,380 °F; 2,130 K) , sublimates at 1500 °C
Solubility in water	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
Magnetic susceptibility (χ)	-39.0·10−6 cm3/mol
Thermochemistry
Specific heat capacity (C)	66.9 J/mol K
Std molar entropy (So298)	80.8 J/mol K
Std enthalpy of formation (ΔfHo298)	-1254 kJ/mol
Gibbs free energy (ΔfG˚)	-832 kJ/mol
Hazards
Main hazards	Xi
Safety data sheet	See: data page
EU classification (DSD)	Repr. Cat. 2
NFPA 704	0 2 1
Flash point	noncombustible
Lethal dose or concentration (LD, LC):
LD50 (median dose)	3163 mg/kg (oral, mouse)
US health exposure limits (NIOSH):
PEL (Permissible)	TWA 15 mg/m3
REL (Recommended)	TWA 10 mg/m3
IDLH (Immediate danger)	2000 mg/m3
Supplementary data page
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

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

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 properties in excellent agreement with experiment. It is now recognized, from experimental and theoretical studies, that the fraction of boron atoms belonging to boroxol rings in glassy B₂O₃ is somewhere between 0.73 and 0.83, with 0.75 ( ³⁄₄) corresponding to a 1:1 ratio between ring and non-ring units.

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 (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:

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.