Boron trioxide

Names
IUPAC name Diboron trioxide
Other names boron oxide, diboron trioxide, boron sesquioxide, boric oxide, boria Boric anhydride
Identifiers
CAS Number	1303-86-2 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30163 Y
ChemSpider	452485 Y
ECHA InfoCard	100.013.751
EC Number	215-125-8
Gmelin Reference	11108
PubChem CID	518682
RTECS number	ED7900000
UNII	483W67CPF4 Y
CompTox Dashboard (EPA)	DTXSID7034387
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) ,[2] sublimes at 1500 °C[3]
Solubility in water	1.1 g/100mL (10 °C) 3.3 g/100mL (20 °C) 15.7 g/100mL (100 °C)
Solubility	partially soluble in methanol
Acidity (pKa)	~ 4
Magnetic susceptibility (χ)	−39.0·10−6 cm3/mol
Thermochemistry
Heat capacity (C)	66.9 J/(mol⋅K)
Std molar entropy (S⦵298)	80.8 J/(mol⋅K)
Std enthalpy of formation (ΔfH⦵298)	−1254 kJ/mol
Gibbs free energy (ΔfG⦵)	−832 kJ/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Irritant[4]
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H360FD
Precautionary statements	P201, P202, P281, P308+P313, P405, P501
NFPA 704 (fire diamond)	2 0 0
Flash point	noncombustible
Lethal dose or concentration (LD, LC):
LD50 (median dose)	3163 mg/kg (oral, mouse)[5]
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 15 mg/m3[4]
REL (Recommended)	TWA 10 mg/m3[4]
IDLH (Immediate danger)	2000 mg/m3[4]
Supplementary data page
Boron trioxide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Boron trioxide or diboron trioxide is the oxide of boron with the formula B₂O₃. It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called boric oxide^[6] or boria.^[7] It has many important industrial applications, chiefly in ceramics as a flux for glazes and enamels and in the production of glasses.

Boron trioxide has three known forms, one amorphous and two crystalline.

The amorphous form (g-B₂O₃) is by far the most common. It 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 many boroxol rings, this view was initially controversial, but such models have recently been constructed and exhibit properties in excellent agreement with experiment.^[8][9] It is now recognized, from experimental and theoretical studies,^{[10][11][12][13][14]} 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 number of boroxol rings decays in the liquid state with increasing temperature.^[15]

The crystalline form (α-B₂O₃) is exclusively composed of BO₃ triangles. Its crystal structure was initially believed to be the enantiomorphic space groups P3₁(#144) and P3₂(#145), like γ-glycine;^[16][17] but was later revised to the enantiomorphic space groups P3₁21(#152) and P3₂21(#154) in the trigonal crystal system, like α-quartz^[18]

Crystallization of α-B₂O₃ from the molten state at ambient pressure is strongly kinetically disfavored (compare liquid and crystal densities). It can be obtained with prolonged annealing of the amorphous solid ~200 °C under at least 10 kbar of pressure.^[19][1]

The trigonal network undergoes a coesite-like transformation to monoclinic β-B₂O₃ at several gigapascals (9.5 GPa).^[20]

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 steam, (H₂O_(g)) and metaboric acid (HBO₂) at around 170 °C, and further heating above 300 °C will produce more steam and diboron trioxide. The reactions are:

H₃BO₃ → HBO₂ + H₂O

2 HBO₂ → B₂O₃ + H₂O

Boric acid goes to anhydrous microcrystalline B₂O₃ in a heated fluidized bed.^[21] Carefully controlled heating rate avoids gumming as water evolves.

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)^[22]

Molten boron oxide attacks silicates. Containers can be passivated internally with a graphitized carbon layer obtained by thermal decomposition of acetylene.^[23]

• Major component of borosilicate glass
• Fluxing agent for glass and enamels^{[citation needed]}
• An additive used in glass fibres (optical fibres)
• 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
• As a starting material for the production of other boron compounds, such as boron carbide

• Boron suboxide
• Boric acid
• Sassolite
• Tris(2,2,2-trifluoroethyl) borate

• 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