Chromium trioxide

Chromium trioxide
IUPAC name Chromium trioxide Chromium(VI) oxide
Other names Chromic anhydride, chromium(VI) oxide, chromic acid, anhydride, chromic acid (misnomer)
Identifiers
CAS number	1333-82-0 Y
PubChem	14915
ChemSpider	14212 Y
UNII	8LV49809UC Y
UN number	1463
ChEBI	CHEBI:48240 Y
RTECS number	GB6650000
Jmol-3D images	Image 1
SMILES O=[Cr](=O)=O
InChI InChI=1S/Cr.3O Y Key: WGLPBDUCMAPZCE-UHFFFAOYSA-N Y InChI=1/Cr.3O/rCrO3/c2-1(3)4 Key: WGLPBDUCMAPZCE-YFSAMUSXAF
Properties
Molecular formula	CrO3
Molar mass	99.99 g mol−1
Appearance	dark red granular solid
Odor	odorless
Density	2.70 g/cm3 (20 °C)
Melting point	197 °C, 470 K, 387 °F
Boiling point	251 °C, 524 K, 484 °F (decomposes)
Solubility in water	61.7 g/100 mL (0 °C) 63 g/100 mL (25 °C) 67 g/100 mL (100 °C)
Solubility	soluble in sulfuric acid, nitric acid
Hazards
MSDS	ICSC 1194
EU Index	024-001-00-0
EU classification	Oxidizer (O) Carc. Cat. 1 Muta. Cat. 2 Repr. Cat. 3 Very toxic (T+) Dangerous for the environment (N)
R-phrases	R45, R46, R9, R24/25, R26, R35, R42/43, R48/23, R62, R50/53
S-phrases	S53, S45, S60, S61
NFPA 704	1 3 1 OX
LD50	80 mg/kg
Y trioxide (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Chromium trioxide is the inorganic compound with the formula CrO₃. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.^[1] This compound is a dark-red/orange brown solid, which dissolves in water concomitant with hydrolysis. Millions of kilograms are produced annually, mainly for electroplating.^[2]

Production, structure, and basic reactions

Chromium trioxide is generated by treating sodium chromate or the corresponding sodium dichromate with sulfuric acid:^[1]

H₂SO₄ + Na₂CrO₄ → CrO₃ + Na₂SO₄ + H₂O

Approximately 100M kg are produced annually by this or similar routes.^[2]

The solid consists of chains of tetrahedrally coordinated chromium atoms that share vertices. Each chromium center, therefore, shares two oxygen centers with neighbors. Two oxygen atoms are not shared, giving an overall stoichiometry of 1:3.^[3][4]

The structure of monomeric CrO₃ has been calculated using density functional theory, and is predicted to be pyramidal (point group C_3v) rather than planar (point group D_3h).^[5]

Chromium trioxide decomposes above 197°C liberating oxygen eventually giving Cr₂O₃:

4 CrO₃ → 2 Cr₂O₃ + 3 O₂

It is used in organic synthesis as an oxidant, often as a solution in acetic acid,^[3] or acetone in the case of the Jones oxidation. In these oxidations, the Cr(VI) converts 1.5 equivalents of alcohols to the corresponding ketones or aldehydes:

2 CrO₃ + 3 RCH₂OH → Cr₂O₃ + 3 RCHO + 3 H₂O

Applications

Chromium trioxide is mainly used in chrome-plating. It is typically employed with additives that affect the plating process but do not react with the trioxide. The trioxide reacts with cadmium, zinc, and other metals to generate passivating chromate films that resist corrosion. It is also used in the production of synthetic rubies.

Safety

Chromium trioxide ignition of ethanol

Chromium trioxide is highly toxic, corrosive, and carcinogenic.^[6] It is the main example of hexavalent chromium, an environmental hazard. The related chromium(III) derivatives are not particularly dangerous; thus, reductants are used to destroy chromium(VI) samples.

Chromium trioxide, being a powerful oxidizer, will ignite some organic materials (such as ethanol) on contact.

References

1. ^ ^{a b} "Chromium Trioxide". Chemicalland21. http://www.chemicalland21.com/industrialchem/inorganic/CHROMIUM%20TRIOXIDE.htm. 
2. ^ ^{a b} Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinge. Chromium Compounds. in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, 2002. doi:10.1002/14356007.a07_067
3. ^ ^{a b} Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 
4. ^ Stephens, J. S.; Cruickshank, D. W. J. (1970). "The crystal structure of (CrO₃)_∞". Acta Cryst. B 26: 222–226. doi:10.1107/S0567740870002182. 
5. ^ Zhai, Hua-Jin; Li, Shenggang; Dixon, David A.; Wang, Lai-Sheng (2008). "Probing the Electronic and Structural Properties of Chromium Oxide Clusters (CrO₃)n⁻ and (CrO₃)n (n = 1–5): Photoelectron Spectroscopy and Density Functional Calculations". J. Am. Chem. Soc. 130 (15): 5167–5177. doi:10.1021/ja077984d. 
6. ^ "Chromium Trioxide (MSDS)". J. T. Baker. http://hazard.com/msds/mf/baker/baker/files/c4400.htm. Retrieved 2007-09-13. 

External links

• ATSDR Case Studies in Environmental Medicine: Chromium Toxicity U.S. Department of Health and Human Services
• http://www.gemstonebuzz.com/flame-fusion