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| Chromium(III) oxide | |
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| Other names | Chromium sesquioxide Chromia Chrome green |
| Identifiers | |
| CAS number | 1308-38-9 |
| Properties | |
| Molecular formula | Cr2O3 |
| Molar mass | 151.99 g/mol Cr=68.43%, O=31.57% |
| Melting point |
2435 °C (4415 °F) |
| Boiling point |
approx. 4000 °C (7250 °F) |
| Solubility in other solvents | Negligible (Insoluble) |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references |
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Chromium(III) oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment. In the nature it occurs as the rare mineral eskolaite.
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Structure and properties
Cr2O3 adopts the corundum structure, consisting of cubic close packed oxides with 2/3 of the octahedral holes occupied by chromium. It is antiferromagnetic up to 307 K, the Neel temperature.12 It not readily attacked by acids or bases, although molten alkali gives chromites.
Production
The Parisians Pannetier and Binet first prepared Cr2O3 in 1838 via a secret process.citation needed It is derived from the mineral chromite, (Fe,Mg)Cr2O4. The conversion of chromite to chromia proceeds via Na2Cr2O7, which is reduced with sulfur at high temperatures:3
- Na2Cr2O7 + S → Na2SO4 + Cr2O3
Chromium oxide can be converted into elemental chromium metal through a thermite-like reaction: unlike iron oxide thermites, chromium oxide thermites creates few or no sparks, smoke or sound, but glow brightly. Because of the very high melting point of chromium, chromium thermite casting is impractical.
Applications
Because of its considerable stability, chromia is commonly used pigment and was originally called viridian. It is used in paints, inks, and glasses. It is the colourant in "chrome green" and "institutional green." Chromium(III) oxide is the precursor to the magnetic pigment chromium dioxide, according to the following reaction:3
- Cr2O3 + 3 CrO3 → 5 CrO2 + O2
References
- ^ J.E Greedon, (1994), Magnetic oxides in Encyclopedia of Inorganic chemistry R. Bruce King, Ed. John Wiley & Sons. ISBN 0471936200
- ^ A. F. Holleman and E. Wiberg "Inorganic Chemistry" Academic Press, 2001, New York. ISBN 0-12-352651-5.
- ^ a b Gerd Anger, Jost Halstenberg, Klaus Hochgeschwender, Christoph Scherhag, Ulrich Korallus, Herbert Knopf, Peter Schmidt, Manfred Ohlinger, "Chromium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
See also
External links
