Chemistry:Chromium(III) oxide
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| Names | |
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| Other names | |
| Identifiers | |
3D model (JSmol)
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| ChEBI | |
| ChemSpider | |
| EC Number |
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| 11116 | |
PubChem CID
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| RTECS number |
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| UNII | |
| UN number | 3077 |
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| Properties | |
| Cr2O3 | |
| Molar mass | 151.9904 g/mol |
| Appearance | light to dark green, fine crystals |
| Density | 5.22 g/cm3 |
| Melting point | 2,435 °C (4,415 °F; 2,708 K) |
| Boiling point | 4,000 °C (7,230 °F; 4,270 K) |
| insoluble | |
| Solubility in alcohol | insoluble in alcohol, acetone, acids |
| +1960.0×10−6 cm3/mol | |
Refractive index (nD)
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2.551 |
| Structure[1] | |
| Corundum | |
| R3c (No. 167) | |
a = 495 pm, c = 1358 pm
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| Thermochemistry | |
Std molar
entropy (S |
81 J·mol−1·K−1 |
Std enthalpy of
formation (ΔfH⦵298) |
−1128 kJ·mol−1 |
| Hazards | |
| GHS pictograms |  
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| GHS Signal word | Danger |
| H302, H317, H319, H360 | |
| P201, P202, P261, P264, P270, P272, P280, P281, P301+312, P302+352, P305+351+338, P308+313, P321, P330, P333+313, P337+313, P363, P405, P501 | |
| NIOSH (US health exposure limits): | |
PEL (Permissible)
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TWA 1 mg/m3[2] |
REL (Recommended)
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TWA 0.5 mg/m3[2] |
IDLH (Immediate danger)
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250 mg/m3[2] |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
 verify (what is   ?)
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| Infobox references | |
Chromium(III) oxide (or chromia) is an inorganic compound with the formula Cr2O3. It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.
Structure and properties
Cr2O3 has the corundum structure, consisting of a hexagonal close packed array of oxide anions with 2⁄3 of the octahedral holes occupied by chromium. Similar to corundum, Cr2O3 is a hard, brittle material (Mohs hardness 8 to 8.5).[3] It is antiferromagnetic up to 307 K, the Néel temperature.[4][5] It is not readily attacked by acids.
Occurrence
Cr2O3 occurs naturally as the mineral eskolaite, which is found in chromium-rich tremolite skarns, metaquartzites, and chlorite veins. Eskolaite is also a rare component of chondrite meteorites. The mineral is named after Finnish geologist Pentti Eskola.[3]
Production
The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr2O3 in 1838 via a secret process, sold as a pigment.[6] 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:[7]
- Na2Cr2O7 + S → Na2SO4 + Cr2O3
The oxide is also formed by the decomposition of chromium salts such as chromium nitrate, or by the exothermic decomposition of ammonium dichromate.
- (NH4)2Cr2O7 → Cr2O3 + N2 + 4 H2O
The reaction has a low ignition temperature of less than 200 °C and is frequently used in “volcano” demonstrations.[8]
Applications
Because of its considerable stability, chromia is a commonly used pigment. It was originally called viridian. It is used in paints, inks, and glasses. It is the colorant in "chrome green" and "institutional green." Chromium(III) oxide is a precursor to the magnetic pigment chromium dioxide, by the following reaction:[7]
- Cr2O3 + 3 CrO3 → 5 CrO2 + O2
Along with many other oxides, it is used as a compound when polishing (also called stropping) the edges of knives, razors, surfaces of optical devices etc. on a piece of leather, balsa, cloth or other material. It is available in powder or wax form, and in this context it is known as "green compound".
It is used as a component of refractories due to its high melting point.
From 1962, it has been used as an inert marker in livestock intake research. However, its use in animal science research has been diminishing with the increased use of Titanium dioxide due to the latter being consider more food safe, allowing for the public sale of animals used in research trials that use an inert digestion marker.[9]
Reactions
Chromium(III) oxide is amphoteric. Although insoluble in water, it reacts with acid to produce salts of hydrated chromium ions such as [Cr(H2O)6]3+.[10] It is also attacked by concentrated alkali to yield salts of [Cr(OH)6]3−.
When heated with finely divided carbon or aluminium, it is reduced to chromium metal:
- Cr2O3 + 2 Al → 2 Cr + Al2O3
Unlike the classic thermite reaction involving iron oxides, the chromium oxide thermite creates few or no sparks, smoke or sound, but glows brightly. Because of the very high melting point of chromium, chromium thermite casting is impractical.
Heating with chlorine and carbon yields chromium(III) chloride and carbon monoxide:
- Cr2O3 + 3 Cl2 + 3 C → 2 CrCl3 + 3 CO
Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment:
- 2 Cr2O3 + 4 MO + 3 O2 → 4 MCrO4
See also
- Chromium(II) oxide
- Chromium(IV) oxide
- Chromium trioxide
- Chromic acid
- Green pigments
- List of inorganic pigments
- Refractory
References
- ↑ Abdullah, M. M.; Rajab, Fahd M.; Al-Abbas, Saleh M. (2014). "Structural and optical characterization of Cr2O3 nanostructures: Evaluation of its dielectric properties". AIP Advances (AIP Publishing) 4 (2): 027121. doi:10.1063/1.4867012. ISSN 2158-3226. Bibcode: 2014AIPA....4b7121A.
- ↑ 2.0 2.1 2.2 NIOSH Pocket Guide to Chemical Hazards. "#0141". National Institute for Occupational Safety and Health (NIOSH). https://www.cdc.gov/niosh/npg/npgd0141.html.
- ↑ 3.0 3.1 "Eskolaite". Webminerals. http://webmineral.com/data/Eskolaite.shtml.
- ↑ J.E Greedan, (1994), Magnetic oxides in Encyclopedia of Inorganic chemistry R. Bruce King, Ed. John Wiley & Sons. ISBN:0-471-93620-0
- ↑ A. F. Holleman and E. Wiberg "Inorganic Chemistry" Academic Press, 2001, New York. ISBN:0-12-352651-5.
- ↑ Eastaugh, Nicholas; Chaplin, Tracey; Siddall, Ruth (2004). The pigment compendium: a dictionary of historical pigments. Butterworth-Heinemann. p. 391. ISBN 0-7506-5749-9. https://archive.org/details/pigmentcompendiu00east_788.
- ↑ 7.0 7.1 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. doi:10.1002/14356007.a07_067
- ↑ "Ammonium dichromate volcano". http://www.rsc.org/learn-chemistry/resource/res00001709/ammonium-dichromate-volcano.
- ↑ Williams, C.H. (1962). "The determination of chromic oxide in feces samples by atomic absorption spectrophotometry". Journal of Agricultural Sciences 59: 381-385.
- ↑ R. Scholder "Sodium Hexahydroxochromate(III)" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 2, 1688ff.
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