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Chromium(III) oxide
Names
	Other names Chromium sesquioxide; Chromia; Chrome green; Eskolaite
Identifiers
CAS Number	1308-38-9 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:48242 ;
ChemSpider	451305 ;
ECHA InfoCard	100.013.783
EC Number	215-160-9;
Gmelin Reference	11116
PubChem CID	517277;
RTECS number	GB6475000;
UNII	X5Z09SU859 ;
UN number	3077
CompTox Dashboard (EPA)	DTXSID4043721 ;
	InChI InChI=1S/2Cr.3O Key: QDOXWKRWXJOMAK-UHFFFAOYSA-N ; InChI=1/2Cr.3O/rCr2O3/c3-1-5-2-4 Key: QDOXWKRWXJOMAK-LUXALHLMAR;
	SMILES O=[Cr]O[Cr]=O;
Properties
Chemical formula	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)
Solubility in water	insoluble
Solubility in alcohol	insoluble in alcohol, acetone, acids
Magnetic susceptibility (χ)	+1960.0×10−6 cm3/mol
Refractive index (nD)	2.551
Structure
Crystal structure	Corundum
Space group	R3c (No. 167)
Lattice constant	a = 495 pm, c = 1358 pm
Thermochemistry
Std molar; entropy (S⦵298)	81 J·mol−1·K−1
Std enthalpy of; formation (ΔfH⦵298)	−1128 kJ·mol−1
Hazards
	GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H317, H319, H360
Precautionary statements	P201, P202, P261, P264, P270, P272, P280, P281, P301+P312, P302+P352, P305+P351+P338, P308+P313, P321, P330, P333+P313, P337+P313, P363, P405, P501
	NIOSH (US health exposure limits):
PEL (Permissible)	TWA 1 mg/m3
REL (Recommended)	TWA 0.5 mg/m3
IDLH (Immediate danger)	250 mg/m3
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Chromium(III) oxide (or chromia) is an inorganic compound with the formula Cr
₂O
₃. 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

Cr
₂O
₃ 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, Cr
₂O
₃ 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

Cr
₂O
₃ 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 Cr
₂O
₃ in 1838 via a secret process, sold as a pigment.^[6] It is derived from the mineral chromite, (Fe,Mg)Cr
₂O
₄. The conversion of chromite to chromia proceeds via Na
₂Cr
₂O
₇, which is reduced with sulfur at high temperatures:^[7]

Na
₂Cr
₂O
₇ + S → Na
₂SO
₄ + Cr
₂O
₃

The oxide is also formed by the decomposition of chromium salts such as chromium nitrate, or by the exothermic decomposition of ammonium dichromate.

(NH
₄)
₂Cr
₂O
₇ → Cr
₂O
₃ + N
₂ + 4 H
₂O

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]

Cr
₂O
₃ + 3 CrO
₃ → 5 CrO
₂ + O
₂

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(H
₂O)
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:

Cr
₂O
₃ + 2 Al → 2 Cr + Al
₂O
₃

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:

Cr
₂O
₃ + 3 Cl
₂ + 3 C → 2 CrCl
₃ + 3 CO

Chromates can be formed by the oxidation of chromium(III) oxide and another oxide in a basic environment:

2 Cr
₂O
₃ + 4 MO + 3 O
₂ → 4 MCrO
₄

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. 4 (2). AIP Publishing: 027121. Bibcode:2014AIPA....4b7121A. doi:10.1063/1.4867012. ISSN 2158-3226.
^ ^a ^b ^c NIOSH Pocket Guide to Chemical Hazards. "#0141". National Institute for Occupational Safety and Health (NIOSH).
^ ^a ^b "Eskolaite". Webminerals. Retrieved 2009-06-06.
^ 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.
^ ^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. doi:10.1002/14356007.a07_067
^ "Ammonium dichromate volcano". www.rsc.org. Retrieved 2019-02-26.
^ 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.