Chromium(III) oxide

Chromium(III) oxide
Other names Chromium sesquioxide Chromia Chrome green Eskolaite
Identifiers
CAS number	1308-38-9 Y
PubChem	517277
ChemSpider	451305 Y
UNII	X5Z09SU859 Y
ChEBI	CHEBI:48242 Y
RTECS number	GB6475000
Jmol-3D images	Image 1
SMILES O=[Cr]O[Cr]=O
InChI InChI=1S/2Cr.3O Y Key: QDOXWKRWXJOMAK-UHFFFAOYSA-N Y InChI=1/2Cr.3O/rCr2O3/c3-1-5-2-4 Key: QDOXWKRWXJOMAK-LUXALHLMAR
Properties
Molecular formula	Cr2O3
Molar mass	151.99 g/mol
Density	5.22 g/cm3
Melting point	2435 °C, 2708 K, 4415 °F
Boiling point	4000 °C, 4273 K, 7232 °F
Solubility in water	insoluble
Solubility in alcohol	soluble in acids
Refractive index (nD)	2.5
Y oxide (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(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 nature, it occurs as the rare mineral eskolaite.

Structure and properties

Cr₂O₃ adopts 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 8-8.5).^[1] It is antiferromagnetic up to 307 K, the Neel temperature.^[2][3] It is not readily attacked by acids or bases, although molten alkali gives chromites (salts with the Cr
2O2−
4 anion, not to be confused with the related mineral chromite). It turns brown when heated, but reverts to its dark green color when cooled. It is also hygroscopic.

Occurrence

Eskolaite mineral

Cr₂O₃ occurs naturally in 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. ^[1]

Production

The Parisians Pannetier and Binet first prepared the transparent hydrated form of Cr₂O₃ in 1838 via a secret process, sold as a pigment.^[4] 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:^[5]

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.^[6]

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 a 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:^[5]

Cr₂O₃ + 3 CrO₃ → 5 CrO₂ + O₂

It is one of the materials that are used when polishing the edges of knives on a piece of leather (also called stropping). In this context it is often known as green compound.

Reactions

Chromium(III) oxide is amphoteric. Although insoluble in water, it dissolves in acid to produce hydrated chromium ions, [Cr(H₂O)₆]³⁺. It dissolves in concentrated alkali to yield chromite ions. When heated with finely divided aluminium or carbon, it is reduced to chromium metal and aluminum oxide:

Cr₂O₃ + 2 Al → 2 Cr + Al₂O₃

Heating with chlorine and carbon yields chromium(III) chloride:

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

References

1. ^ ^{a b} "Eskolaite". Webminerals. http://webmineral.com/data/Eskolaite.shtml. Retrieved 2009-06-06. 
2. ^ J.E Greedan, (1994), Magnetic oxides in Encyclopedia of Inorganic chemistry R. Bruce King, Ed. John Wiley & Sons. ISBN 0471936200
3. ^ A. F. Holleman and E. Wiberg "Inorganic Chemistry" Academic Press, 2001, New York. ISBN 0-12-352651-5.
4. ^ Eastaugh, Nicholas; Chaplin, Tracey; Siddall, Ruth (2004). The pigment compendium: a dictionary of historical pigments. Butterworth-Heinemann. p. 391. ISBN 0750657499. 
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.
6. ^ Ammonium dichromate volcano Retrieved 2009-06-06.