Chromium(III) oxide

Chromium(III) oxide is the inorganic compound of the formula Cr₂O₃. It is one of principal oxides of chromium, it is widely used precursor to other chromium compounds.

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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 an amphoteric oxide, dissolving in acids to give chromium(III) salts and in molten alkali to give chromites.

Production

The Parisians Pannetier and Binet first prepared Cr₂O₃ in 1838 via a secret process. It is manufactured from the mineral chromite, Fe(CrO₂)₂, which is mined in southern Africa, Asia, Turkey, and Cuba. The conversion of chromite to chromia entails air oxidation to Na₂Cr₂O₇, which is subsequently reduced with carbon or sulfur.

Chromium oxide can be converted into elemental chromium metal through the thermite reaction, although this metal oxide is much less commonly used than Fe₂O₃ and Fe₃O₄. Unlike iron oxide thermites, chromium oxide thermite creates little or no sparks, smoke or sound, but it glows with a blinding light. Because of the very high melting point of chromium, chromium thermite casting is impractical.

Pigment

It is commonly called chrome green or institutional green when used as a pigment; however it was referred to as viridian when it was first discovered.

References

• A. F. Holleman and E. Wiberg "Inorganic Chemistry" Academic Press, 2001, New York.

See also

• Chromium(II) oxide
• Chromium(IV) oxide
• Chromic acid