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.

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. 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. 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:

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:

Cr₂O₃ + 3 CrO₃ → 5 CrO₂ + O₂

References

See also

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

External links

• National Pollutant Inventory: Chromium (III) compounds fact sheet