Chromium(III) oxide

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OTC Cr2O3 sold as pigment. It is impure, and also has other compounds.

Chromium(III) oxide, also known as chrome green, chromia, or chromium sesquioxide is the inorganic compound with chemical formula Cr2O3.

Properties

Chemical

Chromium(III) oxide reacts with acids to form chromium(III) salts, though samples that have been heavily annealed at high temperature, such as commercial varieties, are not readily attacked by acids or bases. Molten alkalis react with chromium(III) oxide to form chromates or chromites. Molten potassium nitrate reacts with it to form potassium dichromate. Chromium(III) oxide can be used to produce a low-energy thermite with aluminum or magnesium powder.

Physical

Chromium(III) oxide is a dark green powder and is slightly hygroscopic. It turns brown when heated, but reverts to its dark green color when cooled.

Availability

Chrome green is available at pottery shops as a green pigment, sometimes with traces of calcium carbonate. This material, however, is generally too unreactive to produce other chromium compounds.

Preparation

Chromium(III) oxide can be prepared by reducing potassium dichromate with sulfur:

K2Cr2O7 + S → K2SO4 + Cr2O3

It can also be prepared from the thermal decomposition of ammonium dichromate:

(NH4)2Cr2O7 → Cr2O3 + N2 + 4 H2O

or by reacting chromium trioxide with ethanol.

It is the most common byproduct of the reduction of chromium(VI) compounds, especially in neutral solution.

Projects

Handling

Safety

Chromium(III) oxide is not extremely reactive. If ingested it may cause nausea, vomiting and diarrhea. As it stains, gloves and protective clothing should be worn.

Storage

Chromia doesn't require any special storage.

Disposal

While not as toxic as chromium(VI) compounds, it's best to avoid dumping it in the environment.

References

Relevant Sciencemadness threads