Chromium(III) oxide

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 are not readily attacked by acids or bases. Molten alkalis react with chromium(III) oxide to form chromates or chromites. 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, usually containing traces of calcium carbonate.This material, however, may be 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

• Make chromium metal
• Make sodium chromate

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.