Chromium Dioxide, CrO₂

Chromium Dioxide, CrO₂, or chromic chromate, Cr₂O₃.CrO₃, may be obtained6 by heating the sesquioxide in air or oxygen; or by interaction of chromic hydroxide and chromic acid, or of a chromic salt and a chromate. Thus:

Cr₂(SO₄)₃ + K₂CrO₄ + 2H₂O = Cr₂O₃.CrO₃ (or 3CrO₂) + K₂SO₄ + 2H₂SO₄.

This reaction may be compared with the following:

3MnSO₄ + 2KMnO₄ + 2H₂O = (MnO₃)₃.Mn₂O₇ (or 5MnO₂)

+ K₂SO₄ + 2H₂SO₄.

The dioxide is also formed by suitable reduction of chromates, for example, by means of sodium hyposulphite, alcohol, nitric oxide, or a mixture of nitric and oxalic acids; or by interaction of chromic chloride and bleaching powder.

When prepared by the first method, the product is a light, hygroscopic, black powder, which at a red heat evolves oxygen and leaves a residue of the sesquioxide. It is only slightly attacked by hydrochloric or hydriodic acids, and possesses the properties of a dioxide, O=Cr=O, rather than those of a chromate. Prepared by wet methods, it is a brown powder from which soluble CrO₃ can be removed by repeated washing with water, leaving a residue of insoluble Cr₂O₃, and which dries at 250° C. to a black hygroscopic powder. The latter, when heated with hydrochloric acid or in a current of hydrogen chloride, evolves chlorine.

When an aqueous solution of chromic nitrate is treated with ammoni a, a dark brown, almost black precipitate is produced, analyses of which point to the formula Cr₂(OH)₈.H₂O, or CrO₃.Cr₂O₃.7H₂O. Its behaviour on drying is more readily explained by the first formula, since it loses 1H₂O over sulphuric acid, 2H₂O when heated at 105° C., and 6H₂O at 205° C.; however, Jovitschitsch favours the second formula as more nearly representing its chromium content. It is an unstable body and loses chromium trioxide on repeated washing with warm water. Pure chromium hydroxide free from alkali may be obtained from it by boiling its solution in concentrated hydrochloric acid with alcohol, until reduction is complete, and then adding ammonia in excess. Anhydrous chromium chromate cannot be obtained from it by heating, since it decomposes before the last molecule of water is removed.

The readiness with which chromium chromate is converted by ammonia, alkali carbonate, or hydroxide, or by boiling water, into chromium hydroxide and chromate is considered to be evidence of its salt-like constitution.