Chromic Oxides

compounds of chromium and oxygen, such as CrO, Cr₂O₃, CrO₂, and CrO₃.

Chromous oxide, CrO, is a black crystalline compound, with a melting point of 1550°C. A strong reducing agent, it is insoluble in water and in hot concentrated HCl and H₂SO₄. The hydrate of chromous oxide, Cr(OH)₂, is oxidized upon dehydration to chromic oxide, Cr₂O₃. Chromous oxide is produced by the decomposition of chromium hexacarbonyl, Cr(CO)₆, at 300°C in a vacuum. It has no practical use.

Chromic oxide, Cr₂O₃, is a dark-green crystalline compound, with a density of 5.21 g/cm³ and a melting point of 1990°C. It is insoluble in water and has amphoteric properties. Upon melting with sulfates of alkali metals, it yields chromium sulfate, while upon melting with alkalies, it yields chromites. Chromic oxide corresponds to the hydroxide Cr₂O₃·6H₂O; other hydrated forms may also be obtained, for example, Cr₂O₃·5H₂O and Cr₂O₃·7H₂O. Chromic oxide is the final product of the thermal decomposition of most chromium compounds. In industry, it is produced by the decomposition of chromium trioxide, CrO₃, at 500°C or by the roasting of dichromates, such as potassium dichromate, K₂Cr₂O₇, and sodium dichromate, Na₂Cr₂O₇, with charcoal. Chromic oxide is the starting material in the aluminothermic production of chromium. It is also used in the production of lightfast paints and colored glass and ceramics. In addition, it is used as a polishing material and as a catalyst in inorganic and organic synthesis—in dehydrogenation, aromatization, hydration, and cracking.

Chromium dioxide, CrO₂, is a black crystalline compound, with a density of 4.8 g/cm³. It is produced by heating CrO₃ or chromyl chloride, CrO₂Cl₂, to 360°–400°C at high pressures in an oxygen atmosphere.

Chromium trioxide, or chromic anhydride, CrO₃, is a dark-red crystalline compound, with a density of 2.8 g/cm³ and a melting point of 196°C. It is a hygroscopic compound and disperses in air. With water, it forms chromic acids. Chromium trioxide is a strong oxidizing agent. It is produced by the action of H₂SO₄ on sodium dichromate, Na₂Cr₂O₇ or, less frequently, potassium dichromate, K₂Cr₂O₇. It is used in the electrolytic production of chromium and in chromium plating. Chromium trioxide, like other Cr(VI) compounds, is toxic.