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.
REFERENCESRode, T. V. Kislorodnye soedineniia khroma i khromovye katalizatory. Moscow, 1962.
See also references under .
A. B. SUCHKOV