Interconversion of CrO₂ formed by decomposition of chromium(III) nitrate nonahydrate

The products of the decomposition of chromium(III) nitrate nonahydrate (CNN) and interconversions occurring in the system CrO₂CrOOH have been investigated using thermal analysis, mass spectrometry, X-ray powder diffraction, and magnetic resonance. The studies indicate that changes in the conditions of CNN calcination, especially in the temperature, greatly influence the ratio of Cr(IV) to Cr(III) oxides in the products. CrO₂ formed during slow heating to ca. 350°C is very stable and its complete transformation into Cr₂O₃ requires a temperature higher than 600°C. The rapid crystallization of the amorphous products of CNN calcination, occurring at ca. 380°C, leads to the decomposition of all intermediates during this strongly exothermal reaction, finally resulting in Cr₂O₃. The interconvertability of the components of the redox cycle CrO₂CrOOH was proven experimentally. Due to its unique characteristics, this redox cycle is suggested for the difficult identification of CrO₂ in amorphous chromium-oxygen systems.