ABSTRACT
The most common and stable oxidation states of chromium (Cr) are +2, +3, and +6. Several Cr(IV) and Cr(V) species are known as intermediates in oxidation-reduction reactions. These species are unstable with respect to disproportionation to Cr(III) and Cr(IV) or to reaction with solvents under most conditions. The most stable and important oxidation state is Cr(III). Cr(III) is amphoteric, forming compounds of the Cr(III) ion with acids and chromites with base, and forms a large number of relatively kinetically substitution inert complexes. The redox potentials of the Cr(III)/Cr and Cr(III)/Cr(II) systems have been investigated by direct cell measurements of many redox reactions of chromium species. Latimer noticed from the disagreement between electrochemical data and thermal data that chromium electrodes are irreversible and relied entirely on the third-law calculation in the compilation of redox potentials. The redox reaction of the Cr(II)/Cr couple is irreversible, so that the thermodynamic properties of Cr(II)(aq) ion rely entirely on the third-law calculation.
