It is found that sometimes a relatively less stable oxidation state undergoes an oxidation−reduction reaction in which it is simultaneously oxidised and reduced. This is called disproportionation.
For example,
(a) \[CrO_4^{3 – } + 8{H^ + } \to 2CrO_4^{2 – } + C{r^{3 + }} + 4{H_2}O\]
Cr(V) is oxidized to Cr(VI) and reduced to Cr(III).
(b) \[3MnO_4^{2 – } + 4{H^ + } \to 2MnO_4^ – + Mn{O_2} + 2{H_2}O\]
Mn (VI) is oxidized to Mn (VII) and reduced to Mn (IV).