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.
3CrO43+ + 8H+ → 2CrO42- + Cr3++ 4H2O
Cr(V) Cr(VI) Cr(III)
Cr(V) is oxidized to Cr(VI) and reduced to Cr(III).
(ii) 3MnO42- + 4H+ → 2MnO4- + MnO2 + 2H2O
Mn(VI) Mn(VII) Mn(IV)
Mn (VI) is oxidized to Mn (VII) and reduced to Mn (IV).