The value of EM2+∕M∘ for given metal ions are EMn2+∕Mn∘=−1.18V, ECr2+∕Cr∘=−0.9V, EFe2+∕Fe∘=−0.44V and EC02+∕Co∘=−0.28V. The correct order of EM2+∕M∘ values without considering negative sign would be Mn2+>Cr2+>Fe2+>Co2+