acidified KMnO4,K2Cr2O7

Use the hydrogen ion to combine with the oxygen of the complex ion, to release the core metal atom, e.g. if MnO4- is no acidified the resltant substance of the reaction is MnO2 the oxidation number only change from +7 to +4 and the reaction potential is lesser than +7 to +2 which Mn2+ is formed in acidified mixture.