Effects of pH on redox reactions

What are the effects of pH on redox reactions?

pH affects the strength of an oxidising agent, for example, the oxidising strength of potassium manganate (VII), as measured by its standard electrode potential, is as follows:

Acidic medium

MnO₄^– + 8H⁺+ 5e^– → Mn²⁺ + 4H₂O               E^o_red  = +1.51 V

Neutral/slightly basic medium

MnO₄^– + 2H₂O + 3e^– → MnO₂ + 4OH^–              E^o_red  = +1.23 V

Strongly basic medium

MnO₄^– + e^– → MnO₄^2-               E^o_red  = +0.56 V

Hence, potassium manganate (VII) is most strongly oxidising in an acidic medium and least oxidising in a strongly basic medium. Note that the manganate (VI) ion is stable only in very basic conditions. It disproportionates in acidic, neutral and slightly basic conditions as follows:-

3MnO₄^– + 4H⁺ → 2MnO₄^– + MnO₂ + 2H₂O

 

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