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 Eored = +1.51 V
Neutral/slightly basic medium
MnO4– + 2H2O + 3e– → MnO2 + 4OH– Eored = +1.23 V
Strongly basic medium
MnO4– + e– → MnO42- Eored = +0.56 V
Hence, potassium manganate (VII) is most strongly oxidising in an acidic medium and least oxidising in a strongly basic medium.
The behaviour of MnO4– in different pH conditions is best explained using the the reaction MnO4– + 2H2O + 3e– → MnO2 + 4OH– as a reference. If the pH of this reaction is lowered, excess H+ allows manganese dioxide to be further reduced to the more stable +2 oxidation state ion:
MnO2 + 4H++ 2e– → Mn2+ + 2H2O
Combining the half reactions MnO4– + 2H2O + 3e– → MnO2 + 4OH– and MnO2 + 4H++ 2e– → Mn2+ + 2H2O gives the overall reduction reaction of MnO4– to Mn2+ in acidic medium.
On the other hand, if the pH of the same reaction MnO4– + 2H2O + 3e– → MnO2 + 4OH– is increased, excess OH– will shift the equilibrium towards the left. As the solution becomes more strongly basic, the manganate (VII) ion is instead reduced to the manganate (VI) ion. This implies that the manganate (VI) ion is stable only in very basic conditions. It disproportionates in acidic, neutral and slightly basic conditions as follows:-
3MnO42- + 4H+ → 2MnO4– + MnO2 + 2H2O
Incidentally,
| MnO4– | purple |
| MnO42- | green |
| MnO2 | brown solid |
| Mn2+ | pale pink |