The Influence of pH on Cyclic Voltammetry

For the following reaction schemes, describe how the voltammetric response for the redox reactions will vary as a function of pH over the full aqueous pH range (0-14). Assume that the rate of electron transfer is fast, such that the electrode kinetics are fully reversible, and that the protonation/deprotonation steps are so fast as to be equilibrated throughout. Note that the chemical steps have been written as deprotonations so that the equilibrium constants are readily related to the pKa of the species. 

For system (i), below pH 11 the reaction involves the transfer of one proton and one electron, as the A formed is protonated to give AH. Hence the voltammetric peaks will shift with approximately 59 mV per pH (at 25°C). Above pH 11 the reaction does not involve proton transfer as A is not protonated at these pH values. Consequently, the peak position is invariant with pH. 

For system (ii), below pH 3 species A is protonated in solution. Hence the voltammogram shows the reduction of AH+ to AH. No protons are transferred during the electrochemical process and hence the peak does not shift with pH. Above pH 3, species A is present in solution and hence for it to be reduced, it must first be protonated. Consequently, the voltammetric peak will shift with approximately 59 mV per pH (at 25 C). 

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