Effect of Concentration on Electrode Potentials

It is obvious from the Nemst Equation 7-5 that the value of the electrode potential of a redox couple will depend on the values of the activities of the reaction components. This means of course that changes in the concentrations of any of the components will likewise affect the value of the electrode potential. The precise way in which the concentration affects the potential is related to the type of half-cell reaction involved, i.e., in the half-cell reactions 

Another type of concentration dependence occurs in the half-cell reactions  

It is useful to visualize the way in which the electrode potential of a redox couple varies with the concentrations of the reaction components. A logarithmic diagram analogous to the type used in acid-base systems can be constructed by using the Nemst equation written in the format of Equation 7-1. 

To follow the analogy further, let us define a set of fractions for the redox system. These will be analogous to the a values used previously for acid-base and metal complexation, but to avoid confusion they will be designated as f. In the usual meaning of a, the sum of the f values of all of the species in the system equals unity. Thus, in the Fe -Fe system, f3 + f2 = l(the subscript denotes the oxidation state. 

Compare the forms of fs in Equation 7-21 with the as for the acetic acid system Equation 4-16. 

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