Predominance Areas of a Redox Couple

Predicting redox reactions with the aid of graphical means involves the knowledge of the predominance areas of the redox couples that participate in the equilibria. 

The predominance-areas diagram in Fig. 15.1 comes from the above relations. 

The example of the couple Fe +/Fe + is particularly simple for several reasons. First, both members of the couple are in solution. Second, the exponents of the activities of both members are the same in the corresponding Nernst equation. Finally, there is no supplementary species (with respect to both members of the couple) in the argument of the logarithm. All these reasons lead to the fact that, in this example, we do not have to adopt some conventions fixing one (or several) concentration(s) in order to limit the predominance areas. This is not the case with the following examples. 

The numerator and the denominator of the argument of the logarithm do not have the same exponent. In order to draw preponderance-area diagrams in such cases, we must adopt a convention concerning the concentrations of the involved species. The literature mentions two conventions that appear with the same frequency, conventions also adopted in the case of the drawing of Pourbaix diagrams (see Sect. 15.3). In the first one, we set 

In this example, both conventions (with the value C = 10 moI/L) give the same frontier straight line. This is not always the case. 

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