Dependence of Potential on pH

Owing to the high charge of the anions, the effect of ionic strength is also large (E° = 0.46 V for 0.1 M solutions of both potassium ferrocyanide and ferricyanide), and the potential shift is in a direction opposite that observed for the aquo Fe(III)-Fe(II) couple because the more highly charged species is the reductant rather than the oxidant. 

Whenever hydrogen or hydroxyl ions appear in the half-reaction, the electrode potential varies with pH. Consider the reaction 

The relation between potential and pH often is depicted graphically in the form of Pourbaix or predominance-region or potential-pH diagrams. To illustrate 

FIGURE 12-1 Simplified predominance-region or potential-pH diagram for the system Fe, Fe(II), Fe(III). Left, predominant species present right, type of reaction occurring in passing from one region to another. 

Potential-pH diagrams are often convenient as an aid in predicting the direction and course of redox reactions. If two such diagrams are superimposed, the system of higher potential at any pH will act as an oxidant. If the lines of two systems intersect, the direction of reaction may be reversed by a change in pH. As an example, a comparison of the iodine-iodide and As(V)-As(III) tems (Section 19-5) shows that iodine acts as an oxidant to produce As(V) at higher pH values and As(V) acts as an oxidant to produce iodine at low pH values. 

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The individual lines in Fig. 14.1 are developed by assuming a reaction and calculating the dependence of potential on pH and concentration, using published free energy data. For example, line 1 refers to the equilibrium (10) ... 

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