Nonselective Interface with High Rct

Even for uni-univalent electrolytes, the solution of these equations leads to intractable algebra and some drastic assumptions have to be made in order to obtain an approximate solution (Cammann, 1985). Here we show the origin of the mixed potential graphically (Fig. 6.7). 

The mixed potential accounts for a large portion of reported artifacts in the unorthodox potentiometric sensors, particularly biosensors, and can be rightfully called evil potential . The physical origin of such artifacts can be illustrated using a simple example. Let us assume that a multiple electron transfer takes place simultaneously at the interface of a lump of Zn immersed in dilute HC1. Because this metal is not externally connected the net current is zero. The redox reactions taking place are as follows. 

In this experiment, the dissolution of Zn is accompanied by the evolution of hydrogen. 

Let us now add a small amount of Cu2+ ions. They adsorb at the Zn surface and catalyze the reduction reaction (6.26). The ic will increase (Fig. 6.8b), resulting in the shift of the Emix in the positive direction. (Note Nernst potential should not shift with adsorption or with change of the heterogeneous rate constant for the interfacial process ) 

Therefore, changes of zero-current potential with stirring, adsorption, or changes in the electrode surface are unmistakable signs of mixed potential and can serve as the first warning of experimental artifact. 

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