Cathodic-reactant reduction polarization curves

Fig. 6.2 Schematic experimental polarization curves (solid curves) assuming Tafel behavior for the individual oxidation and cathodic-reactant reduction polarization curves (dashed curves)...

Knowledge of the parameters of the individual electrode reactions permits writing expressions for the individual oxidation or reduction curves (see the section Complete Polarization Curves for a Single Half-Cell Reaction in Chapter 3). Thus, the expression for the cathodic-reactant reduction reaction ... 

Twin polarized platinum microelectrodes are conveniently used for endpoint detection for oxidation-reduction titrations. Consider a titration curve for oxidation-reduction titration where both reactants behave reversibly at the electrodes. An example of this kind of titration is titration of iron(II) with cerium(IV) (Fig. 14A). At the starting point of the titration, no current is observed because no suitable cathode reactant is available. With addition of cerium(IV), a mixture of iron(II) and iron(III) is produced, which permits the passage of current. Beyond the midpoint in the titration, iron(III) becomes in excess, and the current is then regulated by decreasing iron(II) concentration. At the equivalence point, the current approaches zero because iron(III) are present, and the applied potential is not great enough to cause these to react at the electrode. Beyond the equivalence point, the current rises again because both cerium(III) and cerium(IV) are present to react at the electrodes. 

The above relationship is equally applicable if either the metal oxidation-rate curve or the reduction-rate curve for the cathodic reactant does not obey Tafel behavior. To illustrate this point, three additional schematic pairs of individual anodic and cathodic polarization curves are examined. In Fig. 6.3, the metal undergoes active-passive oxidation behavior and Ecorr is in the passive region. In Fig. 6.4, where the total re-... 

The fact that for the same electrode potential the work functions for a metal in solution are the same for different metals was directly proved by Pleskov and Rotenberg in photoemission experi-ments[34,35] (see also [36]). The same result was also obtained for a dark cathodic generation of electrons[37,38] (see [39] for a review). From the above reasoning, it is clear that the nature of the metal of an electrode may influence the activation energy of the process only when the reactants are adsorbed at the electrode (see (1.32)). This conclusion was convincingly confirmed by the experiments of Frumkin and Nikolaeva-Fedorovich on reduction of a number of anions after corrections had been made for i/ j-effects, the polarization curves for various electrode materials coincided[40-42] (see [43] for a review). 

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