Cathodic-reactant reduction polarization

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-... 

When the mass-transport process cannot meet the demand for reactant, the IK drop in Equation 22-16 becomes smaller than the theoretical value, and a diffusion overvoltage appears that just offsets the decrease in H. Here, we consider an electrolytic cell to which we apply a negative voltage to produce a reduction at the cathode. We assume that the anode is nonpolarized, Thus, with the appearance of concentration polarization, Equation 22-16 becomes... 

The adsorption of ions and molecules on the surface of mercury electrodes is a thoroughly investigated phenomenon [51 ]. Surface-active substances are either electroactive [52] or electroinactive [53]. The former can be analyzed by adsorptive stripping voltammetry [54]. This is the common name for several electroanalytical methods based on the adsorptive accumulation of the reactant and the reduction, or oxidation, of the adsorbate by some voltammetric technique, regardless of the mechanisms of the adsorption and the electrode reaction [55, 56]. Frequently, the product of the electrode reaction remains adsorbed to the electrode surface. Hence, the term stripping should not be taken literally in all cases. Besides, some adsorbates may be formed by electrosorption reactions, so that their reduction includes covalently bound mercury atoms. The boundary between adsorption followed by reduction, on the one hand, and electrosorption, on the other, is not strictly defined. Moreover, it is not uncommon that, upon cathodic polarization, the current response is caused by a catalytic evolution of hydrogen, and not by the reduction of the adsorbate itself [57]. However, what is common to all methods is a hnear relationship between the surface concentration of the adsorbate and the concentration of analyte at the electrode surface ... 

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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