Isosurface concentration voltammetry

We have already mentioned that linear Tafel plots cannot be obtained by simple elimination of one of the exponents in Eq. (5.2) or (5.6), because the remaining EAC surface concentration still depends on the overvoltage. In this connection, the possibilities to carry out experiments at constant EAC concentration have attracted considerable attention. RDE voltammetry making it possible to realize this condition was suggested by Miller et al. [12, 13] and was referred to as isosurface concentration voltammetry (ICV). This method was substantiated theoretically for quasi-reversible reduction in relatively simple systems (metal- metal ions and redox systems) for low overpotentials [12-14] and for the Tafel regions [15,16]. Later, complex systems were considered in this respect [17]. 

Conditions for constant surface concentration can be formulated on the basis of equations describing the mass transport. For instance, it follows from Eq. (3.12) that iS should be kept constant under steady-state conditions. This condition 

An experimental verification of the method under consideration was performed for the Ag Ag(I), NHj system, because certain kinetic data are available in [18]. To prepare working electrodes, silver was deposited on Pt RDE from a cyanide bath. Current density and the rate of rotation varied simultaneously in such way that quantity i/ /i2 remained constant. 

In analyzing the RDE voltammograms, the following equilibria were taken into account NH3 + H NHJ, Ag + NHg Ag(NH3) , Ag + 2NH3 = Ag(NH3)2, for which the logarithms of the constants and 2 respec- 

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Miller, B., Bellavance, M., and Bruckenstein, S. (1971) Application of isosurface concentration voltammetry at a rotating disk electrode to simple electron transfer kinetics. /. Electrochem. Soc., 118 (7), 1082-1089. 

Surface concentrations of complex species and ligands can be determined by means described in Chapter 3 further, the normalized Tafel plots (NTPs) can be obtained. However, initially, we shall make use of isosurface concentration voltammetry presented in Section 5.3.1. Using experimental data obtained at constant surface concentrations, one can estimate the value of without any tentative assumptions as to the mechanism of the charge transfer step, that is, for unknown composition of the EAC. It follows from Eq. (4.3) that for t/r = const, the surface concentrations of the components do not depend on time. Selected in this way, data (Figure 8.7) are satisfactorily approximated by the lines with similar slopes, which yield = 0.37 0.03 (see Eq. (8.17)). This value canbe checked by the analysis of individual transients of the potential, but then the composition of the EAC must be established. 

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