Voltammogram rate-independent

For example, for two well-separated waves (AE < 0), is small, and reaction (12.3.39) lies to the left (i.e., the comproportionation of A and C dominates the disproportionation of B). Thus, at potentials of the second wave, C diffusing away from the electrode can reduce A diffusing towards it, so that the concentration profiles of A, B, and C are perturbed from those that would exist if the solution phase reaction did not occur. However, for the Ej-Ej. reaction scheme, the observed voltammogram is independent of the rates of the forward and back reactions in (12.3.39), because, at any given potential, the average oxidation state in any layer of solution near the electrode remains the same (53). At potentials of the second wave, species A, which would take two electrons, is removed by the comproportionation reaction, but two B molecules are produced, and each of these would take one electron for no net change. This is not true, however, if the heterogeneous rate constants for the electron transfer are slow (Section 12.3.7) or in ECE reactions (Section 12.3.8). 

As expected for reversible processes, values obtained from the cyclic voltammograms are independent of the scan rates (0.1-0.7 V s . Fig. 3.8) and concentrations (3. 5 mmol/kg) studied. The peak current for the reduction of DmCc varies linearly with the square root of the scan rate (0.1-0.7 V s ), characteristic of diffusion-controlled electrode processes [65]. 

The cyclic voltammograms of ferrlcyanlde (1.0 mM In 1.0 M KCl) In Fig. 2 are Illustrative of the results obtained for scan rates below 100 mV/s. The peak separation is 60 mV and the peak potentials are Independent of scan rate. A plot of peak current versus the square-root of the scan rate yields a straight line with a slope consistent with a seml-lnflnlte linear diffusion controlled electrode reaction. The heterogeneous rate constant for the reduction of ferrlcyanlde was calculated from CV data (scan rate of 20 Vs using the method described by Nicholson (19) with the following parameter values D 7.63 X 10 cm s , D, = 6.32 X 10 cm s, a 0.5, and n =1. The rate constants were found to be... 

If A" is small, as in the preceding case, an easily recognizable S-like curve voltammogram is obtained having a limiting current independent from the scan rate ... 

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