ABSTRACT
Figure 22. Variations in the current trace in a potentiostatic experiment for an EC sequence for different values of the dimensionless chemical rate constant k0 (numbers on the curves).
the corresponding i - E curve* (Fig. 23b) obtained for the reduction process in Eq. (221). Eye inspection of the cyclic voltammogram in Fig. 23b shows the presence of two current peaks, in contrast to the monotonous variations of the current in Fig. 21c. These peaks result from the existence of two opposite effects that successively control the magnitude of the current. For example, for the cathodic scan in Fig. 23, the gradual decrease in R concentration at the electrode surface, when the potential is made more and more cathodic, tends to increase the current by making the R concentration profile steeper. Y ct the propagation of the perturbation into the solution, as shown in Fig. 21 b, tends to flatten the concentration profile, which results in a progressive decay of the current. In practice it is easily understood that the large variations in the surface concentrations for potentials in the vicinity of E0 overwhelm the diffusion effect, the current then tending to rise. Yet when [R]x=O is close to zero, the potential variations affect the current magnitude less than the diffusion propagation in the solution, which explains why the current progressively decays. For the backward scan the same phenomenology applies to the product concentration [P]x=O, which then results in the appearance of an inverted peak on the lower trace in Fig. 23b. When P reacts chemically, the magnitude of the reverse peak gradually decreases (Fig. 23c) because there is less and less P present in the solution. Again this allows determination of mechanisms and their pertinent rate constants, as explained for the double-step chronamperometric method. From this brief presentation it is seen that an important aspect of cyclic voltammetry is that the shapes of cyclic voltammograms are extremely indicative of the chemical processes occurring at the electrode or in the solution. As such it is an extremely useful tool for kinetic diagnosis. Yet the much more complicated shapes, when compared with those in Fig. 22, for example, makes quantitative information on the current difficult to obtain .
