Anodic current:
This causes anodic current (i → k). The anodic current rapidly increases until the surface concentration of [FeII(CN)6]4- is diminished, resulting the current to peak (j). The current then decays (j → k) as the solution around the electrode is depleted of [FeII(CN)6]4-. The first cycle is done when the potential reaches back to + 0.80 V. Now the cyclic voltammogram is obtained. That is clear in which any potential positive of ~0.40 V would be appropriate as the initial potential in the reduction of [FeIII(CN)6]3- would not occur when the potential is applied. This process avoids inadvertent electrolysis as a result of applying the initial potential. In the forward scan [FeII(CN)6]4- is electrochemically generated from [FeIII(CN)6]3- as indicated by the cathodic current while in the reverse scan [FeII(CN)6]4- is oxidized back to [FeIII(CN)6]3- as indicated by the anodic current. A more detailed understanding can be obtained by considering the Nernst equation and the changes in concentration that occur in solution adjacent to the electrode during electrolysis. The potential excitation signal depends on the ratio of [FeIII(CN)6]3- /[FeII(CN)6]4- at the electrode surface as described by Nernst equation for a reversible system:
E =Eº([FeII(CN)6]4-,[FeIII(CN)6]3- )+ 0.0595/1 log [FeIII(CN)6]3- /[FeII(CN)6]4-
where E0′ is the formal reduction potential of the sample. An initial value of E which is sufficiently positive of E0′ maintains a ratio in which [FeIII(CN)6]3- greatly predominates. Thus, application of +0.80 V at the initial potential causes negligible current. However, as E is scanned negatively, conversion of [FeIII(CN)6]3- to [FeII(CN)6]4- by reduction is mandatory for satisfaction of the Nernst equation. The ratio of iron redox states that must exist at the electrode surface at several potentials during the scan is shown on the lower horizontal axis in Figure. The logarithmic relationship between E and [FeIII(CN)6]3- /[FeII(CN)6]4- is reflected by a rapid rate of change in the region where E = Eo i.e., [FeIII(CN)6]3- /[[FeII(CN)6]4- = 1. This causes the diametric rise in cathodic current (b → d) during the forward scan.