ECE vs DISP 1 Voltammetry of Fluorescein

Explain why double potential step chronoamperometry is able to distinguish between the two mechanisms. Include a sketch of the concentration profiles immediately before the second step. 

In either the ECE or DISP 1 case, the generation of FH is rate-limiting. The current is therefore controlled by the transport of F to the surface and its protonation, but not by the exact mechanism by which the second electron is transferred. This is because in a potential sweep experiment it is impossible to probe the potential region between the reduction potential of FH and the reduction potential of F, under conditions where FH is present in solution. 

If FH reacts principally by heterogeneous electron transfer at the electrode surface (ECE), its concentration will be depleted there, but depending on the rate at which it is generated, some of the radical anion may have diffused away from the electrode before protonation. Therefore, FH is also generated further from the electrode and will diffuse back towards it to react. 

By comparison, if the disproportionation reaction between F and FH is fast (DISP 1), the radical FH will be consumed as soon as generated whether close to the electrode or not, since the presence of F is a pre-condition for its formation, and therefore it will rapidly disproportionate with a further molecule once formed. 

Consequently, at some short time after a potential step to a potential where the reduction of fluorescein is fast, there is likely to be some FH present in solution if the disproportionation reaction is inactive (ECE), but not if this reaction is fast (DISP 1). On a further potential step to an intermediate potential where the reduction of FH is still driven, but now the oxidation of F back to F is driven, the current wiU be purely oxidative in the absence of FH, but will be compensated by some reductive current if this species is present. 

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