Second-order following reaction, scanning

As for first-order following chemical reactions, if the dimerization reaction (that is a second-order reaction) is slow (i.e. k2 is small), or if the scan rate is very high, only the reversible electron transfer is effectively active. 

However, if the redox couples Ox/Red and Ox /Red have sufficiently different standard potentials, can be also calculated using the working curve reported in Figure 16. In fact, considering the process simply as a reversible electron transfer followed by an irreversible first-order chemical reaction (see Section 1.4.2.2), one measures only the current ratio /pr//pf of the first couple Ox/Red. Obviously, the return peak must be recorded before the second process begins to appear this means that the direction of the potential scan must be reversed immediately after having traversed the first forward peak. 

Typical steady-state tip and substrate current approach curves for the oxidation of different concentrations of ArCT are shown in Figure 23. A general observation is that as the concentration of ArCT increases, the tip and substrate currents—at a particular distance—decrease, due to the second-order nature of the follow-up chemical reaction. The experimental approach curves are shown alongside theoretically derived curves for a spread of normalized rate constants, K2, from which it can be seen that there is reasonable agreement between the observed and predicted trends. From measurements of both feedback currents, for all three ArCT concentrations investigated, and collection efficiencies, for the lowest two concentrations, a radical dimerization rate constant of 1.2 ( 0.3) X 10s M 1 s 1 was determined (5), which was in reasonable agreement with that determined earlier using fast scan cyclic voltammetry (36). 

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