Preliminary studies by cyclic voltammetry

For preliminary work, a circular disc electrode with a diameter in the 1-5 mm range is usually used. The substrate concentrations are typically between 1 and 10 mM and the voltage sweep rates are usually between 50 mV s 1 and 500 V s 1. Sometimes the voltage sweep is continued to include several E — t half-cycles. 

Examples of experimental voltammograms are shown in Figs 6.22-6.24. These are all from our current research on the oxidation of substituted pyrroles [39,42]. Numerous other examples may be found in the literature [1, 2]. The voltammograms shown are recorded with a potentiostat without electronic compensation of the solution resistance in order to illustrate better some of the problems caused by the solution resistance. The peak potentials below are given with a precision of 10 mV, which is typical for values reported in the literature. These values originate from the data files and cannot be determined with this precision directly from the figures reproduced below. 

The first example (Fig. 6.22) shows the voltammogram resulting from the oxidation of 2,3,4,5-tetraphenylpyrrole (2345tpp) at v = 0.2 V s-1. An oxidation peak, Ol, with °x = 0.95 Vis observed during the potential sweep in the positive direction, and a reduction peak, Rl, with ped = 0.88 Vis observed during the potential sweep in the negative direction. 

When the sweep in the positive direction is extended to higher potentials, the further oxidation of the radical cation to the dication is observed at Ep = 1.63 V (02). In this case, the current corresponding to the reduction of the dication back to the radical cation is not observed, indicating that the dication undergoes a (fast) chemical reaction. The voltammogram provides no information of the fate of the dication. 

In addition to investigating the effect of changing the voltage sweep rate and of addition of nucleophiles/bases (oxidation) or electrophiles/acids (reductions), the preliminary work often also includes investigation of how the voltammograms are affected by changes in the substrate concentration. 

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