Reversible systems Potential sweep methods

The scan rate is an important parameter for potential sweep methods such as CV or LSV The current is proportional to the square root of the scan rate in all electrochemical systems—irreversible, reversible, and quasi-reversible systems. Figure 4.4 shows the LSV for EMI—TFSl using a glassy carbon (GC) [49]. Note in this figure... 

A useful adjunct of linear potential sweep methods is called cyclic voltammetry. Rather than stopping an oxidative voltammogram at, say, + 0.8 V, the potential is reversed and scanned backward, i.e., a triangular wave potential is applied. The oxidation product formed is present at and close to the electrode surface. With fairly rapid potential sweeps (ca. >4 V/min) it is almost completely re-reduced back to the starting material on the reverse potential sweep. Figure 14B shows a typical cyclic voltammogram for a reversible system (solid line). The ratio of forward to reverse peak currents is unity. If, however, some rapid process removes the product(s), litde or no reverse current is obtained (dotted lines of Fig. 14B). This happens if the overall oxidation is totally irreversible, or fast chemical reactions intervene. We will also see later that a peculiar property of very small electrodes can eliminate most of the reverse current in a cyclic voltammogram. 

Chronopotentiometry at a dme appeared to be impossible until Kies828 recently developed polarography with controlled current density, i.e., with a current density sweep. He explained the method as follows. The high current density during the first stage of the drop life results in the initiation of a secondary electrolysis process at a more negative electrode potential followed by a reverse reaction with rapid (reversible) systems because of the increase in the electrode potential. 

As a final test system, a linear sweep response of a reversible redox couple is simulated by methods EX, CN, RK2 and UNEQ. Here, we have a choice of test quantity the peak current (normalised peak G) or the potential at which this appears. Oldham (1979) supplies accurate values G 0.44629 at -1.1090 RT/nF units or -28.49/n mV for a sweep starting at large positive potentials. It appears that peak potential is the more sensitive quantity and is a better evaluation criterion both are used here. In all programs, the peak potential is computed by parabolic interpolation between the three current/potential points around the peak. The 6-point current approximation was used and sweeps started at +8 normalised potential (RT/nF) units. The sweep was terminated when the peak was reached. In the system equations, time was normalised by the time taken for the linear sweep to move through one RT/nF unit, thus making the results independent of sweep speed. 

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