Cyclic voltammetry kinetic potential shift

If k[ = n-F-v/R-T (i.e. if the chemical complication is neither too slow nor too fast and, consequently, the kinetics of the chemical complication are of the same order as the time scale of cyclic voltammetry) the potential of the forward peak, which has been localized at more anodic potentials than E0 by the chemical complication, shifts towards less anodic values with the scan rate according to the relationship ... 

In the electrochemical case, using, for example, cyclic voltammetry, one way of driving the potential toward more negative values is to increase the scan rate. This is true whether the linearization procedure or the convolution approach is followed. In the first case, equation (3.4) shows that the activation free energy at the peak, AG, is a decreasing function of the scan rate as a result of the kinetic competition between electron transfer and diffusion. The larger the scan rate, the faster the diffusion and thus the faster the electron transfer has to be in order to compete. This implies a smaller value AG, which is achieved by a shift of the peak potential toward more negative values. 

The simplest way of generating and observing aryl halide anion radicals is to use an electrochemical technique such as cyclic voltammetry. With conventional microelectrodes (diameter in the millimetre range), the anion radical can be observed by means of its reoxidation wave down to lifetimes of 10" s. Under these conditions, it is possible to convert, upon raising the scan rate, the irreversible wave observed at low scan rates into a one-electron chemically reversible wave as shown schematically in Fig. 9. Although this does not provide any structural information about RX , besides the standard potential at which it is formed, it does constitute an unambiguous proof of its existence. Under these conditions, the standard potential of the RX/RX " couple as well as the kinetics of the decay of RX-" can be derived from the electrochemical data. Peak potential shifts (Fig. 9) can also be used... 

Cyclic voltammetry is a well-adapted electrochemical technique to elucidate the mechanism and kinetics of reversible hydrogen storage. An example of voltammetry characteristics, using a microporous activated carbon cloth (ACC) from viscose (ACC . S BEX = 1390 m2 g-1) in 3 mol L 1 KOH, is shown in Figure 8.17. The minimum potential is shifted of -100 mV for each cycle, i.e., toward hydrogen evolution. 

The thermochemical cycle in Scheme 4 can be used to estimate the effect of one-electron oxidation on metal-hydride acidities. The method is analogous to one that has been extensively used to investigate organic cation radicals [10c]. Eq. 29 shows that measurements of °ox(MH) and °ox(M ) provide relative p a data for metal hydrides and their cation radicals. Absolute values for p a(MH +) are obtained if the acidities of the neutral hydrides are known. The oxidation potentials of 18-electron hydrides can be readily obtained by cyclic voltammetry. In our experience, the waves that are obtained are frequently chemically irreversible, even at rather high scan rates. Consequently, the oxidation peak potentials will be kinetically shifted and represent minimum values for the true °ox(MH) data, the estimates for p a(MH +) represent maximum values, and calculated Ap a are minimum values. 

Kinetic Currents. In kinetic currents, the limiting current is determined by the rate of a chemical reaction in the vicinity of the electrode, provided this precedes the cell reaction. Electrochem-ically inactive compounds are converted into reducible or oxidizable forms (time-dependent protonation and deprotonation processes, formation and decomposition of complexes, etc.). Conversely, during a chemical reaction after the cell reaction, the product of the electrode reaction is converted to an electrochemically inactive form without influence on the current. However, owing to the changed equilibria between the concentrations of the oxidized and reduced forms at the electrode surface, the half-wave and peak potentials are shifted (Section 25.2.1). In evaluating kinetic effects, cyclic voltammetry can be helpful (Section 25.2.4). 

Fukuzumi reported a detailed mechanistic study of quinone reduction catalyzed by protonated amino acids [245]. Kinetic experiments, EPR spectroscopy and cyclic voltammetry were employed to illustrate the role protonated histidine plays in mediating electron transfer between NADH analog 9,10-dihydro-10-methy-lacridine (AcrH2) and l-(p-tolylsulfinyl)-2,5- benzoquinone (TolSQ). Cyclic voltammetry experiments demonstrate a 0.55-V positive shift in the one-electron reduction potential of TolSQ in the presence of 5.0 x 10 M of protonated... 

---