Two-electron electrochemical reaction

Two-Electron Electrochemical Reactions at Different Electrode Geometries. 180... 

The treatment of non-reversible two-electron electrochemical reactions is much more complex than the reversible one. The general scheme of the process is... 

Equation (6.15) is now applied to SWV in order to characterize a two-electron electrochemical reaction. The effects of the difference between the formal potentials, the frequency, the square wave potential, and the staircase potential of the SWV are discussed and procedures for the determination of the formal potentials of both electrochemical reactions are proposed. 

The behavior of the forward, 7f, reverse, 7r, and net, 7SW, currents of a two-electron electrochemical reaction with A= —200 mV is shown in Fig. 7.30 where a whole square wave cycle with f sw = 50mV is applied to spherical electrodes of different sizes. 

The effect of frequency on the SWV voltammograms of a two-electron electrochemical reaction is shown in Fig. 7.31 for different AEf1 values at T = 298 K. The responses for disc (solid line) and (hemi)spherical (dashed line) microelectrodes calculated from Eq. (7.65) are considered. 

When the formal potential of the first step is much more positive than that of the second, AE f < — 200mV (Fig. 7.31c), the intermediate species 02 is stable and two well-separated peaks are obtained, centered on the formal potential of each process and with the features of the voltammograms of one-electron electrochemical reaction. When the A7 ° value increases, the stability of the intermediate decreases and the two peaks are closer, and the transition from two peaks to a single peak is observed (AE —71.2 mV). Eventually, when the formal potential of the second electron transfer is much more positive than that of the first one, AE > 200 mV, the characteristics of the voltammograms are those of an apparently simultaneous two-electron electrochemical reaction (Fig. 7.31a). Note that the... 

As in the case of a reversible one-electron electrochemical reaction, the halfpeak width of the SWV does not depend on the electrode geometry. For a two-electron electrochemical reaction, Wy2 is only a function of the difference between formal potentials AE f and of the square wave amplitude sw. The evolution of the half-peak width Wy2 with AEcspherical electrodes has been plotted in Fig. 7.32. These curves give a very general criterion for the characterization of the EE process through Wy2. 

In order to analyze the influence of the chemical kinetics on the SWV response of this mechanism when the chemical reaction behaves as irreversible (Keq —> oo), it can be compared with that obtained for a reversible two-electron electrochemical reaction (EE mechanism) at the same values of the difference between the formal potentials of the electrochemical steps, A= E 2 — E (which is always centered atE-mA 1L = (E +E 2)/2). 

Calculate the value of K for a two-electron electrochemical reaction at 25°C in which... 

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