Controlled potential quasireversible

The oxidation product obtained by controlled potential electrolysis, [Ni(3,5-Cl2saloph)] +, shows an EPR spectrum typical of a Ni(III) ion (d7 - low spin) having an octahedral coordination, which is attributed to the axial coordination of two solvent molecules.153 Therefore, from a speculative viewpoint, it could be assumed that the electrochemical quasireversibility is due to the change in coordination from square planar to octahedral. 

The solid complex is diamagnetic and not air-sensitive. The cyclic voltammo-gram in acetonitrile shows two quasireversible waves at 0.67 V (A ), = 70 mV) and 0.86 V (A p = 80 mV) versus SSCE, and controlled potential coulometry at 1.0 V shows that each quasireversible wave corresponds to an overall two-electron process for the Fe /Fe process." The IR spectrum (KBr disk) has three j/cN bands at 2083, 2112, and 2129 cm. ... 

In Chap. 5, the two-species cases were described for the explicit method. Here we add those for the implicit case. Both Dirichlet and derivative boundary conditions are of interest, the latter both with controlled current or quasireversible and systems under controlled potential. 

The physical meaning of the kinetic parameter m is identical as for surface electrode reaction (Chap. 2.5.1). The electrochemical reversibility is primarily controlled by 03 (Fig. 2.71). The reaction is totally irreversible for log(m) < —3 and electrochemically reversible for log(fo) > 1. Between these intervals, the reaction appears quasireversible, attributed with a quasireversible maximum. Though the absolute net peak current value depends on the adsorption parameter. Fig. 2.71 reveals that the quasireversible interval, together with the position of the maximum, is independent of the adsorption strength. Similar to the surface reactions, the position of the maximum varies with the electron transfer coefficient and the amphtude of the potential modrrlation [92]. 

One of the first reports on the quasireversible electrochemistry of redox proteins appeared in 1977 when Eddowes and Hill demonstrated (10) cyclic voltammetry of horse heart cytochrome c at a gold electrode in the presence of 4,4 -bipyridyl (Bipy) in solution. In the voltammo-grams (Fig. 1), the peak-to-peak separations were close to 60 mV and the faradaic currents varied linearly with (scan rate), indicating a quasireversible one-electron transfer process controlled by linear diffusion of redox species to the electrode surface. The midpoint potential... 

Equation 5.5.24 is a very compact representation of the way in which the current in a step experiment depends on potential and time, and it holds for all kinetic regimes reversible, quasireversible, and totally irreversible. The function Fi(A) manifests the kinetic effects on the current in terms of the dimensionless parameter A, which can be readily shown to compare the maximum current supportable by the reductive kinetic process at a given step potential FAkfC% vs. the maximum current supportable by diffusion at that potential [/d/(l+ )] Thus a small value of A implies a strong kinetic influence on the current, and a large value of A corresponds to a situation where the kinetics are facile and the response is controlled by diffusion. The function Fi(A) rises monotoni-cally from a value of zero at A = 0 toward an asymptote of unity as A becomes large (Figure 5.5.2). 

There is a significant contrast here with Section 5.4.2(e), where we found that the results for reversible systems observed at spherical electrodes could be extended generally to electrodes of other shapes. This is true for a reversible system because the potential controls the surface concentration of the electroactive species directly and keeps it uniform across the surface. Mass transfer to each point, and hence the current, is consequently driven in a uniform way over the electrode surface. For quasireversible and irreversible systems, the potential controls rate constants, rather than surface concentrations, uniformly across the surface. The concentrations become defined indirectly by the local balance of interfacial electron-transfer rates and mass-transfer rates. When the electrode surface is not uniformly accessible, this balance varies over the surface in a way that is idiosyncratic to the geometry. This is a complicated situation that can be handled in a general way (i.e., for an arbitrary shape) by simulation. For UME disks, however, the geometric problem can be simplified by symmetry, and results exist in the literature to facilitate the quantitative analysis of voltammograms (12). 

For quasireversible (Butler-Volmer) kinetics at the tip and a diffusion-controlled mediator regeneration at the substrate, one can obtain an approximate equation for the tip current at any potential assuming uniform accessibility of the tip surface (i.e., the current density is uniform over the tip surface) [17] ... 

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