Cyclic voltammetry quasi-reversible systems

Square-wave voltammetry of Osteryoung s type Application of SW techniques Linear sweep and cyclic voltammetry Principles of the linear sweep voltammetry Linear sweep voltammetry of reversible systems Irreversible and quasi-reversible processes Systems of two components and two-step charge transfers Distorting effects in LSV analysis... 

Cyclic Voltammetry, Fig. 3 Cyclic voltammograms for a quasi-reversible system for different values of reduction and oxidation rate constants... 

In contrast to classical cyclic voltammograms, AC-cyclic voltammograms have a clear baseline, which is advantageous for quantitative measurements. By extending AC linear sweep voltaimnetry by a reverse scan, AC cyclic voltammetry is obtained. If the surface concentrations of the electroactive species are the same at the same potential for forward and reverse scans, the peaks for forward and reverse scans are expected to be identical. If the DC process is not fully reversible, the surface concentrations of the electroactive species are different at a given DC potential for forward and reverse scans— that is, for quasi-reversible systems a displacement of the jjeaks for forward and reverse scan can be observed. This displacement can be used to derive kinetic parameters of the electrode reaction. For instance, the derivation (1, p. 393] (or Eq. 5-33) for sluggish one-step heterogeneous quasireversible and/or... 

Obviously, therefore there must be an intermediate case in which the kinetics of both the forward and reverse electron-transfer processes have to be taken account of. Such systems are described as being quasi-reversible and as would be expected, the scan rate can have a considerable effect on the nature of the cyclic voltammetry. At sufficiently slow scan rates, quasi-reversible processes appear to be fully reversible. However, as the scan rate is increased, the kinetics of the electron transfer are not fast enough to maintain (Nernstian) equilibrium. In the scan-rate region when the process is quasi-reversible, the following observations are made. 

All potentials refer to SCE, except for Pd(5-H ) for which the potential refers to Fc" /Fc. The systems are reversible except in few cases indicated by qrev (quasi-reversible) or irr (irreversible). The potentials have been determined by cyclic voltammetry on both Pt and Hg electrodes, leading to identical values, unless otherwise noted. 

Redox potential (V) measured in CH3CN versus the saturated calomel electrode (SCE), determined by cyclic voltammetry at the platinum electrode, 0.1 M (nC4H9)4N. BF4 room temperature under argon scan rate, 100 mV s" the systems are reversible except in a few cases indicated by qrev (quasi reversible) and irr (irreversible). 

A phthalocyanine core was functionalized with four, eight, and 16 ferrocenyl units (compounds 49-51) [67]. Cyclic voltammetry experiments in DMF or CH2CI2 showed the presence of many quasi-reversible waves. All the ferrocenyl units behave independently, giving rise in all three compounds to a single oxidation wave, which is accompanied by one or two oxidation and by two or three reduction processes due to the phthalocyanine rt-system. For 51, the presence of a stripping peak indicates precipitation of the oxidized product in CH2CI2, as often observed in ferrocene-functionalized dendrimers (see Section 9.4.1). 

This equation is often used to determine the formal potential of a given redox system with the help of cyclic voltammetry. However, the assumption that mid-peak potential is equal to formal potential holds only for a reversible electrode reaction. The diagnostic criteria and characteristics of cyclic voltammetric responses for solution systems undergoing reversible, quasi-reversible, or irreversible heterogeneous electron-transfer process are discussed, for example in Ref [9c]. An electro-chemically reversible process implies that the anodic to cathodic peak current ratio, lpa/- pc equal to 1 and fipc — pa is 2.218RT/nF, which at 298 K is equal to 57/n mV and is independent of the scan rate. For a diffusion-controlled reduction process, Ip should be proportional to the square root of the scan rate v, according to the Randles-Sevcik equation [10] ... 

Vitamin B 2 (cyanocob(III)alamin) is an example of a quasi-reversible redox system that exhibits slow heterogeneous electron-transfer kinetics. Cyclic voltammetry alone suggests that the reduction of vitamin B 2 is a single two-electron process at = -0.93 V vs SCE to the Co(I) redox state (Figure lOA). However, thin-layer spectroelectrochemistry using a... 

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