Reversible electron transfer cyclic voltammetry

FIGURE 1.25. Successive reversible electron transfers in cyclic voltammetry of attached reactants. Normalized charge (a) and current (b) as a function of the separation between the standard potentials, at 25°C, from right to left A ° — E — E = 0.4, 0.1, 0.0356, —0.2 V. The middle of each curve corresponds to — )/2. (c) Variation of the normalized peak current with AE° in the range where a single wave is observed. 

FIGURE 1.26. a Successive reversible electron transfers in cyclic voltammetry as a function of the separation between the standard potentials, at 25°C, from right to left AE° =... 

If kfchemical reaction is very slow compared to the intervention times of cyclic voltammetry) the response is very similar to that of a simple reversible electron transfer and occurs at the formal potential, E°, of the couple Ox/Red. 

In the case of dissociative electron transfer to aromatic compounds, electron transfer is not necessarily concerted with bond dissociation. The substrate 7t-radical-anion may be an intermediate whose existence can be demonstrated by fast scan cyclic voltammetry in aptotic solvents. At fast scan rates, reversible electron transfer occurs. At slower scan rates, die anodic peak height falls and a second reversible electron transfer step appears due to formation of the radical-anion of the compound formed by replacement of the substituent by hydrogen. Cleavage of the... 

Cyclic Voltammetry. However, experimental use of this technique has been restricted almost exclusively to the analysis of the limiting currents of the signals obtained. One reason for this could be that when a quasi-reversible electronic transfer is analyzed in RPV, two very close waves are obtained, which are difficult to resolve from an experimental viewpoint. This problem can be eliminated by using the triple pulse technique Reverse Differential Pulse Voltammetry (RDPV), proposed in references [80, 84, 85] and based in the application of the waveform presented in Scheme 4.5. 

ErevCirrev diagnostics in cyclic voltammetry — Assuming that the product (R) of an electrochemically reversible electron transfer reaction (E step) is involved in additional irreversible chemical reaction (C step), according to the scheme ... 

Randles-5>evcik equation — An equation introduced by - Randles [i] and - Sevcik [ii] describing the magnitude of the voltammetric peak current /p (in - linear scan voltammetry or in - cyclic voltammetry) for a reversible electron transfer ( rev mechanism -> Erev diagnostics in cyclic voltammetry). 

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. 

First, we consider cyclic voltammetry and focus on the non-turnover experiment in which there is no substrate in solution. As the potential is scanned, electrons transfer back and forth between the electrode and the redox-active site(s), producing a current peak in each direction. These two peaks constitute the signal . Provided the scan rate is slow enough to equilibrate all the processes required in the redox reaction, the signal obtained will be as predicted by the Nemst equation that holds for a reversible electron-transfer process. The current peaks will be compact and... 

With faster scan cyclic voltammetry, a new two-electron anodic peak was detected, at more negative potentials, for the first stage of the oxidation process, with an accompanying cathodic peak on the reverse scan (11). The ratio of the forward to the reverse peak currents increased towards unity as the scan rate was raised to —200 V s 1 (Fig. 15). This behavior was attributed to the initial two-electron process being accompanied by a fairly rapid follow-up chemical reaction and was successfully analyzed in terms of an EqCi process (quasi-reversible electron transfer followed by a first-order irreversible chemical process), with a rate constant for the chemical step, k, = 250 s 1. 

In a recent report, we described (15) how encapsulating the YBa Cu O- pellets in an epoxy matrix effectively fills the outer pores ana diminishes the problem with large background currents. These electrodes undergo reversible electron transfers with electroactive solutes in a variety of nonaqueous solvents, as observed by cyclic voltammetry. This reversible voltammetry was used as a phenomenological method for estimating the lifetime of such electrodes, in a variety of corrosive media, based on monitoring the voltammetric response as a function of exposure time to corrosive media. 

When the formal potentials of the adsorbed and non-adsorbed redox couples are similar, a single wave is obtained where the contribution of the electron transfer involving the adsorbed species increases with the scan rate. Thus, there is a transition from diffusional-shaped voltammograms at slow scan rates to adsorptive-shaped at fast scan rates. To understand this behaviour and illustrate the characteristics of adsorptive voltammograms let us consider the response in cyclic voltammetry of a monolayer of species A that undergoes a one-electron, fully reversible electron transfer ... 

Redox-active species that are immobilized on the electrode and undergo simple reversible electron transfer give rise to a peak-type cyclic voltammetry response that is not influenced by diffusion effects, but which is, instead, much more sensitive to the characteristic properties of the protein [1]. Provided the sample is homogeneous and there are no interactions between molecules in the layer, the peaks for oxidation and reduction should have the Nernstian characteristics defined by Laviron that is, they comprise finite passed charge with no tailing current, with a peak separation close to zero [29]. This is depicted in Fig. 3(a), where the capacitive background that is observed in real experiments is not shown. The peak widths... 

In cyclic voltammetry, an ideal (homogeneous) immobilized layer of molecules undergoing a reversible electron transfer that is uncomplicated by coupled chemical reactions gives a pair of reduction and oxidation peaks that are compact at all scan rates (the half-height width for a simple n-electron transfer is approximately... 

Cyclic voltammetry (CV) can provide information about the thermodynamics of the redox process, kinetics of heterogeneous electron transfer reactions and coupled chemical reactions [32]. The reversible electron transfer steps inform us about the compound s ability to accept electrons however, experimental conditions, such as solvent and temperature also influence the voltammogram. The structure of the lowest unoccupied molecular orbital (LUMO) levels of the compound can be determined from the number of CV waves and reduction potentials ( 1/2)- Moreover, the CV can serve as a spectroscopy as demonstrated by Heinze [32], since the characteristic shapes of the waves and their unequivocal positions on the potential scale are effectively a fingerprint of the individual electrochemical properties of the redox system. 

In theory, D, c, or n can be calculated from the peak current (Ip) of a dc linear-sweep or cyclic voltammogram by employing the relationships described by the Randles-Sevcik equation for a reversible electron-transfer process of the kind Ox+ne" Red, when the effect of is negligible [6]. When woiking in a viscous RTIL, however, very few of the electrochemical systems of interest fulfill these criteria, which severely limits the direct application of dc voltammetry for this purpose. Assuming the Randles-Sevcik equation is not applicable, a best-fit approach with numerical simulation may be a viable alternative, although even this method has its limitations, as mechanistic complexities or other uncertainties can make the modeling process difficult [10]. 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

On the basis of theoretical calculations Chance et al. [203] have interpreted electrochemical measurements using a scheme similar to that of MacDiarmid et al. [181] and Wnek [169] in which the first oxidation peak seen in cyclic voltammetry (at approx. + 0.2 V vs. SCE) represents the oxidation of the leucoemeraldine (1 A)x form of the polymer to produce an increasing number of quinoid repeat units, with the eventual formation of the (1 A-2S")x/2 polyemeraldine form by the end of the first cyclic voltammetric peak. The second peak (attributed by Kobayashi to degradation of the material) is attributed to the conversion of the (1 A-2S")x/2 form to the pernigraniline form (2A)X and the cathodic peaks to the reverse processes. The first process involves only electron transfer, whereas the second also involves the loss of protons and thus might be expected to show pH dependence (whereas the first should not), and this is apparently the case. Thus the second peak would represent the production of the diprotonated (2S )X form at low pH and the (2A)X form at higher pH with these two forms effectively in equilibrium mediated by the H+ concentration. This model is in conflict with the results of Kobayashi et al. [196] who found pH dependence of the position of the first peak. 

Cyclic voltammetry, square-wave voltammetry, and controlled potential electrolysis were used to study the electrochemical oxidation behavior of niclosamide at a glassy carbon electrode. The number of electrons transferred, the wave characteristics, the diffusion coefficient and reversibility of the reactions were investigated. Following optimization of voltammetric parameters, pH, and reproducibility, a linear calibration curve over the range 1 x 10 6 to 1 x 10 4 mol/dm3 niclosamide was achieved. The detection limit was found to be 8 x 10 7 mol/dm3. This voltammetric method was applied for the determination of niclosamide in tablets [33]. 

The first reports on direct electrochemistry of a redox active protein were published in 1977 by Hill [49] and Kuwana [50], They independently reported that cytochrome c (cyt c) exhibited virtually reversible electrochemistry on gold and tin doped indium oxide (ITO) electrodes as revealed by cyclic voltammetry, respectively. Unlike using specific promoters to realize direct electrochemistry of protein in the earlier studies, recently a novel approach that only employed specific modifications of the electrode surface without promoters was developed. From then on, achieving reversible, direct electron transfer between redox proteins and electrodes without using any mediators and promoters had made great accomplishments. 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

The two cyclic voltammograms shown in Fig. 13 of [Scm(LBu2)] (b) and Scln(LMe-)] (a) show an important feature. Whereas the cyclic voltammetry (CV) of the former compound displays three reversible one-electron transfer waves, the latter shows only two irreversible oxidation peaks. Thus methyl groups in the ortho- and para-positions of the phenolates are not sufficient to effectively quench side reactions of the generated phenoxyls. In contrast, two tertiary butyl groups in the ortho- and para-positions stabilize the successively formed phenoxyls, Eq. (5)... 

The current responses may be displayed as a function of time, as in Figure 1.1c, or as a function of potential, as in Figure 1.1c. The latter presentation is generally preferred and is what is meant in short by the phrase cyclic voltammetry. The fact that the response is symmetrical about the potential axis provides a clear indication of the reversibility of the system, in both the chemical sense (the electron transfer product is chemically stable) and the electrochemical sense (the electron transfer is fast). If the electron transfer product were unstable, the anodic current would be less than the cathodic current, eventually disappearing for high instabilities. For a slow electron transfer and a chemically stable product, the current-potential pattern is no longer symmetrical about the vertical axis, the anodic peak potential being more positive than the cathodic peak potential. 

As mentioned in the introduction to controlled potential electrolysis (Section 2.3), there are various indirect methods to calculate the number of electrons transferred in a redox process. One method which can be rapidly carried out, but can only be used for electrochemically reversible processes (or for processes not complicated by chemical reactions), compares the cyclic voltammetric response exhibited by a species with its chronoamperometric response obtained under the same experimental conditions.23 This is based on the fact that in cyclic voltammetry the peak current is given by the Randles-Sevcik equation ... 

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