Cyclic voltammetry coupled homogeneous reactions

Cyclic voltammetry is a powerful tool for investigating electrode processes involving coupled homogeneous reactions. We exemplify with the EC mechanism ... 

Voltammetric methods produce current-voltage curves with features characteristic of the reaction mechanism and kinetic conditions. Combining this with the ease of changing the waveform parameters, cyclic voltammetry is nearly always the first technique used to study a new systan. It is particularly usefril for assessing reaction mechanisms, even when there are additional complications such as coupled homogeneous reactions, or surface adsorption. These techniques also provide quantitative information as will be shown below with a selection of theoretical expressions. 

PROCESSES WITH COUPLED HOMOGENEOUS REACTIONS 15.4.1 Linear sweep and cyclic voltammetry... 

Linear scan voltammetry (LSV) and cyclic voltammetry (CV) (see Chapter 11) are among the most common electrochemical techniques employed in the laboratory. Despite their utility, however, they are not particularly well suited to careful measurements of diffusion coefficients when using electrodes of conventional size. We will briefly discuss techniques for measuring D with LSV and CV, but the reader should be cautioned that these measurements under conditions of planar diffusion (i.e., at conventional electrodes) are probably useful to only one significant digit, and then only for nemstian systems with no coupled homogeneous reactions and with no adsorption. For more reliable results with LSV and CV, UMEs should be used. 

As with the other reaction schemes involving the coupling of electron transfer with a follow-up homogeneous reaction, the kinetics of electron transfer may interfere in the rate control of the overall process, similar to what was described earlier for the EC mechanism. Under these conditions a convenient way of obtaining the rate constant for the follow-up reaction with no interference from the electron transfer kinetics is to use double potential chronoamperometry in place of cyclic voltammetry. The variations of normalized anodic-to-cathodic current ratio with the dimensionless rate parameter are summarized in Figure 2.15 for all four electrodimerization mechanisms. 

SEV is an effective means of probing homogeneous chemical reactions that are coupled to electrode reactions, especially when it is extended to cyclic voltammetry as described in the next section. Considerable information can be obtained from the dependence of ip and Ep on the rate of potential scan. Figure 3.20 illustrates the behavior of ip and Ep with variation in scan rate for a reversible heterogeneous electron transfer reaction that is coupled to various types of homogeneous chemical reactions. The current function j/p is proportional to ip according to the equation... 

As discussed in Sects. 3.4 and 4.5, electrode processes coupled with homogeneous chemical reactions are very frequent and their study is of interest in many applied fields, such as organic electrosynthesis, ecotoxicity, biosciences, environmental studies, among others [15-17]. In this section, multipulse techniques (with a special focus on Cyclic Voltammetry) are applied to the study of the reaction kinetics and mechanisms of electrogenerated species. 

Simulations that have been carried out for a vast number of different rate parameters and mechanistic variants show that only the slow second-order coupling steps between dimers and the subsequent slow elimination of protons can explain the trace-crossing (Fig. 2). In any case, the so-called nucleation loop in cyclic voltammetry is not the result of a nucleation step, but of slow, homogeneous follow-up reactions of soluble oligomers in the diffusion layer. 

Many interesting processes occurring at the liquid/liquid interface involve coupled homogeneous chemical reactions. In principle, electrochemical methods used for probing complicated mechanisms at metal electrodes (61) can be employed at the ITIES. However, many of these techniques (e.g., rotating ring-disk electrode or fast-scan cyclic voltammetry) are hard to adapt to liquid/liquid measurements. Because of technical problems, few studies of multistep processes at the ITIES have been reported to date (1,62). 

Kong et al. [90] applied the electrochemical approach to the study of a two-phase azo coupling facilitated by reverse PTC. Cyclic voltammetry and chronoamperometry were employed to evaluate quantitatively the rate constants for the reaction. The process was interpreted in terms of an EC mechanism, i.e., diffusion-controlled electrochemical charge transfer followed by a homogeneous chemical reaction. The authors highlighted the usefulness of this approach based on the factors that enable the estimation of the contributions of the chemical reaction, mass transfer, partitioning, and the adsorption of reactants at the interface to the overall two-phase reaction. 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

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... 

Low amount of theoretical, and maybe even less amount of experimental work has been made on double potential step chronoamperometry that, similarly to cyclic voltammetry (see below) is classified as a reversal technique after the forward step potential, WE is polarized at a value at which the electrogenerated species is reoxidized to the starting one. Such a technique is quite effective in studies of electrode reaction mechanisms. As an example, very accurate quantitative data about the kinetics accounting for the stability of electrogenerated species can be gained. However, the issue of how the data should be treated in order to obtain similar information about different homogeneous kinetics coupled to the charge transfer is far beyond the scope of the present book. 

The situation is more complex with the Fe(III)/(Fe II) and Fe(II)/Fe(I) waves which show coupling of the electron transfer with homogeneous ligation/deligation reactions in a number of cases. Cyclic voltammetry and thin-layer spectroelectrochemistry were then used jointly for obtaining the characteristic standard potentials and equilibrium constants as well as the rate constants for the complexation of iron(II) by chloride ions. The procedures employed in this purpose have been described in detail , requiring in several cases an extension of the available theory2 26 of... 

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