A comparison of the multistep consecutive reduction mode with the multicomponent system reduction mode in cyclic voltammetry
Introduction
The cyclic and linear sweep voltammetry is a very important tool for both analytical and mechanism-revealing purposes. In this paper, we present a new way of analysis of cyclic voltammetry (CV) curves in order to reveal their subtle structures and simultaneously to pass from experimental consecutive reduction to simulated multicomponent system reduction. The transformation seems to be of particular importance for multi-electron reactions studied by voltammetry methods where one-electron steps are very close to each other and unrevealed. The electroreduction of intermediates is displayed on the transformed voltammogram. After the transformation, advantages of simulation are particularly well seen. The problem of multistep charge transfer (kinetic problem) overlaps the problem of multicomponent charge transfers (analytical problem) (Bard and Faulkner, 1980). The aim of this paper is an attempt of unification of both reduction modes. The procedure presented and the example helps to understand the relationship between the consecutive multistep reduction mode and multicomponent reduction mode. It shows how to pass from the consecutive multistep reduction, interesting for kinetic investigations, to the multicomponent reduction interesting for analytical purposes. In practice we have a consecutive experimental CV multielectron curve and we are interested to see the respective CV curve of multicomponent system. Not everyone seems aware that on the consecutive reduction voltammogram, the peak potential of an intermediate rarely presents its true value as for the single free substance in a solution. It takes place even for two-electron peaks numerically resolved into one-electron ones (Sanecki and Kaczmarski, 1999, Sanecki and Kaczmarski, 2001). When kh,n+1>kh,n, Ep does not reflect the real electrochemical properties of the intermediate. After transformation into the multicomponent system, the picture corresponds to that of a single intermediate reduced separately or to that of a mixture of all species occurring in the sequence. Peak potentials (naturally or numerically resolved) of all species correspond to that of pure substances.
The first simple example of such a transformation was shown in (Sanecki, 2001) and now another example as well as more general approach is presented.
The case considered here is an example of reductive cleavage of halogen in derivatives analogous to those investigated recently in detail (see Andrieux et al., 1997, Antonello and Maran, 1999, Savéant, 1994)
Section snippets
Kinetics
The following consecutive ECE–ECE reaction sequence is consideredwhere kA=k1, kC=k2, kD=k3, kF=k4 are the heterogeneous rate constants (cm s−1) and kfBC=kf1, kfEF=kf2 are the chemical rate constants (s−1). The numerical treatment of the kinetic sequence was described in Sanecki and Kaczmarski (1999). Further information on the subject can be found (e.g. Speiser, 1996, Bott, 1997, Bott et al., 1996, Alden and Compton, 1997a, Alden and Compton, 1997b,
Results
An ECE–ECE reductive process of cleavage of SF bond in m-benzenedisulfonyldifluoride m-C6H4-(SO2F)2 (m-BDF), investigated and modeled in Savéant (1994) was selected as an example to be presented. It involves two separate two-electron stages (Andrieux et al., 1997). Each two-electron stage consists of one-electron step followed by another one and separated by the chemical step:
The sequence (Eq. (2)) corresponds to the part and again
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