Quasi-reversible reactions, cyclic

The reversibility of electrochemical reactions is determined by the reaction rates for a reversible reaction, k° > 0.3 v /2 cm/s, for a quasi-reversible reaction, 0.3 v172 > k° > 2 x 10 V12 cm/s, while for an irreversible reaction, k° < 2 x KTV° cm/s. Figure 1.16 shows the cyclic voltammograms for irreversible and quasi-reversible redox processes. 

For non>Nemstian systems the shape of the cyclic voltammogram changes. For the irreversible case the forward peak ceases to be symmetric, and of course there is no reverse peak. For quasi-reversible reactions there will be a reverse peak but both peaks will be asymmetric and the peak potentials will not be coincident. There is insufficient space here to consider these systems more fully, but further details can be found in the literature [12,13]. 

In case of quasi-reversible systems, the cyclic voltammograms show considerably different behavior from their reversible counterparts. Figure 3 shows the voltammogram for a quasi-reversible reaction for different values of the reduction and oxidation rate constants. 

Fig. 8 Typical cyclic voltammograms of pure electron transfer reactions (a) effect of quasi-reversibility ks decreases from solid to dashed line) (b) effect of relative values of...

Macrocyclic N-donor ligands. Nickel complexes of macrocyclic ligands have been studied by cyclic voltammetry, and the irreversible or quasi-reversible couples Ni" L Ni L Ni L have been established. The structure of (124) has been reported and the co-ordination is essentially square-planar with a slight tetrahedral distortion. The reaction of [Ni(pn)3] with... 

Electron transfer reactions are classified as reversible, quasi-reversible or irreversible depending on the ability of the reaction to respond to changes in E, which, of course, is related to the magnitude of k°. The distinction is important, in particular, for the (correct) application of linear sweep and cyclic voltammetry, and for that reason further discussion of this classification will be postponed until after the introduction of these techniques in Section 6.7.2. 

Thus, in order to obtain a sensitive response in SCV, it is necessary to decrease the pulse time length, t, to transform the response to quasi-reversible or irreversible. This behavior contrasts with that observed in Cyclic Voltammetry for which a current-potential response is obtained even for fast charge transfer reactions (see Sect. 6.4.2). 

Cyclic Square Wave Voltammetry (CSWV) is very useful in determining the reversibility degree and the charge transfer coefficient of a non-Nemstian electrochemical reaction. In order to prove this, the CSWV curves of a quasi-reversible process with Kplane = 0.03 and different values of a have been plotted in Fig. 7.17. In this figure, we have included the net current for the first and second scans (Fig. 7.17b, d, and f) and also the forward, reverse, and net current of a single scan (first or second, Fig. 7.17a, c, e) to help understand the observed response. 

The electrochemistry of both rans-dioxoosmium(VI) and rutheni-um(VI) macrocyclic tertiary amine complexes has been studied in great detail. Both trans-[RuVI(b)2(0)2]2+ and rans-[RuVI(14TMC)(0)2]2 + and its related complexes display similar cyclic voltammograms in aqueous solution (132, 212). At pH < 7, three reversible/quasi-reversible redox couples, corresponding to the redox reactions are observed ... 

The general behavior expected in LSV may then be computed by digital simulations [20] and the results can be conveniently visualized in a zone diagram as shown in Figure 9. For log 2 < — 1, the dimerization reaction has no effect whatsoever, and the cyclic voltammetric behavior pertains to the reversible, quasi-reversible (QR), or totally irreversible charge transfer (IR), depending on the value of A. For very large... 

Likewise, Komorsky-Lovric et al. investigated the behavior of lutetium bisphtha-locyanine with the voltammetry of microparticles [108]. This solid-state reaction (which may be studied with either square-wave or cyclic voltammetry) was shown to proceed via the simultaneous insertion/expulsion of anion ions. The oxidation was found to have quasi-reversible characteristics in electrolyte solutions containing perchlorate, nitrate, and chloride, whereas bromide and thiocyanate... 

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. 

---