Irreversible electrode reaction cyclic voltammetry

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... 

Although cyclic voltammetry could fruitfully be applied to the kinetic analysis of these catalytic systems, it has mostly been investigated by means of rotating disk electrode voltammetry (Section 1.3.2). The simplest case is that of an irreversible catalytic reaction at a monolayer coating. The next section is devoted to the analysis of these systems by the two techniques. 

The electrode reaction of triamterene 15 was elucidated by means of DCP, Tast polarography, cyclic voltammetry, microcoulometry, controlled potential electrolysis, and spectroscopy (ultraviolet/visible (UVA is), NMR). Two steps of reduction independent of pH were observed two-electron reduction of 15 resulted in the formation of 17. The first reduction wave of 15 was assumed to be due to irreversible two-electron reduction forming unstable 16, which tautomerized to 17, and the second reduction wave was ascribed to two-electron reduction of 17 to the tetrahydro product, 18 (Scheme 2). 

Here, the electrode reaction is followed by a first-order irreversible chemical reaction in solution that consumes the primary product B and forms the final product C. The rate of this chemical reaction can be measured conveniently with cyclic voltammetry, double-potential-step chronoamperometry, reverse pulse voltammetry, etc. However, this is only true if the half-life of B is greater than or equal to the shortest attainable time scale of the experiment. 

It will be clear that cyclic voltammetry is a powerful tool for a first analysis of an electrochemical reaction occurring at the surface of an electrode because it will reveal reversibility. Depending on whether the system is reversible, information will be obtained about half wave potential, number of electrons exchanged in the reaction, the concentration and diffusion coefficient of the electroactive species. However, these data can also be obtained for an irreversible system1113 but, in this case, the equations describing the current-potential curves differ somewhat from Equations 2.21 to 2.27. 

The electrochemical oxidation of NADH in aqueous solutions is seen as a single peak by cyclic voltammetry and takes place at 0.4, 0.7, and IV vs SCE at carbon, Pt, and Au electrodes, respectively (37,38). No re-reduction of NADH related intermediates is observed in cyclic voltammetry even at fast scan rates (30 V/s) (39), reflecting the high chemical irreversibility of the reaction. It was early recognized that the oxidation of NADH resulted in electrode fouling, necessitating... 

In general the electrochemical stability of an electrolyte is experimentally evaluated by means of cyclic voltammetry. However, the determination of the electrochemical windows exhibits several problems. First, the electrochemical degradation or breakdown of an electrolyte is an irreversible reaction, thus there is no theoretical redox potential [40, 41], Passivation of the electrodes often makes it difficult to identify the onset of the reaction due to inhibition of further reactions [40, 42],... 

Fig. 9.10. EC mechanism in cyclic voltammetry (reversible electrode reaction and irreversible homogeneous reaction), (a) Plot of the ratio /p,a//p,cl with lg r) where vt = EV2 - Ex (from Ref. 6 with permission), (b) Variation of cathodic peak potential, EPjC9 as a function of lg (k Jo).

Studies by cyclic voltammetry of anodic oxidation of organotin compounds at a platinum electrode in acetonitrile show that the primary irreversible process is the outer sphere oxidation of R4Sn to the radical cation, followed by rapid fragmentation into R3Sn+ and R which is then rapidly oxidised further to R+. The rate constants of the reactions correlate with those for the oxidation by Fe(III) complexes. Values for the oxidation potentials and the ionisation energies are given in Table 5-3. 

Mechanistic studies can employ CPE if the coupled chemical reactions are slow. Conventional bulk electrolyses require typically 10-30 min for completion, longer than the typical longest time for voltammetric techniques (ca. 20 s maximum for cyclic voltammetry, CV, ca. 8 s for polarography, etc.). This is important to recall when comparing CPE with voltammetry data. An electrode reaction that is chemically reversible in a slow CV experiment may be irreversible in bulk electrolysis if the electrode product has a half-life of, e.g., a minute or two. Conversely, an electron transfer that is quasi- or irreversible in a relatively fast voltammetric experiment may be electrochemically reversible in the long timescale of bulk electrolysis. 

Theory (SIT) for more accurate consideration of activity coefficients [5, 49, 50b]. Kihara et al. [49] calculated values of 0.956 0.010 V for PiiOr+/Pu()2+ and 1.026 0.010 V for Pu" +/Pu + from the cyclic voltammetry data of Riglet et al. [50] in perchlorate solutions. In an earlier study Capdevila and Vitorge determined of0.938 0.010 V for PuO2 +/PuO2+ and 1.044 0.010 V for Pu +/I u + also from voltammetric data in perchlorate solutions [116]. Because of the irreversibility of the PuO22" /Pu " electrode reaction limited formal potential data exist in the literature for calculation of the standard potential. However, a value of0.867 V has been calculated from the existing data and appropriate correction factors [49],... 

Since the Co(II) to Co(III) complexes were redox active, an electrochemical method of analysis seemed viable for the quantification of the two species in the reaction. The specific electrochemical technique developed to monitor the activation reaction allowed the simultaneous quantitative measurement of (salen)Co(II) and (salen)Co(III) species in the medium. The principle of the method is based on the electro-oxidation of both species on a platinum-rotating electrode linearly polarized with respect to a standard electrode [7]. The electrochemical reactions operative with this cyclic voltammetry technique involve the single electron oxidation of each species and occur at the revolving surface of the electrode. With this salen ligand system, the Co(II) to Co(III) transformation was determined as being fully reversible, while the Co(III) to Co(IV) reaction was irreversible. 

SECM SG/TC experiments were carried out to prove that the product of the initial two-electron oxidation process diffused into the solution, where it would react homogeneously and irreversibly. For these measurements, a 10 /xm diameter Au tip UME was stationed 1 /xm above a 100 /xm diameter Au substrate electrode. With the tip held at a potential of —1.3 V versus saturated mercurous sulfate electrode (SMSE), to collect substrategenerated species by reduction, the substrate electrode was scanned through the range of potentials to effect the oxidation of borohydride. The substrate and tip electrode responses for this experiment are shown in Figure 16. The fact that a cathodic current flowed at the tip, when the substrate was at a potential where borohydride oxidation occurred, proved that the intermediate formed in the initial two-electron transfer process (presumed to be mono-borane), diffused into the solution. An upper limit of 500 s 1 was estimated for the rate constant describing the reaction of this species (with water or OH ), based on the diffusion time in the experimental configuration. This was consistent with the results of the cyclic voltammetry experiments (11). 

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