Electrode potentials processes, consecutive

As mentioned, DPV is particularly useful to determine accurately the formal electrode potentials of partially overlapping consecutive electron transfers. For instance, Figure 40 compares the cyclic voltammogram of a species which undergoes two closely spaced one-electron oxidations with the relative differential-pulse voltammogram. As seen in DPV the two processes are well separated. 

These complexes undergo two consecutive one-electron reductions with features of chemical and electrochemical reversibility both in MeCN and CH2C12 solution.2 Table 2 summarizes the formal electrode potentials for the first reduction process and the sums of the Taft constants of the various substituents for each complex. 

It should be noticed that, unlike consecutive electron transfer reactions whose kinetics are determined by the slowest process, mixed potentials are determined by the fastest of several possible occurring electrode reactions. 

When dehydration occurs as a consecutive reaction, its effect on polarographic curves can be observed only, if the electrode process is reversible. In such cases, the consecutive reaction affects neither the wave-height nor the wave-shape, but causes a shift in the half-wave potentials. Such systems, apart from the oxidation of -aminophenol mentioned above, probably play a role in the oxidation of enediols, e.g. of ascorbic acid. It is assumed that the oxidation of ascorbic acid gives in a reversible step an unstable electroactive product, which is then transformed to electroinactive dehydroascorbic acid in a fast chemical reaction. Theoretical treatment predicted a dependence of the half-wave potential on drop-time, and this was confirmed, but the rate constant of the deactivation reaction cannot be determined from the shift of the half-wave potential, because the value of the true standard potential (at t — 0) is not accessible to measurement. 

Multi-electron (multistep) electrode processes will be studied in Sect. 3.3, underlining the key role of the difference of the formal potentials between each two consecutive electrochemical steps on the current-potential curves and also the comproportionation/disproportionation reactions that take place in the vicinity of the electrode surface. In the case of two-step reactions, interesting aspects of the current-potential curves will be discussed and related to the surface concentrations of the participating species. 

Mass transport limitation is more often encountered in electrode kinetics than in any other field of chemical kinetics because the activation-controlled charge-transfer rate can be accelerated (by applying a suitable potential) to the point that it is much faster than the consecutive step of mass transport, and therefore no longer controls the observed current. From the laboratory research point of view, mass transport is an added complication to be either avoided or corrected for quantitatively, in order to obtain the true kinetic parameters for the charge-transfer process. 

Reactions that take place consecutive to the electrode process can be studied polarographioally only in those cases in which the electrode process is reversible. In these cases the wave-heights and the wave-shape remain unaffected by the chemical processes. However, the half-wave potentials are shifted relative to the equilibrium oxidation-reduction potential, determined e.g. potentiometrically. Hence, whereas in all above examples, limiting currents were measured to determine the rate constant, it is the shifts of half-wave potentials which are measured here. First- and second-order chemical reactions will be discussed in the following. 

At n-type electrodes, the complete reaction already occurs in the dark because sufficient electrons are available in the conduction band. In the latter case the participation of the valence band has been proved by luminescence measurements. Since in the second reaction step electrons are transferred from the valence band to the OH radicals, hole are injected into the valence band of the n-type electrode which finally recombine with the electrons (majority carriers). In the case of n-GaP, this recombination is a light-emitting process, as has been found experimentally. The same result has been obtained with 820 [68] and for quinones [69]. Since the reduction of H2O2 consists of two consecutive steps, it is reasonable to describe its redox properties by two standard potentials, given by... 

Consider the case in which two reducible substances, O and O, are present in the same solution, so that the consecutive electrode reactions O -h ne R and O + n e R can occur. Suppose the first process takes place at less extreme potentials than the second and that the second does not commence until the mass-transfer-limited region has been reached for the first. The reduction of species O can then be studied without interference from O, but one must observe the current from O superimposed on that caused by the mass-transfer-limited flux of O. An example is the successive reduction of Cd(II) and Zn(II) in aqueous KCl, where Cd(II) is reduced with an E1/2 near —0.6 V v. SCE, but the Zn(II) remains inactive until the potential becomes more negative than about —0.9 V. 

In aqueous solutions, the EC mechanism proposed by Ruiz [13] for the oxidation of AA at low pH is widely accepted. It involves two consecutive one-electron transfer processes to form dehydroascorbic acid immediately followed by irreversible hydration to give the final product 2-3 diketogluco-nic acid. Although the electrochemical reaction is reversible at Hg electrodes [13], the large overpotential needed at carbon electrodes renders the oxidation of AA to be irreversible and the anodic potential (--300 mV at pH 3.9) is considerably higher than its standard value [14, 27], Figure 1. 

FAST REACTIONS ACCOMPANYING THE ELECTRODE PROCESS AND RATES OF ELECTRODE PROCESS PROPER From the measurements of polarographic limiting kinetic currents (and sometimes of their half-wave potentials), and their dependence on certain parameters (mainly pH, buffer composition, drop-time etc.), it is possible to compute rate constants for the fast chemical reactions, antecedent, parallel or consecutive to the electrode process proper. Rate constants of the second order reactions of the order 10 to 10 1. mol. sec have been determined in this way. The mathematical basis and the method of computation of the rate constants is beyond the scope of this text, and the reader is referred to other texts. 

Using the Berzins and Delahay equation for the case in which a single substance undergoes two consecutive charge transfer reactions (diffusion-controlled electrode processes) at sufficiently different potentials [19] ... 

Cyclic voltammetry and controlled potential coulometry investigations on a MeCN solution, made 0.1 M in BU4NCIO4, have indicated that cis-[Ptn(FcPy)2Cl2] imdergoes a two-electron oxidation process at the platinum electrode, according to the two consecutive one-electron steps (4) and (5) ... 

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