Reduction and oxidation peak potentials

Redox-Mediated Metal Deposition. A reduced polyimide surface can function as a reducing substrate for subsequent deposition of metal ions from solution. For metal reduction to occur at a polymer surface, the electron transfer reaction must be kinetically uninhibited and thermodynamically favored, i.e., the reduction potential of the dissolved metal complex must be more positive than the oxidation potential of the reduced film. Redox-mediated metal deposition results in oxidation of the polymer film back to the original neutral state. The reduction and oxidation peak potential values for different metal complexes and metal deposits in nonaqueous solvents as measured by cyclic voltammetry are listed in Table III. 

Table III. Reduction and Oxidation Peak Potentials for Catalytic Metals...

The half-wave potential, as obtained from the average of the reduction and oxidation peak potentials, is very close to that reported for the more hydrophilic methylviologen (—0.69 V vs the same SSCE reference electrode) [15]. This probably reveals that the electroactive 4,4 -bipyridinium moiety is essentially unaffected by the complexation, arguing in favor of an inclusion complex between the lipophilic tail of the surfactant and the cyclodextrin host. 

The formal or midpoint potential (E°) of a redox species is calculated by averaging the reduction and oxidative peak potentials. The rate at which an ET occurs between electrode and cyt P450-heme center can be determined by acquiring the cyclic voltammograms with increasing scan rates and by applying the Buder-Volmer and Marcus theories. One can also obtain information about non-electrochemical processes associated with an electrochemical event from cycUc voltammetry. One example is proton-coupled ET. 

Franke [47] undertook a comprehensive electroanalytical study of K2S207 mixtures with K2S04, which is formed by Eqs. (47) and (48) and V2Os, a widely-used oxidation catalyst for S02. Pure pyrosulfate under N2 or air (Fig. 38a,b) shows only the reduction to S02 and sulfate, Eq. (48) (all potentials are vs. Ag/Ag+). When S02 is added, a new reduction and oxidation peak appear (Fig. 38c,d). When the electrolyte was pre-saturated with K2S04 (ca. 4 wt.%) (Fig. 39) the gas composition had no direct effect on the voltammetry. Although the equilibrium for Eq. (49) lies well to the right at this temperature, 400 °C, the kinetics are quite slow in the absence of a catalyst. The equilibrium between pyrosulfate and sulfate, Eq. (47), lies well to the left (K = 2 x 10-6), but will proceed to the right in the absence of S03. Thus, the new peaks are sulfate oxidation, Eq. (43), and S03 reduction to sulfite ... 

The difference in potentials of the reduction and oxidation peaks on cyclic voltam-mograms measured by derivative techniques... 

The potential difference between the reduction and the oxidation peaks is theoretically 59 mV for a reversible reaction. In practice, the difference is typically 70-100 mV. Larger differences, or nonsymmetric reduction and oxidation peaks, are an indication of a nonreversible reaction. These parameters of cyclic voltammo-grams make CV most suitable for characterization and mechanistic studies of redox reactions at the electrodes. 

Figure 2 shows cyclic voltammograms and observed redox potentials [Eobs° = (E,° + E2° )/2] for [W2(n-SPh)2(CO)g] - (15) and [W2( -PPh2)2(CO)8] (18) at a sweep rate of 0.1 V s. The overall two-electron character of the reactions has been confirmed by controlled potential coulometry and comparative voltammetric peak current measurements. Only single reduction and oxidation peaks are observed at sweep rates ranging from 0.005 to 1000 V s" thus, the one-electron intermediate in reactions 1 and 2 is not detectable by simple electrochemical means. 

At low surface concentrations, the redox process is nearly reversible, by taking in account the areas and the potentials of the peaks. At higher, the differences in the areas between reduction and oxidation peaks may be attributed to differences in the adsorption of cystine and desorption of cysteine residues at the positively or negatively charged mercury respectively. At the same time the protein molecule as a whole remains adsorbed by hydrophobic interactions. 

TABLE 4. Oxidation and reduction of benzoylsilanes and benzoylgermanes peak potential values Ep, from CV in acetonitrile, TEAP 0.1 M on GC vs Ag/AgCl102... 

Fig. 1 shows the cyclic voltammetry of an FePc/XC-72 dispersion, heated at 280°C in an inert atmosphere, in the form a thin porous Teflon bonded coating electrode in a 1 M NaOH solution. A description of the methodology involved in the preparation of this type of electrode may be found in Ref. 3. As can be clearly seen, the voltammetry of this specimen exhibits two sharply defined peaks separated by about 330 mV. The potentials associated with these features are essentially identical to those found by other workers for the reduction and oxidation of films of iron oxy-hydroxide formed on a number of host surfaces, including iron and carbon.(5)... 

The biggest fullerene isolated and studied using electrochemistry is C92, which shows eight reversible reductions and one broad irreversible oxidation. The intensities of the eight reduction peaks can be grouped into two distinct sets, thus indicating that the electrochemistry corresponds to a mixture of two isomers The reduction and oxidation potentials are shown in Table 8.1. 

In CHjCl, unless otherwise noted. The potentials are reversible potentials (taken as the midpoint between the peak potentials for the reduction and oxidation processes) for one-electron exchanges, unless otherwise noted, and the designations F 3i,F 4,R d3> o i refer to the 1., 2. and 3. reduction and 1. and 2. oxidation process, respectively, relative to the oxidation state of the complex given in the first column. All potentials are referred to NHE. 

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