Cyclic redox potential

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes, on the kinetics of heterogeneous electron-transfer reactions, and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species, and convenient evaluation of the effect of media upon the redox process. 

Fig. 18. pH dependence of the oxidized Rieske fragment from bovine heart mitochondria (ISF). (a) Redox potential determined by cyclic voltammetry. The line was fitted to the data points, giving = 7.6 and pi a, x2 = 9-2. (b) CD intensity of the oxidized... 

The redox potentials of the ITO electrodes modified with CgoN -MePH clusters were measured by cyclic voltammetry and differential pulse voltammetry in the absence and presence of magnetic processing. 

The redox potential of Fc obtained from the cyclic voltammetry experiments at the water-DCE interface can be verified by evaluating the thermodynamic cycle given by Eq. (4). It follows that... 

Looking back at Table 4.4 the anaerobic cyclic steady state is from the first product of C0(C02) reduction, formaldehyde (HCHO), to [CH2OH] . This step requires a kinetic pathway in redox potential from -0.2 to -0.6 V versus the H2/H+ potential at pH 7, where sulfur is the oxidised waste ... 

Cyclic voltammetry is an excellent tool to explore electrochemical reactions and to extract thermodynamic as well as kinetic information. Cyclic voltammetric data of complexes in solution show waves corresponding to successive oxidation and reduction processes. In the localized orbital approximation of ruthenium(II) polypyridyl complexes, these processes are viewed as MC and LC, respectively. Electrochemical and luminescence data are useful for calculating excited state redox potentials of sensitizers, an important piece of information from the point of view of determining whether charge injection into Ti02 is favorable. 

Fig. 8 Plots of cyclic voltammograms of abcp-substituted triruthenium species 48 and the parent triruthenium complex [Ru30(0Ac)6(py)3]+ in chloromethane solution of (Bu4N)(PFg), showing anodic shifts of redox potentials in 48 relative to those in [Ru30(0Ac)6(py)3] +...

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

Electrochemistry. The redox potentials of 66a and 67b were measured by cyclic voltammetry. Both systems undergo two reversible, one-electron ring reductions. These reductions are compared to data for 47 and H2(pc), Table XVIII. 

Electrochemistry-EPR. The redox potentials of the porphyrazines, 69a, 69b, 70a, and 70b were measured using cyclic voltammetry (Table XX). The redox potentials of the molybdocene appended porphyrazines 70a and 70b showed marked changes from that observed for the parent ligands 69a or 69b suggesting that the peripheral metalation by molybdocene profoundly alters the physiochemical properties of the macrocycle by more than just the sum of the two parts (6). 

Electrochemistry. The redox potentials, as measured by cyclic voltammetry, of 76 (trans) and 77 (cis) porphyrazines are given in Table XXII, where they are compared to all other isomeric products (A4, B4, A3B, AB3). These redox processes show no obvious correlations between the amount of thioether functionality and redox potential. 

Despite the problems that can afflict experimental cyclic voltammograms, when the method for deriving standard redox potentials is used with caution it affords data that may be accurate within a few tens of mV (10 mV corresponds to about 1 kJ mol-1), as remarked by Tilset [335]. Kinetic shifts are usually the most important error source The deviation (A If) of the experimental peak potential from the reversible value can be quite large. However, it is possible to estimate AEp if the rate constant of the chemical reaction is available. For instance, in the case of a second order reaction (e.g., a radical dimerization) with a rate constant k, the value of AEV at 298.15 K is given by equation 16.24 [328,339] ... 

As already stated, other electrochemical techniques have been used to derive thermodynamic data, some of them considered to yield more reliable (reversible) redox potentials than cyclic voltammetry. This is the case, for instance, of second harmonic alternating current voltammetry (SHACV) [219,333], Saveant and co-workers [339], however, concluded that systems that appear irreversible in slow-scan CV are also irreversible in SHACV experiments. We do not dwell on these matters, important as they are. Instead, we concentrate on a different methodology to obtain redox potentials, which was developed by Wayner and colleagues [350-352]. 

The evolution of a new set of electrochemical waves (as opposed to the gradual shifting of the redox couple) on addition of guest species may be due to a number of factors. If the complex formed has a particularly high stability constant and has a redox potential which is markedly different from that of the free ligand, a new set of waves may be observed. However, if the decomplexation kinetics of the complex formed is particularly slow on the electrochemical time scale then, as the potential is scanned between the vertex points during a cyclic voltammetric experiment, the solution complexed species will be stable over this time period and the two sets of waves will correspond to free ligand and complex. Therefore care should be taken to determine the cause of the evolution of a new set of electrochemical waves and... 

Table 2 Synthesis of phenazine ethers 10,44a-g by etherification of 2-hydroxyphenazine (9) with ROMs(-Br) and their redox potentials vs. SHE as determined by cyclic voltammetry in phosphate buffer at pH 7 using HMDE...

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