Redox parameters

Figure 6.23. Changes of the redox parameter (pe + pH) in two Israeli arid soils during saturated paste incubation (after Han and Banin, 1996. Reprinted from Soil Sci Soc Am J, 60, Han F.X., Banin A., Solid-phase manganese fractionation changes in saturated arid-zone soils Pathways and kinetics, p 1076, Copyright (1996), with permission from Soil Sci Soc Am)...

The data of Figures 1-4 have been used to calculate formal potentials for Reactions 7 and 8 for copper, silver, and gold these are summarized in Table I. Analogous redox parameters for aqueous solutions also are included (14). ... 

The interpretation of Eh-pH diagrams implies assumption of complete equilibrium among the various solutes and condensed forms. Although this assumption is plausible in a compositionally simple system such as that represented in figure 8.20, it cannot safely be extended to more complex natural systems, where the various redox couples are often in apparent disequilibrium. It is therefore necessary to be cautious when dealing with the concept of the system Eh and the various redox parameters. 

However, in the literature we often encounter the redox parameter pe, which, like pH, represents the cologarithm of the activity of the hypothetical electron in solution ... 

A related series of experiments were described by Hailstone in 1984 [189]. A reinterpretation of his experimental data was given by Muenter et al. in 1985 [190] and in 1999 [158], using more accurate redox parameters, and recognizing that his... 

Redox parameters analogous to those for acid-base chemistry can be defined for all aqueous systems. The redox intensity factor pE is an energy parameter in non-dimensional form that describes the ratio of electron acceptors (oxidants) and donors (reductants) in a redox couple. The redox potential (Ej ) of the system is an alternative and equivalent intensity factor. Table I summarizes the complete thermodynamic analogy between pH and pE. An analogy between acid-base and redox systems can also be made for capacity factors. 

Summary of 4Fe-4S Cluster Redox Parameters for Selected Mutant Chromatium vinosum HiPIPs°... 

The intention of this paper is to evaluate as to whether redox parameters derived from the concepts outlined above are predictive with respect to the degradation rate of chlorinated ethenes observed in the field. We used data from 13 field studies where in-situ degradation rates had been determined together with a (partly incomplete) analytical protocol of redox sensitive species. A novel approach will be derived to predict redox conditions based on the assumption that reductive dechlorination of chlorinated ethenes is at steady-state. 

As mentioned above, the performance of the VPP catalyst is largely dependent on the transient operating conditions. Some of the redox parameters studied in the hter-ature are feed compositions, solids residence time and temperature, or in rare cases pressure. The feed compositions for fixed-bed or fluidized-bed reactors vary between 1.8. 0% -butane, while in an industrial CFB reactor the feed may contain up to 20% -butane. Catalyst regeneration times may vary from 40 seconds to 1 minute... 

Os(bipy) displays properties similar to those of Ru(bipy) " [10] however, the excited state has a smaller lifetime and the redox properties are quite different from those of Ru(bipy) " . In particular, the MLCT state is a more powerful reductant than that of its ruthenium equivalent. It is noteworthy that the redox parameters can be adjusted by introducing electron withdrawing or electron donating substituents at the appropriate positions of the diimine ligands. Copper(I) complexes are and some of them show intense charge transfer transitions in the visible [11]. In particular, using properly substituted phenanthrolines, it has been possible to obtain long lived MLCT... 

In general, low level detection is masked by the noise level inherent in any measuring device. Electrochemical methods are susceptible to electrical interference from external sources, variations in reference electrode parameters resulting from aging or contamination, and interference from redox... 

Computer simulations of electron transfer proteins often entail a variety of calculation techniques electronic structure calculations, molecular mechanics, and electrostatic calculations. In this section, general considerations for calculations of metalloproteins are outlined in subsequent sections, details for studying specific redox properties are given. Quantum chemistry electronic structure calculations of the redox site are important in the calculation of the energetics of the redox site and in obtaining parameters and are discussed in Sections III.A and III.B. Both molecular mechanics and electrostatic calculations of the protein are important in understanding the outer shell energetics and are discussed in Section III.C, with a focus on molecular mechanics. 

Measurement of some of these parameters identifies the risk of a particular type of corrosion, for example pH measurements assess the risk of acid attack and redox potential measurements is used to assess the suitability of the soil for microbiological corrosion, a low redox potential indicates that the soil is anaerobic and favourable for the life cycle of anaerobic bacteria such as to sulphate-reducing bacteria. Other measurements are more general, resistivity measurements being the most widely quoted. However, as yet no single parameter has been identified which can confidently be expected to assess the corrosion risk of a given soil. It is therefore common practice to measure several parameters and make an assessment from the results. 

Other tests to determine bacterial-notably sulphate reducing-activity, soil resistivity, pH, redox potential, etc., will provide valuable data to supplement the results obtained with test specimens. A useful account of some of these was given in Reference 336 and they are also discussed in Sections 2.6 and 10.7. A scheme for assessment of corrosivity of soils based on some of the above parameters has been given by Tiller . 

Peroxides are used most commonly either as thermal initiators or as a component in a redox system. While peroxides are photochemically labile, they seldom find use as photoinitiators other than in laboratory studies because of their poor light absorption characteristics. They generally have low extinction coefficients and absorb in the same region as monomer. Kinetic parameters for decomposition of some important peroxides are given in Table 3.5,... 

In principle it should be possible to predict quantitatively the reactivity of such species containing nucleophilic homolytic leaving groups towards diazonium ions, by using a dual parameter equation. One parameter serves as a measure of the donor property of the particle the other parameter is the redox potential. However, the complex nature of kinetics of homolytic dediazoniations is likely to be a great obstacle in attempts to calculate rate constants referring only to the radical-generation step. 

The second direction in which redox properties of sulfones and sulfoxides could manifest themselves in photochemistry is redox photosensitization108,110-114. In such a photosensitization the photosensitizer is transformed by light into a short-lived oxidant or reductant able to react with the substrate to be activated. Tazuke and Kitamura115 have discussed the parameters to play with when one... 

Stationary microwave electrochemical measurements can be performed like stationary photoelectrochemical measurements simultaneously with the dynamic plot of photocurrents as a function of the voltage. The reflected photoinduced microwave power is recorded. A simultaneous plot of both photocurrents and microwave conductivity makes sense because the technique allows, as we will see, the determination of interfacial rate constants, flatband potential measurements, and the determination of a variety of interfacial and solid-state parameters. The accuracy increases when the photocurrent and the microwave conductivity are simultaneously determined for the same system. As in ordinary photoelectrochemistry, many parameters (light intensity, concentration of redox systems, temperature, the rotation speed of an electrode, or the pretreatment of an electrode) may be changed to obtain additional information. 

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