Oxidation-reduction potentials factors determining

The principles outlined above are, of course, important in electro-synthetic reactions. The pH of the electrolysis medium, however, also affects the occurrence and rate of proton transfers which follow the primary electron transfer and hence determine the stability of electrode intermediates to chemical reactions of further oxidation or reduction. These factors are well illustrated by the reduction at a mercury cathode of aryl alkyl ketones (Zuman et al., 1968). In acidic solution the ketone is protonated and reduces readily to a radical which may be reduced further only at more negative potentials. 

This paper summarized our current understanding of the factors that determine selectivity for dehydrogenation versus formation of oxygen-containing products in the oxidation of light alkanes. From the patterns of product distribution in the oxidation of C2 to C6 alkanes obtained with supported vanadium oxide, orthovanadates of cations of different reduction potentials, and vanadates of different bonding units of VO in the active sites, it was shown that the selectivities can be explained by the probability of the surface alkyl species (or the... 

Iron-sulfur proteins belong to the class of electron-transport proteins [29]. They contain an iron sulfur cluster, e.g. [4Fe-4S], which shuttles between different oxidation states. The structure of the cluster is quite consistent among a series of these proteins, but their redox potentials vary widely. Synthetic models of iron-sulfur proteins have been designed [30] to investigate the factors that determine the reduction potential of the core and to mimic other biologically... 

In a fluid such as milk, which contains several oxidation-reduction systems, the effect of each system on the potential depends on several factors. These include the reversibility of the system, its E0 value or position on the scale of potential, the ratio of oxidant to reductant, and the concentration of active components of the system. Only a reversible system gives a potential at a noble metal electrode, and this measured potential is an intensity factor analogous to the potential measured on a hydrogen electrode in determining hydrogen ion concentrations. 

Reactivity patterns in widely varying oxidants are seldom considered, the reduc-tant patterns being more often compared. Such studies can be approached in the same way as that of reductants, but because the [CoCNHjjjX]"" oxidants are so common, there may be more difficulties in determining both the self-exchange rate and reduction potential. Table 1 lists values of and for several oxidants, as well as the calculated oxidation factors (OF) [using (f) in 12.2.5.1.1]. These OF values can be corrected to give effective oxidation (actors, but because fewer reversals of trends appear, the effective oxidation factors are not included here. The OF values suggest a reactivity pattern with a reductant of = 0.3 of [Ru(bipy)3] > [Fe(l,10-phen)3 + > [IrBr ] ",... 

The function of the external electronic circuitry - or the potentiostat - in amperometric experiments is to supply the desired voltage difference between the working and reference electrodes and measure the resulting redox current instantaneously. The magnitude of the working electrode potential necessary to initiate the oxidation or reduction is ideally determined by the Nemst equation (shown in Eq. 3) but can also be affected by factors such as the conductivity of the carrier fluid, the electron-transfer kinetics of the specific redox reaction, the composition of the detection electrode, and the relative distance between the working and reference electrodes. 

The quantitative analysis requires knowledge of the rate(s) of the heterogeneous electrode reaction(s), reagent diffusion coefficients and the transfer coefficient. If the electrode reaction is reversible, most of these parameters can be determined from the CV experiments. The formal reduction potential, differs from the standard potential, °, because the latter is obtained by extrapolation to infinite dilution, while the former refers to the actual experimental conditions of ionic strength and temperature. For a fast, reversible process, E° => j,2 10 mV if the diffusion coefficients of the oxidized and reduced forms are within a factor of two. Potentials are reported relative to some standard electrode, such as ferrocene/ferrocinium ion, saturated calomel, SCE, or Ag/AgCl, and this must be taken into account in comparing results from different sources. 

Very recently, Kadish et al have shown that porphyrin ring distortion is an important factor to determine redox potentials. According to their observations, highly distorted Fe(BrgTPP)Cl exhibits smaller first oxidation potential than that for Fe(TPP)Cl, while the first reduction potentials are independent to the ring distortion [114]. 

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