Values of Redox Potentials

The values of redox potentials were tabulated in numerous collections [43-47], the latest collections being critically selected. The potentials of redox systems with participation of radicals and species in excited electronic states are discussed in Refs. [56, 57]. There is no need to measure the potentials for all redox pairs (and, correspondingly, AG for all known reactions). If we obtain the partial values for a number of ions and compounds, the characteristic values for any other reactions can be computed. The idea of calculation is based on the fact that the emf value only depends on the initial and final states of the system, being independent of the existence of any intermediate states. This fact is of great importance for systems, for which it is impossible, or extremely difficult, to prepare a reversible electrode (redox couples containing oxygen or active metals). From considerations of the equilibria with participation of a solvated electron, the value that determines the value of the constant in Eq. (7) was estimated - 2.87 V (SHE) [56]. 

It should be mentioned that no standard electrode can be experimentally achieved, because the standard conditions are hypothetical (no system is ideal at a unimolal concentration and the atmospheric pressure). Hence, the majority of tabulated values were recalculated from the data for more dilute solutions, usually after the preliminary extrapolation to the zero ionic strength. When dealing with the reverse procedure (recalculation of equilibrium potentials for dilute solutions from tabulated standard values), we should take into account that in real systems the time of equilibrium establishment increases with decreasing concentration, because of the decrease in rates of halfcell reactions. Hence, the possibility to prepare a reversible electrode is limited to a certain concentration value. 

The shifts of reversible potentials of oxide electrodes with composition were first considered in Ref. [12] using the example of hydrated Mn02- c, which demonstrates a wide region of inhomogeneity. Later, the considerations of this sort were carried out also for other systems [58]. 

There are lots of empirical and semiem-pirical correlations of redox potentials with molecular characteristics of substances, especially for the sequences of related 

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As can be seen from the energy level structure diagram, the relative position of the HOMO and LUMO levels are not less important than the energy gap between them, since they control the possibility of charge injection. At this point, however, note, that a MO scheme is often used for illustration, but more properly the total energy states of the molecules and their radical cations and anions that may be subjected to electronic rearrangement have to be considered. Bearing this in mind, the measured values of redox potentials can be translated into the molecular orbital picture. 

Table L Mean values of redox potential, temperature, residence time, pH and DO in the twelve treatments (standard deviations are shown in parentheses).

Effects of Isomerism Geometrical isomers of coordination compounds can exhibit different values of redox potential, as accounted for, in various cases, by simple djr orbital level splitting diagrams or by MO calculations. The dependence of the relative stability and redox behavior of the geometrical isomers on the electronic configuration of the metal is also... 

The chemistry of the alkoxoderivatives of rhenium is very diversified due to the existence of the compounds corresponding to practically all the possible oxidation states. The transitions between them occur usually quite easily, being characterized by comparably low values of redox potentials as it takes place, for example, for rhenium (VI) cateholates [471] ... 

This supports the view that two electrons are transferred to P-cluster and the second electron requires probably very precise complementarity of the two proteins unattainable in the complex Cp2Avl. Perhaps the transfer of this second electron corresponds to particularly strongly negative value of redox potential. 

The strong antioxidation efficiency of dihydric phenols and, at the same time, their facile oxidation are connected with the presence of two hydroxyl groups in positions 1,2 or 1,4 of benzene nucleus. Generally, hydroquinones have lower values of redox potential (500—660 mV185 ) than pyrocatechols (765—773 mV186 ). The values decrease by the gradual alkylation of aromatic nucleus in both isomeric series and may be weE correlated with the steric and electronic factors of substituents. However, the redox potential value is not a sufficient condition to achieve the stabEiza-... 

Correlation with Eh value is associated with their capability to have several oxidation degrees. For instance, selenium can migrate as oxyanions of selenic SeOT" and selenious SeO, acids. Relative domination of these forms depends on the value of redox potential Eh. If Eh value exceeds 100 mV selenious acid goes over into selenic one ... 

The latter equation shows that the nitrate concentration in equilibrium with atmospheric nitrogen is proportionate with the value of redox potential and at Eh < 0.71 should not exceed 1 mmole-l" (62 mg-1 )- Salts of nitrate display high solubility (sodium saltpetre NaNOj - 876 g kg k potassium salpetre KNO - 316 g-kg S nitroammit (NH) NOj - 1,880 g-kg" nitrobarite Ba(NOj)2 - 90.5 g-kg Setc.). The nitrate practically does not take part in complexation and is not adsorbed, and that is why it is removed from water with difficulty. Natural accumulation of sodium salts (saltpetres) are extremely rare and found in conditions of arid climate. An example is sodium saltpetre (NaNO ) on the coast of Chile. 

The values of redox potentials E ci[Pg.255]

The data listed in Table I and depicted in Figure 5 were obtained using a B3P86 frinctional. The calculated values of redox potential in aqueous solvent for a ven compound were subsequently compared with its reactivity (NC values) towards mV-1 nucleocapsid protein (see below). 

There are three important aspects to natural controls of redox conditions. Firstly, depending upon the dominant redox reaction in the system, the absolute value of redox potential may vary considerably. Iron is usually the most important chemical component in this regard, but sulphur. 

Fig. 18.2 illustrates the first decisive steps of the reaction with formation of compounds I and II possessing high oxidizing capacity (the values of redox potentials are Ei = 970 mV and Eu = 950 mV, respectively. Two-electron oxidation of hemin-Fe group to compound I proceeds with the synchronous... 

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