Standard reduction potential summary

Preparation and chemistry of chromium compounds can be found ia several standard reference books and advanced texts (7,11,12,14). Standard reduction potentials for select chromium species are given ia Table 2 whereas Table 3 is a summary of hydrolysis, complex formation, or other equilibrium constants for oxidation states II, III, and VI. 

In aqueous solution, thorium exists as Th(IV), and no definitive data have been presented for the presence of lower-valent thorium ions in this medium. The standard potential for the Th(IV)/Th(0) couple has not been determined from experimental electrochemical data. The values presented thus far for the standard reduction potential have been calculated from thermodynamic data or estimated from spectroscopic measurements. The standard potential for the four-electron reduction of Th(IV) ions has been estimated as —1.9 V in two separate references 12. The reduction of Th(OH)4 to Th metal was estimated at —2.48 V in the same two publications. Nugent et al. calculated the standard potential for the oxidation ofTh(III) to Th(IV) as +3.7 V versus SHE, while Miles provides a value of +2.4 V [13]. The standard potential measurements from studies in molten-salt media have been the subject of some controversy. The interested reader is encouraged to look at the summary from Martinot [10] and the original references for additional information [14]. 

In summary, the potential of the standard hydrogen electrode is set at exactly 0 V. Any electrode at which a reduction half-cell reaction shows a greater tendency to occur than does the reduction of H" " (1 M) to H2 (g, 1 bar) has a positive value for its standard electrode potential, E°. Any electrode at which a reduction half-cell reaction shows a lesser tendency to occur than does the reduction of H" "(l M) to H2 (g, 1 bar) has a negative value for its standard reduction potential, E°. Comparisons of the standard copper and zinc electrodes to the standard hydrogen electrode are illustrated in Figure 19-6. Table 19.1 on page 875 lists some cormnon reduction half-cell reactions and their standard electrode potentials at 25 °C. 

Table 4 Summary of the voltammetric reduction peak potentials ( p), standard dissociative reduction potentials ( "roor/ro ,ro ) for 3 variety peroxides and endoperoxides in DMF/O.l M TBAP at T = 25°C. Also summarized are the BDFEs and the standard potentials of the corresponding leaving group."...

In summary, the preparation of bimetallic catalysts by surface redox reaction using a reductant preadsorbed on the parent monometallic catalyst has been studied in detail. Unfortunately, the method is intricate and time consuming, especially if several successive operations are required. Furthermore, when the modifier has a standard electrochemical potential higher than that of the parent metal (AUCI4 deposited on Pt°), the overall reaction is a complex one involving a reduction by adsorbed reductant but also direct oxidation of the metallic parent catalyst. The relative rate of the two parallel reactions determines the catalytic properties of the resulting bimetallic catalyst. 

Fig. 6. Summary of the standard oxidation-reduction potentials of several dyes and the experimentally determined rate constants for their reduction by P430 . [DP, dipyridyls, preceded by the redox-potential values M(B)V, methyl (benzyl) viologens ST. safranine T MB, methylene blue]. Figure source Ke (1973) The primary electron acceptor of photosystem I. BiochimBiophysActa301 29.

Figure 18.2 Summary of respiratory energy flows. Foods ate converted into the reduced form of nicotinamide adenine dinucleotide (NADH), a strong reductant, which is the most reducing of the respiratory electron carriers (donors). Respiration can he based on a variety of terminal oxidants, such as O2, nitrate, or fumarate. Of those, O2 is the strongest, so that aerobic respiration extracts the largest amount of free energy from a given amount of food. In aerobic respiration, NADH is not oxidized directly by O2 rather, the reaction proceeds through intermediate electron carriers, such as the quinone/quinol couple and cytochrome c. The most efficient respiratory pathway is based on oxidation of ferrocytochrome c (Fe ) with O2 catalyzed by cytochrome c oxidase (CcO). Of the 550 mV difference between the standard potentials of c)Tochrome c and O2, CcO converts 450 mV into proton-motive force (see the text for further details).

In summary, substitution routes have the potential of introducing hitherto unattainable flexibility and subtlety into the preparation of technetium radiopharmaceuticals. As currently being developed, these routes should lead to new classes of technetium radiopharmaceuticals, the properties of which will be considerably different and more easily controlled than those of complexes prepared by the standard Sn(n) reduction of pertechnetate. 

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