Reduction-oxidation chemistry redox potential

Oxidation-reduction potentials for complexes in solution are determined by the relative stabilities of the complexes of the metal ion in the lower and higher oxidation states. The thermodynamic cycle connecting redox potentials and stabifity constants is shown in Fig. 7. This cycle can be useful both in rationalizing aspects of aqueous solution chemistry of complexes and in predicting or estimating values for stabifity constants or redox potentials for systems which are difficult or impossible to access experimentally. Thus knowledge of stabifity... 

The factors which influence the rate of dissolution of iron oxides are the properties of the overall system (e. g. temperature, UV light), the composition of the solution phase (e.g. pH, redox potential, concentration of acids, reductants and complexing agents) and the properties of the oxide (e. g. specific surface area, stoichiometry, crystal chemistry, crystal habit and presence of defects or guest ions). Models which take all of these factors into account are not available. In general, only the specific surface area, the composition of the solution and in some cases the tendency of ions in solution to form surface complexes are considered. 

Finally, Zn also plays an important role in Cu,Zn-SOD (copper-zinc superoxide dismutase) for which the structure is shown in Figure 8 ". Although it is well-known that Zn(II) is not involved in electron-transfer chemistry, it is generally believed that the essential role of Zn(II) ion in SODs is to accelerate both the oxidation and reduction of superoxide by controlhng the redox potential of the Cu(II) ion and superoxide ion in the catalytic cycle. ... 

In the absence of suitable scavengers, recombination occurs within a few nanoseconds (19). Valence band holes (h+(vb)) have been shown to be powerful oxidants (20-231 whereas conduction band electrons (e (cb)) can act as reductants (24,251. The redox potentials of both, e and h+, are determined by the relative position of the conduction and valence band, respectively. Bandgap positions are material constants which have been determined for a wide variety of semiconductors (26). Most materials show "Nernstian" behavior which results in a shift of the surface potential by 59 mV in the negative direction with a pH increase of ApH = 1. Consequently electrons are better reductants in alkaline solutions while holes have a higher oxidation potential in the acid pH-range (26). Thus, with the right choice of semiconductor and pH, the redox potential of the e (cb) can be varied from +0.5 to -1.5 V (vs. NHE) and that of the h+(vb) from +1.0 to more than +3.5 V. This sufficiently covers the full range of redox chemistry of the H20/02 system (271. 

The standard electrode potential for an oxidation-reduction process is often called the standard redox potential of the pair of ions involved. A table of redox potentials finds immediate application in inorganic chemistry. 

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. 

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