The Effect of Redox Potential

Nojeim et al (utilized an electrolytic cell model system, free of oxygen, buffered with phthalate to pH k.2 and designed to provide a redox potential between +300 and +650 mv to evaluate the effect of redox potential on the ionization and valence of four iron compounds El, FS, FOP, and SFEDTA, described previously (1+3.). Data obtained were used to predict ionization and valence trends in actual food systems of different redox potentials. Redox potential was found to have no significant effect on the ionization of any of the four compounds evaluated. However, in the case of El and FS lower potentials in the environment favored the reduced form of iron. This is to be expected since a greater difference between the +770 mv potential of the ferric to ferrous couple and the potential of its chemical environment would cause the reduction to be more spontaneous. 

FIGURE 12.9 The effect of redox potential on the distribution of copper among selected chemical forms in Calcasieu River sediment suspensions at pH 8.0 (total copper = 21.4 mg kg ). 

FIGURE 16.9 The effect of redox potential on relative production of nitrous oxide and nitrogen gas during denitrification. (From Kralova et al., 1992.)... 

Levitt, D.B., Slaughter, W., Pope, G.A., and Jouenne, S. 2010. The Effect of Redox Potential and Metal Solubility on Oxidative Polymer Degradation. Paper SPE 129890 presented at the SPE Improved Oil Recovery Symposium, Tulsa, 24-28 April. SPE-129890-MS. DPI 10.2118/129890-MS. 

In the case of CaCl2 and NaCl, the order corresponds with the corrosion behaviour expected from cathodic polarisation curves . The order of aggressiveness of chlorides can also be explained on the basis of redox potentials of the melts, calculated on thermodynamic grounds from the free energies of formation of the appropriate oxides and chlorides . The order of aggressiveness of nitrates is complicated by passivity effects , while that of alkalis in contact with air is... 

Consider the interface between a semiconductor and an aqueous electrolyte containing a redox system. Let the flat-band potential of the electrode be fb = 0.2 V and the equilibrium potential of the redox system o = 0.5 V, both versus SHE. Sketch the band bending when the interface is at equilibrium. Estimate the Fermi level of the semiconductor on the vacuum scale, ignoring the effect of dipole potentials at the interface. 

Conductance of a solution is a measure of its ionic composition. When potentials are applied to a pair of electrodes, electrical charge can be carried through solutions by the ions and redox processes at the electrode surfaces. Direct currents will result in concentration polarization at the electrodes and may result in a significant change in the composition of the solution if allowed to exist for a significant amount of time. Conductance measurements are therefore made using alternating currents to avoid the polarization effects and reduce the effect of redox processes if they are reversible. 

Assaf-Anid, N., Hayes, K. F. Vogel, T. M. (1994). Reductive dechlorination of carbon tetrachloride by cobalamin(II) in the presence of dithiothreitol mechanistic study, effect of redox potential and pH. Environmental Science Technology, 28, 246-52. Ballard, T. M. (1971). Role of humic carrier substance in DDT movement through forest soil. Soil Science Society of America Proceedings, 35, 145-7. 

The order of redox potentials for oxidation of (4) (F>C1 Br) has been reported and found most consistent with a detectable resonance contribution through the a-framework. The most difficult oxidation of (5) (despite the fluoro substituent being one carbon atom more removed from the double bond) is consistent with the Whiffer effect (cr-hyperconjugative destabilization proceeding through two pathways is more than double tiie same effect through one pathway), in consonance with the AMI prediction. The facial selectivity of epoxidation and diazetidme formation from (4) proved to be hi... 

The analysis of redox potential modulation in heme proteins has been undertaken through both experimental and theoretical strategies. In particular, the use of simple models such as microperoxidase (MP) and the design of artificial heme proteins or biomimetics has allowed to single out the effect of different factors on redox potential [17, 18], There are a number of relevant interactions, listed on Table 4.2, related to the thermodynamics terms mentioned above and that have been shown to influence the redox potential of heme proteins and biomimetics. Although they may not entirely explain redox potential modulation, they are the best understood and several examples may be found in the literature. 

The effect of mutations or chemical modification is mostly measured in terms of changes in peroxidase activity. However, the electron transfer rate depends on the redox potential of the enzyme, so that a more integral characterization should include the electrochemistry of the enzyme. Thus, the modulation of redox potential could also serve for the design of more efficient biocatalysts. 

Figure 8.26. Representative Fe(II)/Fe(III) redox couples at pH = 7. (phen = phen-anthroline sal = salicylate porph = porphyrin = valid for [HCO = 10 M.) Complex formation with Fe(II) and Fe(III) both on solid and solute phases has a dramatic effect on the redox potentials thus electron transfer by the Fe(II),Fe(III) system can occur at pH = 7 over the entire range of the stability of water (-0.5 to 1.1 V). (= Fe 0)2 Fe refers to Fe adsorbed inner-spherically to a surface of a hydrous ferric oxide. The range of redox potentials for heme derivatives given on the right illustrates the possibilities involved in bioinorganic systems.

The use of ozone os on oxidant for industrial wastes containing cyanides and other reducible toxic substances appears worthy of careful investigation. The oxidation of simple cyanides by ozone is rapid and complete. Mass transfer controls the absorption. The use of packed towers or sieve plate towers is indicated, and the maintenance of a pH of at least 9.0 is recommended. The destruction of cyanates and cyanide complexes is slower than the cyanide oxidation. These substances are destroyed if sufficient contact time and proper pH control are maintained so that these slower reactions can take place. The use of redox potential to control the degree of oxidation appears promising. Proper interpretation of the redox potential of the treated waste will give an excellent indication of the effectiveness of the treatment and the degree of removal of cyanide and cyanate. 

How much lower the system is may be related to the level of completion to which the reaction is carried. Surprisingly, as pointed out by Nojeim (56), the study of reduction potentials as possible predictors of the chemical fate of iron in food has largely been neglected. In fact, literature discussing any effects of redox potential on food chemistry is sparse. Most of... 

E Xu. Effects of redox potential and hydroxide inhibition on the pH activity profile of fungal laccases. J. Biol. Chem. 272(2) 924-928, 1997. 

Elrashidi MA and Adriano DC (1987) Effect of Redox Potential and pH on Chemical Speciation of Inorganic Selenium ill Soils. Sixth International Conference on Heavy Metals in the Environment, New Orleans. Proceedings Vol I, pp. 107— 109. CEP Consultants Ltd., Edinburgh. 

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