Measurement of Redox Potentials

Many experimental approaches have been appHed to the deterrnination of stabihty constants. Techniques include pH titrations, ion exchange, spectrophotometry, measurement of redox potentials, polarimetry, conductometric titrations, solubiUty deterrninations, and biological assay. Details of these methods can be found in the Hterature (9,10). 

In the high temperature limit where all the nuclear motions coupled to the process can be described classically, the nuclear factor is expressed in terms of only two parameters the driving force of the reaction AG°, and the whole reorganization energy X (expressions (13) and (14)). Detailed calculations carried out in the case of cytochrome c have demonstrated that AG° is a complex quantity, which depends not only on the electronic properties of the redox centers but also on those of the protein and of the surrounding solvent [100]. Usually, AG can be evaluated from measurements of redox potentials and of eventual interaction energies between the different parts of the systems (Appendix). 

Electrochemical cell for measurement of redox potentials of hydrogenase by EPR spectroscopy... 

The identities of the solid phases that form remain a mystery. Direct identification is difficult because Fe(II) and Mn(II) solid phases are readily oxidized by O2 and it is therefore necessary to maintain scrupulously anoxic conditions to ensure that the material examined actually represents that in anoxic soil. An alternative is to make indirect assessments through measurements of pe, pH and [Fe +] in solution, but these too are difficult (see section on measurement of redox potential in soil). 

These factors rather constrain the nsefnlness of Uh measnrements in soil solutions. Inferences about the thermodynamics of redox processes in soils that rely heavily on measurements of redox potential shonld be treated with caution. Nonetheless soil h measnrements provide a ready measnre of redox status, for example in experiments in which constant and pH are reqnired (Patrick et al, 1973). 

Although many studies of Mo—S complexes make use of electrochemical techniques, and particularly of cyclic voltammetry (CV), this is often restricted to measurements of redox potentials, and to discussions of their variation with structure or of their relationships with spectroscopic properties. Such examples can be found in the above-mentioned reviews. In this article, we will comment... 

Deeble DJ, Schuchmann MN, Steenken S, von Sonntag C (1990) Direct evidence for the formation of thymine radical cations from the reaction of SO/" with thymine derivatives a pulse radiolysis study with optical and conductance detection. J Phys Chem 94 8186-8192 DeFelippis MR, Murthy CP, Faraggi M, Klapper MH (1989) Pulse radiolytic measurement of redox potentials the tyrosine and tryptophan radicals. Biochemistry 28 4847-4853 Delatour T, Douki T, D Ham C, Cadet J (1998) Photosensitization of thymine nucleobase by benzo-phenone through energy transfer, hydrogen abstraction and one-electron oxidation. J Photo-chem Photobiol 44 191-198... 

Grundl, T. and D. Macalady. 1989. Electrode measurement of redox potential in anaerobic aqueous iron systems. J. Contam. Hydrol. 5, 97-117. 

Liss, P.S., Herring, J.R. and Goldberg, E.D. (1973) The iodide/iodate system in seawater as a possible measure of redox potential. Nature Phys. Sci., 242(120), 108-109. 

In another technique, the solution is circulated through a capillary cell by a peristaltic pump (37). If part of the circulating loop is immersed in a constant-temperature bath, it is possible to measure the spectrum over a wide temperature range. A more sophisticated technique (38) allows the measurements of redox potentials and electronic spectra as well as Raman spectra using a circulating cell. 

Figure 18.6. Measurement of Redox Potential. Apparatus for the measurement of the standard oxidation-reduction potential of a redox couple. Electrons, but not X or X% can flow through the agar bridge.

Patch-clamp technique for measurmg channel activity (p.. 163) Measurement of redox potential (p. 506)... 

Since 1973 [3] and 1975 in Dresden 2 [4], on line measurements of redox potential have been performed in the BWRs (Boiling Water Reactors) primary coolant in order to verify the IGSCC (Intergranular Stress Corrosion Craking) susceptibility of austenitic stainless steel components. Such measurements turned out to be fundamental for estimating the effect of hydrogen addition as a remedy for IGSCC [5-6]. 

Shaikh, A.U., Hawk, R.M., Sims, R.A. Scott, H.D. (1985) Graphite electrode for the measurement of redox potential and oxygen diffusion rate in soils. Nuclear and Chemical Waste Management,... 

FOCS are finding increasing applications in analytical laboratories for titration and the measurement of redox potential. Acid-base titration [57, 69, 70] (see Section 17.1.1.3) with glass fibers (bifurcated) is developing in fluorescence as in absorption with inexpensive solid-state components (LEDs and photodiodes) [57], [97]. 

Because it is difficult to make a precise measurement of redox potential in soil, especially in well-aerated soil, reliable data on redox potential in the rhizosphere in this study are lacking. The results presented in Fig. 5 may be inaccurate however, a trend of change toward negative Eh in the rhizosphere soil is inferred. Rappart et al. (1987) indicated that greater amounts of exchangeable copper have been detected in soils at low rather than high redox potential. It is assumed that in the maize rhizosphere in this study, the reduced redox potential was favorable to metal mobilization. 

Measurement of redox potentials is relatively inaccurate. As the redox potential measurements are easily influenced by adsorption of organic substances such as minute amounts of oil, it is essential to clean the electrode surface. This is best done with a mixture of calcium carbonate, detergent, and trisodium phosphate. 

The possibility of formation and existence of the different oxidation states in molten chlorides is confirmed by direct measurements of redox potentials of lanthanides that were performed by a potentiometric method. The ratio between their concentrations ([Ln ]/[Ln ]) is related to the redox potentials through the Eq. 11 [3]. 

Measurements of redox potentials thus enable one to deduce the changes in free energy during redox reactions. 

In corrosion studies [59—61] the central focus was placed on the compatibihty of Ni-based alloys with molten Li,Na,Be/F salt system as applied to primary circuit of MOSART design fueled with different compositions of actinide trilluorides from LWR spent fuel without U-Th support. These studies (see Table 5.5) included (1) compatibility test between Ni-Mo alloys and molten 15 LiF-58 NaF-27 BeF2 (mol%) salt in natural convection loop with measurement of redox potential (2) study on PuFs addition effect in molten 15 LiF-58 NaF-27 BeF2 (mol%) salt on compatibility with Ni-Mo alloys and (3) Te corrosion study for molten 15 LiF-58 NaF-27 BeF2 (mol%) salt and Ni-Mo alloys in stressed and unloaded conditions with measurement of the redox potential. Three Hastelloy N type modified alloys, particularly, HN80M-VI with 1.5% of Nb, HN80MTY with 1% of Al, and MONICR [59] with about 2% of Fe, were chosen for our study in corrosion facilities (see Tables 5.3 and 5.6). 

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