Oxidation reduction potential measurement

A comprehensive series of oxidation-reduction potential measurements have shown the FAD moiety to have the following one-electron couples PFl/PFIH = = —290 mV and PFIH 7PFIH2 = —365 mV while the FMN moiety exhibits the following PFl/PFl- = -110 mV and PFIH /PFIH = -270 mV. The FMN and FAD smiquinones were found to both be the neutral form as judged from absorption and ESR spectral data. The overlap of oxidized/semiquinone potential of the FAD moiety withkhe semiquinone/hydroquinone couple of the FMN moiety demonstrates the thermodynamic facilitation of flavin-flavin electron transfer via a one-electron mechanism. Stopped-flow kinetic data are also consistent with this view in... 

Makino R, Chiang R, Hager LP (1976) Oxidation-reduction potential measurements on chloroperoxidase and its complexes. Biochemistry 15 4748—4754... 

Conroy CW, Tyma P, Daum PH et al (1978) Oxidation-reduction potential measurements of cytochrome c peroxidase and pH dependent spectral transitions in the ferrous enzyme. Biochim Biophys Acta 537 62-69... 

Water quality and oxidation-reduction potential measurements. 

Example 32 A solution contains 0-05m FeCl2 and 01 5m FeCI3. What is the oxidation-reduction potential measurable in the solution ... 

Oxidation-reduction potential measurements were made with a platinum-calomel electrode system connected to a Beckman Model H-2 pH meter. The buffered bacterial suspensions were similar to those in the dose-contact time experiments. To these suspensions, successive equal aliquots of an ozone solution were added at 1°, and the oxidation-reduction potential was measured in millivolts. 

Figure 12-8 summarizes the information available as far as the HOMO/LUMO positions of the compounds is concerned. Being inferred from oxidation/reduction potentials measured by cyclic voltammetry in polar solution and from HOMO/ LUMO gaps, respectively, absolute values should be viewed with some caution. 

Oxidation-reduction potential measurements in natural waters and wastewaters are difficult to interpret. The only potentials that will register in the ORP cell are those from species that can react at the indicator electrode surface—we call these electroactive species. In natural waters only a few reactions proceed at the electrode surface, for example,... 

Cytochrome Oxidase. Reduced cytochrome c is oxidized in particles by cytochrome a. This is consistent with the difference in oxidation-reduction potentials measured by Ball of - -0.25 volts for cytochrome c and -fO.29 volts for cytochrome a. Despite years of intensive investigation, both the chemical nature and physiological functions of cytochrome a remain uncertain. One of the major questions concerns the relation of cytochrome a to the terminal enzyme of the series, cytochrome oxidase, which reacts with molecular oxygen. 

C. W. Conroy, P. Tyma, P. H. Daum, J. E. Erman, Oxidation-Reduction Potential Measurements of Cytochrome c Peroxidase and pH Dependent Spectral Transitions in the Ferrous Enzyme. Biochim. Biophys. Acta, 537 (1978) 62-69. 

Ozone can be analyzed by titrimetry, direct and colorimetric spectrometry, amperometry, oxidation—reduction potential (ORP), chemiluminescence, calorimetry, thermal conductivity, and isothermal pressure change on decomposition. The last three methods ate not frequently employed. Proper measurement of ozone in water requites an awareness of its reactivity, instabiUty, volatility, and the potential effect of interfering substances. To eliminate interferences, ozone sometimes is sparged out of solution by using an inert gas for analysis in the gas phase or on reabsorption in a clean solution. Historically, the most common analytical procedure has been the iodometric method in which gaseous ozone is absorbed by aqueous KI. 

ORP Oxidation reduction potential - the degree of completion of a chemical reaction by detecting the ratio of ions in the reduced form to those in the oxidized form as a variation in electrical potential measured by an ORP electrode assembly. OSHA The Williams-Steiger Occupational Safety and Health Act of 1970 (OSHA) is a law designed to protect the health and safety of industrial workers and treatment plant operators. It regulates the design, construction, operation and maintenance of industrial plants and wastewater treatment plants. The Act does not apply directly... 

Fig. 4. Catalytic activities of metals (as potentials measured at 10-4 A.cm-2) for anodic oxidation of different reductants. Er thermodynamic oxidation-reduction potentials of reductants. H2 reversible hydrogen electrode potential in solution used to study oxidation of each reductant. Adapted from ref. 38.

A different view of the OMT process is that the molecule, M, is fully reduced, M , or oxidized, M+, during the tunneling process [25, 26, 92-95]. In this picture a fully relaxed ion is formed in the junction. The absorption of a phonon (the creation of a vibrational excitation) then induces the ion to decay back to the neutral molecule with emission (or absorption) of an electron - which then completes tunneling through the barrier. For simplicity, the reduction case will be discussed in detail however, the oxidation arguments are similar. A transition of the type M + e —> M is conventionally described as formation of an electron affinity level. The most commonly used measure of condensed-phase electron affinity is the halfwave reduction potential measured in non-aqueous solvents, Ey2. Often these values are tabulated relative to the saturated calomel electrode (SCE). In order to correlate OMTS data with electrochemical potentials, we need them referenced to an electron in the vacuum state. That is, we need the potential for the half reaction ... 

Quantitative structure-chemical reactivity relationships (QSRR). Chemical reactivities involve the formation and/or cleavage of chemical bonds. Equilibrium constants, rate constants and oxidation-reduction potentials are typical examples of quantitative measures... 

Water analysis parameters such as pH, electric conductivity, oxidation-reduction potential and temperature were measured in the field. Ionic chromatography, turbidimetry and ICP-OES were used for anions and metals. 

Most commonly, iron is discussed as being in either the ferrous (Fe2+) or ferric (Fe3+) state. Changes between these two depend on the soil s pH and Eh (where Eh is a measure of the oxidation-reduction potential of soil) as discussed in Chapter 9. Add conditions and low Eh values tend to lead to the production of ferrous ion, while high pH and high Eh values result in the predominance of ferric ion. It should be noted that the ferrous ion is more soluble than the ferric ion and, thus, it will be more available to plants. 

In a manner similar to pH, one can describe the availability or concentration of electrons, abbreviated as Eh, in an environment. This then is the negative log of the electron concentration. As with pH, it is really a measure of the electron activity rather than the concentration and is a measure of the oxidation-reduction potential (often referred to as redox potential) of the soil environment. Aerobic conditions represent electron-losing or oxidizing environments, and anaerobic conditions represent electron-gaining or... 

Zhang TC, Pang H. Applications of microelectrode techniques to measure pH and oxidation-reduction potential in rhizosphere. Soil Environ. Sci. Technol. 1999 33 1293-1299. 

The oxidation-reduction potential or redox potential ( h) is a measure of the tendency of a solution to be oxidizing or reducing. Oxidation and reduction are basically electrical processes that are readily measiued by an electrode potential. All measurements are referred to die standard hydrogen electrode, the potential of which is taken as 0.00 V at 298 K, the H2 pressure as 101325 N/m (1 atm) and activities of H2 and as unity. When the half-cell reaction is written as an oxidation reaction ... 

Oxidation-Reduction Potential Oxidation-reduction potential (ORP) is measured by an ORP probe, which is effective to monitor the redox potential of a bioreactor operated under microaerobic conditions that cannot be successfully measured by a DO probe. The measurements of redox are sometimes influenced by changes in the pH and mineral concentrations of a culture broth. 

The oxidation-reduction potentials of metal ions differ in different solvents due chiefly to differences in the strength of coordination of the solvents to the metal ions. Thus, Schaap and coworkers,33 who measured reduction potentials polarographically in anhydrous ethylenediamine, found the order of half-wave potentials to be Cd2+ > Pb2+ > Cu2+ - Cu+ > Ti+, whereas, in aqueous solution, the order is Cd2+ > Ti+ > Pb2+ > Cu2+ -> Cu+. Oxidation—reduction potentials have been measured in a great variety of non-aqueous solvents, both protonic and non-protonic. Among the former are liquid ammonia and concentrated sulfuric acid.34 Among the latter are acetonitrile, cyanopropane, cyanobenzene, dimethyl sulfoxide, methylene chloride, acetone, tet-rahydrofuran, dimethylformamide and pyridine.34... 

The selectivity inherent in the chemical affinity of one element or compound for another, together wiLli lliedi known sLoicldoiiieliic and llieiiiiodyiiamic behavior, permits positive identification aud analysis under many circumstances. In a somewhat opposite sense, the apparent dissociation of substances at equilibrium in chemical solution gives rise to electrically measurable valence potentials, called oxidation-reduction potentials, whose magnitude is indicative of the conceuliaiiou and composition of llie substance. Wlnle individually all llie above ellecls are unique for eaeli element or compound, many are readily masked by the presence of more reactive substances so they can be applied only to systems of known composition limits. 

Additional difficulties occur with attempts to measure oxidation-reduction potentials electrochemically in aquatic environments. Values obtained depend on the nature and rates of the reactions at the electrode surface and are seldom meaningfully interpretable. Even when suitable conditions for measurement are obtained, the results are significant only for those components behaving reversibly at the electrode surface. 

Since natural waters are generally in a dynamic rather than an equilibrium condition, even the concept of a single oxidation-reduction potential characteristic of the aqueous system cannot be maintained. At best, measurement can reveal an Eh value applicable to a particular system or systems in partial chemical equilibrium and then only if the systems are electrochemically reversible at the electrode surface at a rate that is rapid compared with the electron drain or supply by way of the measuring electrode. Electrochemical reversibility can be characterized... 

Calculated e v values likewise show a reasonably linear relation with polarographic reduction potentials measured on the dyes in solution. The values likewise correlate well with experimental oxidation potentials. 

Ronald Breslow and his collaborators have given some attention to the problem of estimating the degree of destabilization of cyclobutadiene with respect to nonconjugated models. They have concluded from electrochemical measurements of oxidation-reduction potentials of the system 37 38, of which only the quinone 38 has the cyclobutadiene fragment, that the C4H4 ring is destabilized by some 12-16 kcal mole-1 and so is definitely antiaromatic.15... 

The potential of each channel may be composed of two potentials. One is an oxidation-reduction potential generating at the boundary surface between the Ag electrode and the lipid membrane. The other is a Donnan potential at the boundary between the lipid membrane and the aqueous medium or more generally a Gouy-Chapman electrical double-layer potential formed in the aqueous medium [24]. Figure 7 shows a potential profile near the lipid membrane. The oxidation-reduction potential would not be affected by the outer solution in short time, because the lipid membrane had low permeability for water. Then the measured potential change by application of the taste solution is mainly due to the change in the surface electrical potential. 

Holm, T.R. and Curtiss, C.D. (1989) A comparison of oxidation-reduction potentials calculated from the As(V)/As(III) and Fe(III)/Fe(II) couples with measured platinum-electrode potentials in groundwater. Journal of Contaminant Hydrology, 5(1), 67-81. 

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