Applications redox potential measurements

Other electrochemical techniques covered include measurements of the corrosion potential, the redox potential, the polarization resistance, the electrochemical impedance, electrochemical noise, and polarization curves, including pitting scans. A critical review of the literature concerned with the application of electrochemical techniques in the study of MIC is available [1164]. 

The possibility of isolating the components of the two above-reported coupled reactions offered a new analytical way to determine NADH, FMN, aldehydes, or oxygen. Methods based on NAD(P)H determination have been available for some time and NAD(H)-, NADP(H)-, NAD(P)-dependent enzymes and their substrates were measured by using bioluminescent assays. The high redox potential of the couple NAD+/NADH tended to limit the applications of dehydrogenases in coupled assay, as equilibrium does not favor NADH formation. Moreover, the various reagents are not all perfectly stable in all conditions. Examples of the enzymes and substrates determined by using the bacterial luciferase and the NAD(P)H FMN oxidoreductase, also coupled to other enzymes, are listed in Table 5. 

Just as in aqueous solutions, potentiometry is the most fundamental and powerful method of measuring pH, ionic activities and redox potentials in non-aqueous solutions. Here we deal with the basic techniques of potentiometry in non-aque-ous solutions and then discuss how potentiometry is applicable to studies of chemistry in noil-aqueous solutions. Some topics in this field have been reviewed in Ref. [1],... 

Redox potential is measured potentiometrically with electrodes made of noble metals (Pt, Au) (Fig. 12). The mechanical construction is similar to that of pH electrodes. Accordingly, the reference electrode must meet the same requirements. The use and control of redox potential has been reviewed by Kjaergaard [218]. Considerations of redox couples, e.g. in yeast metabolism [47], are often restricted to theoretical investigations because the measurement is too unspecific and experimental evidence for cause-effect chains cannot be given. Reports on the successful application of redox sensors, e.g. [26,191], are confined to a detailed description of observed phenomena rather than their interpretation. 

The Nernst equation is applicable only if the redox reaction is reversible. Not all reactions are completely reversible in natural systems activities of reacting components may be too low or equilibrium may be reached very slowly. In a sediment, the biotic microenvironment may create a redox potential that is different from the surrounding macroenvironment. For this reason, measurements of Eh in natural systems must be cautiously evaluated and not used strictly for calculations of chemical equilibria. Calculations of redox equilibria are in some cases valuable, in the sense that they will give information about the direction of chemical reactions. 

Recently we carried out kinetic studies with Hildenborough and Miyazaki cytochrome c3 using deazariboflavin semiquinone (dRf ), MV +, and propylene diquat (PDQ +), produced by laser flash photolysis, as reductants (37). Initially, all three reactions were accurately second order, consistent with all hemes being reduced with the same rate constant or with a single site reduced, followed by fast intramolecular electron transfer to reduce the remaining three hemes. However, by measuring reduction kinetics with cytochrome c3 poised at different extents of reduction, the kinetics of reduction of individual hemes could be resolved. Thus, reduction of cytochrome c3 in approximately 5% steps and application of the known macroscopic redox potentials (see previous section) enabled calculation of the concentration of each heme (c.) at each stage of reduction. The plot of kohs versus percent reduction can thereby be fitted to solve for the rate constant for each heme (kt) ... 

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]. 

Successful application of cathodic protection depends upon the selection, design, installation, and maintenance of the system. Before designing the cathodic protection systems, adequate field data must be collected, analyzed, and evaluated. Nature and conditions of the soil are reflected by field measurements like soil resistance, hydrogen ion activity (pH), and the redox potential. To understand the nature of the pipeline, potential measurements, coating resistance, and meaningful design current requirement tests must be conducted. 

Habib and Bockris measured the potential differences across bilayer lipid membranes separating two solutions containing redox systems of different redox potentials (Figure 2) under no applied potential condition. They also measured the potential differences when the bilayer membrane was replaced with a Pt membrane, which represents the ideal situation in the presence of electronic condition when the electron transfer hypothesis for the membrane potential is undoubtedly applicable. Correspondingly, they found that the measured membrane potential is, indeed, the difference of the redox potentials on the two sides of the Pt membrane (line I, Figure 5). 

Redox indicators have been widely used to detect the endpoint of titrimetric redox analyses. Potentio-metric detection of endpoints has now largely replaced the use of indicators, but redox indicators are still in use because of their simplicity. Redox indicators can be used to assess redox potentials in many redox systems where visual rather than electrical measurements can sometimes be more helpful. Recent applications of redox indicators include flow-injection analysis with colorimetric monitoring, or the measurement of electrode potentials of solutions using an immobilized redox indicator on the end of a fiber-optic probe. In studies of the metabolism of cells, redox indicators with their color or fluorescence changes are sometimes more convenient than potentiometry. Redox indicators suffer from their dependence on pH changes, and there is not yet a universal redox indicator that can show the redox potential of a solution over a wide range of potentials... 

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