Redox Potential Measurements

Measurement of some of these parameters identifies the risk of a particular type of corrosion, for example pH measurements assess the risk of acid attack and redox potential measurements is used to assess the suitability of the soil for microbiological corrosion, a low redox potential indicates that the soil is anaerobic and favourable for the life cycle of anaerobic bacteria such as to sulphate-reducing bacteria. Other measurements are more general, resistivity measurements being the most widely quoted. However, as yet no single parameter has been identified which can confidently be expected to assess the corrosion risk of a given soil. It is therefore common practice to measure several parameters and make an assessment from the results. 

As a simplification, it was assumed that the interactions between charged oligomeric segments were negligible. These redox data correlate well with potential values obtained by extrapolation from quantum ma hanical calculations and redox potential measurements on oligomers of defined chain length. As expected,... 

In these experiments, AE was obtained from temperature dependent redox potential measurements for the isolated, monomeric couples based on a thermochemical cycle. 

Another way of testing eq 9 is by using the same chemical system in a series of solvents. Clearly the situation is more complicated since there are two solvent dependences in the experimental measurement, one arising from x and one from AE. The latter can be approximated from redox potential measurements... 

What should be emphasized is that the redox potentials measured for 10 and 22 allow for both the reduction of 10 to dihydro-10 via F420H2 and H2, and the oxidation of dihydro-10 to 10 by 22. This finding, supported by electrochemical experiments, also strongly corroborates the hypothesis that 10 plays a prominent role as an electron carrier in the electron transport system of methanogens. 

In spite of this progress, the gaps in our knowledge of the molecular mechanisms of the participation of flavins in one-electron transfer reactions are enormous. Whether the reduction of flavins by obligatory two-electron donors occurs by a concerted two-electron process or by sequential one-electron transfers remains a matter of controversy and is a subject under current active investigation. It is hoped that this review will convince the reader of the usefulness and necessity of redox potential measurements in the understanding of electron transfer reactions in flavoenzymes. These type of measurements have become more numerous in recent years however, more information of this type is needed. We have seen that the apoprotein environment can alter the one-electron potentials of their respective bound flavin coenzymes by several hundred millivolts, yet virtually nothing is known, on a molecular basis, of how this is achieved. 

This is a striking result since it suggests that simple absorption band measurements can be used to calculate Ea for thermal electron transfer. Note that for unsymmetrical cases where AE 0, both Eop and AE must be known AE can sometimes be estimated from temperature dependent redox potential measurements. 

Hannan. JN. and D.M. Gray pH and Redox Potential Measurements, in Process/lndustrial Instruments Controls Handbook, D.M. Considine, Editor, 4th Edition, McGraw-Hill, New York, NY, 1993. 

However, redox potential measurements, ESCA scans, and thermoanalytical studies indicate that manganese(II) has, in fact, undergone oxidation. The redox potential of the old manganese-montmorillonite is more positive than that of the fresh sample, showing an oxidation process. According to the results of ESCA studies, the oxidation state of the fresh manganese-montmorillonite is II, whereas the old samples refer to III, IV oxidation states. 

Potentiometric measurements are simple the redox potential is measured compared to a reference electrode. For pH measurements, commercially available electrodes, comprising the working (glass electrode) and reference electrodes, can be used. For redox potential measurements, the working electrode is usually an inert (e.g., platinum) electrode, and the reference electrode can be a hydrogen electrode, calomel, or other electrodes. Ion-selective electrodes are also based on potential measurements. 

When the RTIL contained in the RE is different from the target RTIL, an experimental error can be generated because of the junction potential between these RTILs. However, the use of an internal reference such as ferrocene can remove the junction potential. This is because the redox potential measured with the RE contains the same junction potential. 

In the absence of oxygen, with extreme care, a reasonably reliable E can be measured, for example, in groundwater (see Example 8.15 and Figure 8.18). But even in the case mentioned (Grenthe et al., 1992) it took more than 20 days to establish a redox potential of the Fe(OH)3(s)-Fe(II) system of that groundwater. For a recent review on redox potential measurements in the laboratory and in the field see Grenthe et al. (1992) and Lindberg and Runnels (1984). 

The reaction of sodium hydroxide with chlorine is strongly exothermic (AH = 103 kJ/mol). Production can be carried out discontinuously and is monitored by redox potential measurements. Since hypochlorite is easily converted to chlorate at high temperatures, the reaction temperature must be kept below 40°C, for which coolers constructed of titanium are used. The chlorination is generally carried out in such as way that a slight excess of alkali is retained so as to increase the stability of the... 

Redox equilibria between the various oxidation states of selenium have been little studied. This is apparently a consequence of slow reaction rates. For instance, it has been repeatedly demonstrated that the redox potential measured by a platinum electrode is not affected by the ratio of Se(VI) / Se(lV) present in solution, [87RUN/LIN]. The values of the standard electrode potentials of the important redox couples Se(VI) / Se(IV) and Se(lV) / Se(0) are essentially based on only one experimental investigation each. A more detailed discussion than would normally be required will therefore be made of the two investigations. 

The potentials listed in Table 7.1 and plotted in Figure 7.1 are thermodynamic values and do not necessarily correspond to actual values measured in solutions, for reasons that will be discussed in this section. For example, aerobic soil solutions typically have measured values of about 400 to 500 mV, and O2 disappears from the soil at about 350 mV. These are much lower values than expected theoretically, since dissolved O2 should in principle maintain potentials closer to 1000 mV (see Figure 7.1). To understand discrepancies between theory and measurement, some knowledge of the principles involved in redox potential measurement is needed. 

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