Redox potential, measured

The conception and building of a photomodulation voltammetry (PV) apparatus was stimulated by the need to measure redox potentials of short-lived... 

Equation (6) links, in a simple way, the thermodynamically important stability constants Kox and /Cred of a complex in different oxidation states with experimentally measurable redox potentials EH and EHa. Therefore it provides an easy way to obtain the ratio of KoxIKted, which is a theoretically useful parameter known as the binding enhancement factor (BEF). We propose that a better description for this ratio would be the reaction coupling efficiency (RCE) since binding by so-called molecular switches may be reduced or enhanced, depending upon the particular system involved. Equation (6) also allows the calculation of Kox if Kted is known or vice versa. 

Since such correlations belong to a series of treatments which are commonly identified as Linear Free Energy Relationships (LFER), and as only the standard potential is an electrochemical quantity directly linked with free energy (AG° = -n F AE°), one can make use of these mathematical treatments only in cases of electrochemically reversible redox processes (or in the limit of quasireversibility). Only in these cases does the measured redox potential have thermodynamic significance. 

Figure 5.9 Example of a redox titration of nickel of hydrogenase from M. marburgensis. The amplitude of the Ni EPR signal is plotted against the measured redox potential. Half of the active sites in the enzyme solution is reduced at a redox potential (midpoint potential) of — 140 mV (at pH 6).The 2H /H2 redox couple has an of —354mV at this pH.The line through the points is a theoretical line assuming a midpoint potential of — 140 mV (Coremans et al. 1989).

A Bucket Brigade of Molecules Carries Electrons from the TCA Cycle to 02 The Sequence of Electron Carriers Was Deduced from Kinetic Measurements Redox Potentials Give a Measure of Oxidizing and Reducing Strengths... 

We now see that mitochondria contain a variety of molecules—cytochromes, flavins, ubiquinone, and iron-sulfur proteins—all of which can act as electron carriers. To discuss how these carriers cooperate to transport electrons from reduced substrates to 02, it is useful to have a measure of each molecule s tendency to release or accept electrons. The standard redox potential, E°, provides such a measure. Redox potentials are thermodynamic properties that depend on the differences in free energy between the oxidized and reduced forms of a molecule. Like the electric potentials that govern electron flow from one pole of a battery to another, E° values are specified in volts. Because electron-transfer reactions frequently involve protons also, an additional symbol is used to indicate that an E° value applies to a particular pH thus, E° refers to an E° at pH 7. 

Bisogni, J.J., Jr. 1989. Using mercury volatility to measure redox potential in oxic aqueous systems. Environ. Sci. Technol. 23, 828-831. 

Figure 4.1 shows the correlation between measured redox potentials of Fe(III)/ Fe(II) presented in Table 4.3 and estimated redox potentials of Compound II/Fe(III) presented in Table 4.4. In spite of the different methodology, there is a clear tendency for the more oxidant enzymes to present more positive Fe(III)/Fe(II) redox potential values. 

Eq. 53 shows the calculation of single redox potentials, unlike the measured redox potential, which may be a mixed potential of several redox reactions not in equilibrium. 

The conversion from pE to the measured redox potential EH follows from ... 

Example 8.15. Solubility of Fe(OH)3 from Redox Potential Data in Deep Groundwaters Figure 8.18 gives measured redox potentials and Fe data (from Grenthe et al., 1992). These latter data were obtained from analysis of dissolved [Fe(II)] and corrected for complex formation with carbonate (Fe -I- C03 = FeC03(aq) log K = 5.56, I = 0). Assuming that the measured redox potential refers to the Fe(II)/Fe(III) system, calculate the solubility constant K for the reaction... 

Figure 8.18. Measured redox potentials in a deep groundwater. Experimental values of the measured redox potentials (recalculated to the standard hydrogen electrode scale) versus (3pH + log[Fe ]). The concentration of [Fe J has been obtained from the analytical determinations by correction for the complex formation with carbonate. The notation refers to the different test sites. The full-drawn line has been calculated using the selected value of the standard potential E. The straight line has the theoretical Nemstian slope of +0.056 V, at the temperature of measurements. (Adapted from Grenthe et al., 1992.)...

The commonest method of measuring redox potential is to immerse a platinum (Pt) electrode along with a reference electrode into a solution the electrodes are connected to a potentiometer that measures the potential difference between the two electrodes. Reduced species in solution tend to donate electrons to the conducting Pt electrode, while oxidized species tend to accept electrons from the electrode. This creates electron flow in the electrode. However, to cause any detectable movement of electrons in the electrode, the reaction between the oxidized (ox) and reduced (red) form of an element in solution ... 

The O2—H2O couple is the redox pair controlling reactions in aerated solutions, so that reaeration of anoxic soils drives reduced species (e.g., Fe " ) toward the oxidized state. The range of redox potentials over which Fe ", and NH4 have been found to oxidize and disappear on aeration of a reduced soil are denoted by the open boxes in Figure 7.5. Nitrate reappearance on aeration is also depicted by an open box. The measured redox potentials that follow re-aeration do not directly reflect the 02—H20 equilibrium state but rather the status of redox couples having faster electron exchange rates. Furthermore, while each redox couple would be expected (in theory) to undergo complete conversion to the reduced form (in flooded soils) or to the oxidized form (in re-aerated soils) before the adjacent redox couple on the Eh scale became active, actual behavior in soils is much less ideal. Several redox reactions are typically active simultaneously. This may reflect spatial variability in the aeration (and redox potential) of soil aggregates, caused by slow diffusion processes in micropores. 

It is clear, after considering all of the factors likely to affect electrode-measured redox potentials, that these potentials are unlikely to be quantitatively meaningful... 

The E% values for a number of indicators useful in measuring redox potentials of soil solutions are listed in Table 7.2. The EX value (adjusted for pH) represents the approximate solution Ef, range for which that indicator is diagnostic. Outside of its range, any particular indicator is fully in the reduced or oxidized state for the Q— HQ indicator, this would mean that the last term in equation 7.13 could not be quantified. In that event, the solution Eh could not be measiued by this single indicator, it would be possible to state only that the Eh is above the E% value (if the indicator is fiilly oxidized) or below the E% value (if the indicator is fully reduced). 

Why are electrode-measured redox potentials and thermodynamically defined redox potentials not necessarily the same in soil solutions ... 

The free enthalpy of electron transfer can easily be calculated from the electrochemically measured redox potentials of the substrates and the excitation energy of the arene using... 

Potentiometry is an electrochemical technique in which the electrical potential of an "inert" electrode is measured against that of a reference electrode while both are immersed in an aqueous solution. A problem in potentiometry is that the measured potential may be slow to achieve a steady value. This is especially common in attempts to measure the Eh of solutions that are poorly poised, as is the case with most natural waters, and it is not uncommon for measured redox potentials to drift for many hours Q, 2). The long equilibration times, together with published reports of large discrepancies between platinum Eh values and actual solution compositions (2), have led to a great deal of uncertainty and skepticism about the use of Eh measurements. 

The redox potential is pH-sensitive and should be measured in conjunction with pH. To convert redox potentials to a common pH, the conversion factor of —59 mV per unit pH is usually employed, although the change of redox potential with pH can be as great as —200 mV per unit pH during rapid redox changes. The value of —59 mV per unit pH is probably reasonable for most measurements. Reporting both the measured redox potential and the pH avoids the conversion problem. Because redox potentials are most often mixed potentials, measurements more precise than 10 mV have little significance... 

In a detailed study, Cu Cl and Cu Br complexes of bpy, dNbpy, BPMOA, BPMODA, TPMA, PMDETA, and MeeTREN were all characterized by cyclic voltammetry (CV) and the measured redox potentials were correlated with the activity of the complexes in the ATRP of MA initiated by ethyl 2-... 

If we consider the energetics of the process, by taking into account the experimentally measured redox potentials of the Cu " /Cu" and O2/O2 couples (see Section VII), it is possible to estimate that the energy associated with the electron transferred from O2 to Cu is at most about 54 kJ mol . ... 

The authors have studied the complex formation between Th(lV) and fluoride using the redox couple Fe(ni)/Fe(ll) as indicator electrode. In a separate experiment they determined the fluoride complexes formed by Fe (the fluoride complex formation of Fe is negligible under the conditions nsed). From the known equilibrium data for the iron system they then calculated the free fluoride concentration from the measured redox potential in test solutions that in addition to Fe(lll)/Fe(ll) also contained Th(lV) the indicator system acts as a fluoride selective electrode. 

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