Oxidation-reduction potential changes

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. 

Oxidation-reduction reactions may affect the mobility of metal ions by changing the oxidation state. The environmental factors of pH and Eh (oxidation-reduction potential) strongly affect all the processes discussed above. For example, the type and number of molecular and ionic species of metals change with a change in pH (see Figures 20.5-20.7). A number of metals and nonmetals (As, Be, Cr, Cu, Fe, Ni, Se, V, Zn) are more mobile under anaerobic conditions than aerobic conditions, all other factors being equal.104 Additionally, the high salinity of deep-well injection zones increases the complexity of the equilibrium chemistry of heavy metals.106... 

Equilibrium considerations other than those of binding are those of oxidation/reduction potentials to which we drew attention in Section 1.14 considering the elements in the sea. Inside cells certain oxidation/reductions also equilibrate rapidly, especially those of transition metal ions with thiols and -S-S- bonds, while most non-metal oxidation/reduction changes between C/H/N/O compounds are slow and kinetically controlled (see Chapter 2). In the case of fast redox reactions oxidation/reduction potentials are fixed constants. 

A quite different set of oxidations/reductions, but not fast, were the equilibria which governed the change of the environment, that is external oxidation/reduction potentials. They involve elements such as S, Se and metals but not all C or N couples. Their slow change in value was due to the slow release of oxygen by... 

Some lines of prokaryote development are shown in Table 6.2 with a guide to oxidation/reduction potential ranges in Table 6.3. In all these and further changes the novel chemistry has to be built into the cooperative whole (see Section 3.9). Note again the necessity that the novel features must become part of a controlled autocatalytic restricted set of reaction paths, which become general to any further evolution. 

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. 

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. 

Buffer Capacities of Natural Waters. Natural waters are buffered in different ways and to varying degrees with respect to changes in pH, metal ion concentrations, various ligands, and oxidation-reduction potential. The buffer capacity is an intensive variable and is thermodynamic in nature. Hydrogen-ion buffering in natural waters has recently been discussed in detail by Weber and Stumm (38). Sillen (32) has doubted... 

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. 

Photoinduced electron transfer (PET) has been widely used as the preferred tool in fluorescent sensor design for atomic and molecular species [52-57], PET sensors generally consist of a fluorophore and a receptor linked by a short spacer. The changes in the oxidation/reduction potential of the receptor upon guest binding can alter the PET process creating changes in fluorescence. 

The term photochromism can be defined as a light-driven reversible transformation between two isomers possessing different absorption spectra.111,21 The two isomers differ from one another not only in their absorption spectra, but also in their geometrical structures, oxidation/reduction potentials, refractive indices, and dielectric constants. When such photochromic chromophores are incorporated into functional molecules, such as polymers, host molecules, conductive molecular wires, or liquid crystals, the functions can be switched by photoirradiation.[3 61 Photostimulated reversible changes in refractive index can also be applied to optical waveguide switching.171 This chapter reviews applications of photochromic chromophores, especially diar-ylethene derivatives, in various photo switching molecular systems. 

The oxidation-reduction potential, E, (or redox potential) of a substance is a measure of its affinity for electrons. The standard redox potential (E0 ) is measured under standard conditions, at pH 7, and is expressed in volts. The standard free energy change of a reaction at pH 7, AG0, can be calculated from the change in redox potential AE0 of the substrates and products. A reaction with a positive AE0 has a negative AG0 (i.e. is exergonic). 

Oxidation-reduction potential (or redox potential, E) is the potential of compounds to accept electrons and is by convention measured relative to that of hydrogen. Thus E is very negative for NADPH (a strong reductant) but positive for 02 (a strong oxidant). Standard redox potentials (Eo values in volts) refer to standard conditions (1M redox components) at neutral pH (pH 7). The standard free energy change at pH 7 for a particular redox reaction (AGo ) is given by ... 

From the absorbance change at certain wavelengths as the function of the given potential, we could obtain the electrochemical characteristics such as the redox potential. From the absorbance change as the function of the applied potential in Figure 13.3f>, the oxidation-reduction potential of PEO-cyt.c dissolved in 1/KCl... 

The oxidation reduction potentials of MMOR have been measured for two MMO systems by monitoring the changes in the optical and EPR spectra. In MMO Bath the potentials for the FAD/FADH [Fe2S2] / [Fe2S2] and FADH/FADH2 couples were found to be nl50, n220, and... 

The oxidation-reduction potential of water is important in controlling the mobility of uranium. In anoxic waters where the aquatic environment is reductive, U(VI) will be reduced to U(IV) (e.g., changed from a soluble compound to an insoluble one). The U(IV) will be deposited into the sediment due to the insolubility of the resulting U(IV) salts (Allard et al. 1979 Herczeg et al. 1988). Mobilization and... 

The slope of the plot of the oxidation-reduction potential, for constant quinone-hydroquinone ratio, against the pH, i.e., against — log oh% thus undergoes changes, as shown in Fig. 84 the temperature is 30 , so... 

Charton (J52) has also applied the extended Hammett equation to the oxidation-reduction potentials of 5-substituted phenanthroline complexes of iron in various acidic media (95, 97, 651) and of bis-5- and 4,7-substituted phenanthroline complexes of copper in 50% dioxane (404). Thus, one should expect an overall similarity between the variations in pAa, stability constant, and oxidation-reduction potential data for the various ligands. The variations in a and )3 values found for various substitution positions and the tautomerism in the LH+ ions show that the correlation need not be good. A similar point may also be made about the comparison of data for the transoid bipyridylium ions and their cis complexes. Plots of A versus pA for various systems (95, 404) show a linear dependence to differing extents. As would be expected, the data for analogous complexes of iron (28), ruthenium (214, 217, 531), and osmium (111, 218, 220) show very good correlation. The assumption (152) that the effects of substituents are additive is borne out by these potential data, where the changes in potential on methyl substitution are additive (97). 

While the effect on oxidation-reduction potentials of substituents on phenanthroline ligands is regular, studies of the oxidation of [Os(bipy)(terpy)X] + species, where X is an alkyl-substituted pyridine molecule (111), do not show a linear dependence of E° on the pA of X. These results have been explained in terms of the Baker-Nathan effect. However, taken in conjunction with the entropy data of Kratochvil and Knoeck (439) for substituted iron complexes, an explanation involving changes in solvation with substituents seems preferable. The potentials of various Os(II)/Os(III) couples (111) and Ru(II)/Ru(III) couples (220) have been used to study the effect of the overall charge on the... 

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