Redox conditions, measurement

In general, experimenters performing recent hydrolysis studies have recognized some of the problems in earlier work (such as the need to work under an inert atmosphere in order to avoid carbonate contamination), and these studies should be more reliable, especially when coupled with direct spectroscopic methods for species determination. The major problems associated with solubility studies remain the attainment of steady-state conditions, the identification of solid phases, and control of the redox conditions. Measurements of hydrolysis reactions and solubilities at temperatures other than 25° C or ambient are almost nonexistent... 

The pros and cons of oxidative dehydrogenation for alkene synthesis using doped cerianites as solid oxygen carriers are studied. The hydrogen oxidation properties of a set of ten doped cerianite catalysts (Ce0.9X0.1Oy, where X = Bi, In, La, Mo, Pb, Sn, V, W, Y, and Zr) are examined under cyclic redox conditions. X-ray diffraction, X-ray photoelectron spectroscopy, adsorption measurements, and temperature programmed reduction are used to try and clarify structure-activity relationships and the different dopant effects. 

While not stated explicitly, in this discussion so far, it has been assumed that all the systems were well defined, at equilibrium, and at a constant 25°C. None of these conditions occur in soil in the environment. Soil is not a pure system and, often, all the components affecting redox reactions are not known, defined, or understood, and a host of different redox couples are likely to be present. Unless it is possible to take into account all couples present, it is not possible to describe the exact redox conditions in a soil without measuring it. 

Redox equilibrium is not achieved in natural waters, and no single pe can usually be derived from an analytical data set including several redox couples. The direct measurement of p thus is usually not meaningful because only certain electrochemically reversible redox couples can establish the potential at an electrode (4, 35). However, p is a useful concept that indicates the direction of redox reactions and defines the predominant redox conditions. Defining pe on the basis of the more abundant redox species like Mn(II) and Fe(II) gives the possibility of predicting the equilibrium redox state of other trace elements. The presence of suitable reductants (or oxidants) that enable an expedient electron transfer is, however, essential in establishing redox equilibria between trace elements and major redox couples. Slow reaction rates will in many cases lead to nonequilibrium situations with respect to the redox state of trace elements. 

Platinum electrodes are sometimes utilized to obtain Eh measurements on water samples. However, they are often ineffective, especially with natural waters (Drever, 1997), 136, 180-182 (Lindberg and Runnells, 1984). Besides problems with chemical disequilibria in many natural waters, platinum electrodes usually fail to quickly respond to the kinetics of the couples that commonly control redox conditions in water samples, including O2-H2O, S02 -H2S, CO2-CH4, NO -N2, and N2-NH (Drever, 1997), 136. For arsenic, large discrepancies may exist between redox values calculated from the As(V)/As(III) couple and Eh measurements with platinum electrodes (Ryu et al., 2002), 2989-2990. Additionally, when collecting... 

Section 5.2 provides the thermodynamic basis for predicting whether or not a specific redox transformation can occur spontaneously in a given environment. The necessary redox halfreactions involving contaminants are usually well characterized because contaminants are the primary motivation for many studies of environmental systems. However, difficulties often arise in selecting the appropriate "environmental" half-reaction with which to balance the overall equation. When an environmental half-reaction cannot be identified, it is tempting to use traditional electrode potential measurements (127,128) as a generic measure of in situ redox conditions. These values (Emeas) then might be used as E°red or E°ox in Equation 14, to assess the thermodynamic potential of a particular contaminant transformation in a particular environment. However, a number of fundamental difficulties arise with this approach, so we do not recommend the procedure. 

As discussed in section 3.3, there is no reason to expect that measures of "overall" redox conditions (such as Pt electrode potentials or concentrations of dissolved H2) will ever provide an improved basis for quantitatively predicting rates of environmental redox reactions. However, extensions and refinements to the simplified bimolecular model can be made when sufficient data are available. 

Procedures. Batch equilibrations of interbed solids (Mabton Interbed, Rattlesnake Ridge sandstone, or tuff), tracers, and groundwaters were used to measure radionuclide distributions between solid and liquid phases. Triplicate measurements were made for each combination of temperature, redox condition, tracer concentration, tracer type, groundwater composition, and interbed sample. Constant temperatures were maintained by placing the... 

Since crushed basalt has been recommended as a major backfill component (1), experiments were completed to evaluate the rate of dissolved oxygen consumption and the redox conditions that develop in basalt-water systems under conditions similar to those expected in the near-field environment of a waste package. Two approaches to this problem were used in this study (l)the As(III)/As(V) redox couple as an indirect method of monitoring Eh and (2) the measurement of dissolved oxygen levels in solutions from hydrothermal experiments as a function of time. The first approach involves oxidation state determinations on trace levels of arsenic in solution (4-5) and provides an estimate of redox conditions over restricted intervals of time, depending on reaction rates and sensitivities of the analyses. The arsenic oxidation state approach also provides data at conditions that are more reducing than in solutions with detectable levels of dissolved oxygen. 

A measure of the oxidation/reduction capability of a solution (liquid or solid) measured with an -> inert electrode. For -> electrochemically reversible systems it is defined by the - Nernst equation. For -> electrochemically irreversible systems it is a conditional measuring quantity, i.e., depending on the experimental conditions. See also -> potential, - redox potential. 

The pH can be measured by the usual potentiometric method. Furthermore, potentiometric measurements give information on the redox conditions of the solution. These can be done in the suspension of rocks/soils in the liquid phase. In both cases, oxidized or reduced species are studied, and the results are introduced into the thermodynamic calculation of stable species. 

Potentiodynamic polarization measurement is an effective electrochemical technique in characterizing film formed on a metal surface under various redox conditions. The electrochemical polarization measurements were... 

The same, hut different. The a 3 P 3 y complex of mitochondrial or chloroplast ATP synthase will function as an ATPase in vitro. The chloroplast enzyme (both synthase and ATPase activity) is sensitive to redox control, whereas the mitochondrial enzyme is not. To determine where the enzymes differ, a segment of the mitochondrial y subunit was removed and replaced with the equivalent segment from the chloroplast y subunit. The ATPase activity of the modified enzyme was then measured as a function of redox conditions. 

Cozzarelli 1. M., Eganhouse R. P., Godsy E. M., WarrenE., and Bekins B. A. (2001b) Measurement of biodegradation of hydrocarbons in situ under different redox conditions. In 20th Int. Mtg of Organic Geochemistry, 10-14 Sept. 2001, 2pp. 

Bjerg P. L., Jakobsen R., Bay H., Rasmussen M., Albrechtsen H.-J., and Christensen T. H. (1997) Effects of sampling well construction on H2 measurements made for characterization of redox conditions in a contaminated aquifer. Environ. Sci. Technol. 31, 3029-3031. 

Although H2S is considered to be a gas commonly generated during the oxidation of sulphide mineral deposits (see Chapter 8), no more than 0.01 mg 1 could be found in the pore spaces of soils above mineral deposits. Therefore simulation experiments were performed to investigate (a) the affinity of soil for HjS adsorption (b) the method of H2S transport through soil and (c) the redox conditions favourable to H2S persistence in soil. These experiments are very simplistic and there are some difficulties in relating them to the natural environment. They do, however, contribute to understanding the behaviour of H2S in soils and how it can be used in mineral exploration. The limit of detection of the method used to measure H2S in these experiments is about 0.01 mg H2S. 

Eh is often confused with the closely related redox potential or oxidation-reduction potential (ORP) measurement. ORP is measured by placing a redox electrode into water or sediment the redox electrode is a piece of metallic platinum, which acquires a more negative potential with respect to its reference electrode under reducing conditions where electron activities are higher. ORP is the voltage measured between this redox electrode and a reference electrode placed in the same environment. It provides a useful, approximate characterization of redox conditions in the aquatic environment, although it... 

Redox conditions in aquatic systems are bounded by the reactions of potential electron donors and acceptors with water, much as water buffers acid/base reactions by accepting or donating protons (Chapter 4). For example, in oxic marine systems, where dissolved O2 concentrations are measurable, the controlling redox couple (vwitten as a reduction) is O2-H2O ... 

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