Redox barrier

Because reductants are present in most water-saturated soils or sediments and oxygen is relatively unavailable, the Eh declines as the water moves into the subsurface. The decline may be from oxic to anoxic sulfidic or nonsulfidic levels (Table 11.5). The rate and extent of Eh decline with distance from the surface depends on the availability and reactivity of sediment organic matter and other reductants. In the sediments of flooded rice paddies, wetlands, estuaries, and shallow lakes, which may be especially rich in fresh organic matter, the redox front or intetface (also termed a redox barrier or boundary by some), which is the zone of abruptly changing Eh values, may be only a few millimeters or centimeters thick. 

Sale T, Petersen M, Gilbert D. (2005). Electrically Induced Redox Barriers for Treatment of Groundwater. Environmental Security Technology Certification Program (ESTCP). Project CU-0112. Final Report. 

Sale T C, GilbotD M, Petersen M A (2005) Cost and performance report electiieally induced redox barriers for treatment of groundwater. ESTCP Project CU0112... 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

Armentrout PB (2003) Threshold Collision-Induced Dissociations for the Determination of Accurate Gas-Phase Binding Energies and Reaction Barriers. 225 227-256 Astruc D, Blais J-C, Cloutet E, Djakovitch L, Rigaut S, Ruiz J, Sartor V, Valerio C (2000) The First Organometallic Dendrimers Design and Redox Functions. 210 229-259 Aug6 J, see Lubineau A (1999) 206 1-39... 

Depending on the electronegativity and the cation s oxidation state, the oxide can be either acidic or basic. Also, doping the ceria lattice does not automatically mean a substitution of a host cerium cation. Inomata and coworkers showed, in the case of Ce(i x)FexOy, that Fe3+ ions were located not only at Ce4+ sites, but also at interstitial sites (49). These interstitial ions bring about an increased barrier for electrons moving between Ce3+ and Ce4+, and influence the redox potential and the ease of reducibility of the material. 

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