Redox concentrations

Current-potential curves at fixed redox concentrations... 

In the practice of electrolysis one mostly deals with altering and even exhausting redox concentrations at the electrode interface, so-called concentration polarization this has been considered already on pp. 100-102 for exhaustion counteracted by mere diffusion. The equations given for partial and full exhaustion (eqns. 3.3 and 3.4) can be extended to the current densities ... 

If C°R is kept constant, then the expression involves five unknowns, k , A , k , C°Rn, and C r Hence, it is necessary to carry out experiments at five different redox concentration ratios. This has been done by using five oxidant concentrations, Co, Cq, Co, C°o and C°oV, keeping the concentration of the reductant constant, i.e., C°R. 

In order to apply the zero-point method, it is necessary to know the value of the frequency at which the rectification voltage tends to zero. The experimental determination of the zero-point frequency has some practical difficulties, because over a small frequency range, the rectification signal is indistinguishable. Hence, the zero-point frequencies have been determined by extrapolating plots of EJ V versus w 1/2 for those redox concentration ratios which intercept the abscissa (i.e., when AE = 0). 

This reaction is fairly stable in a 1.0 N KN03 medium. Earlier studies have been made using equimolar concentrations of the oxidant and reductant.59 This reaction has been studied again by varying the redox concentration ratios.60 The plot of AE / V versus to l/2 is shown in Fig. 8. Although the value of a remains unaltered, the rate constant is found to be of the order of 10 1 cm/s and is... 

The SECM can be used in the feedback mode to probe lateral mass-charge transfer [79-83]. The theory of SECM feedback surveyed in Section IV.A.2 assumes that the substrate surface is uniformly reactive. When lateral mass and/ or charge transfer occurs on the substrate surface, or within a thin film, the surface reactivity of the substrate becomes non-uniform and the SECM feedback theory must be modified. Unwin and Bard [79] developed the theory for adsorption-desorption of a redox species at the substrate that allowed for surface diffusion of the adsorbate. They introduced a new approach, the scanning electrochemical microscope induced desorption (SECMID), as a way to probe surface diffusion. The set of differential equations for the diffusion problem comprise Eqs. (8a,b), and Eq. (26), which relates the redox concentration at the substrate surface and the surface coverage by adsorbed species... 

Zhang et al. [42] carried out SECM studies of the externally biased polarized ITIES. They obtained Tafel plots for different concentrations of the redox couple [Fe(CN)6]4 /3, with same Fe(II)/Fe(III) concentration ratio of 1 1. Essentially the same transfer coefficient (0.38) was found from the entire set of experimental data, indicating that for a wide range of redox concentrations, the... 

Under conditions of permselectivity, i.e., when the electrolyte concentration is much less than the redox concentration in the film, represents the mass of solvent exchanged. 

FIGURE 3.17 (See color insert following page 392.) Redox concentrations shown as (a) nodnles and concretions, (b) pore linings, and (c) soft bodies within the soil matrix. (USDA-NRCS, 2006.)... 

A16. Coast Prairie Redox. For use in MLRA 150A of LRR T. A layer starting within 15 cm (6 in.) of the soil surface that is at least 10 cm (4 in.) thick and has a matrix chroma 3 or less with 2% or more distinct or prominent redox concentrations as soft masses or pore linings. 

Unique characteristics of ferromanganese nodules and associated oxidation-reduction reactions have been used by soil scientists as morphological indicators to help identify hydric soils (see Chapter 3). These characteristics are termed by soil scientists as redoximorphic features however, various terms such as redox concentrations, redox depletions, and reduced matrix are synonymously used for the oxidation-reduction of iron and manganese and their respective concentrations. We prefer not to define these characteristics as redoximorphic features because oxidation-reduction reactions not only involve iron and manganese but also a range of elements that support biotic communities in the biosphere. 

COLOR FIGURE 3.17 Redox concentrations shown as (a) nodules and concretions, (b) pore linings, and... 

Fe(CN)g solution have been studied and it was found that the rate-limiting step of the whole photoinduced electron transfer depends on the applied potential and the redox concentration in solution. The sigmoidal-shaped steady-state photocurrent versus applied potential curve observed is different from the linear-shaped i(ph)/ curve of Cgo modified BLM reported by Bensasson et al. [68]. 

No longer an independent variable, the electrode potential is determined by the redox concentrations. Accordingly, the surface excess of redox components and charge are also not independently determinable. The expressions for the redox components are shown in the following equation... 

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