Redox potential local charges

Cyclic voltammetry is an excellent tool to explore electrochemical reactions and to extract thermodynamic as well as kinetic information. Cyclic voltammetric data of complexes in solution show waves corresponding to successive oxidation and reduction processes. In the localized orbital approximation of ruthenium(II) polypyridyl complexes, these processes are viewed as MC and LC, respectively. Electrochemical and luminescence data are useful for calculating excited state redox potentials of sensitizers, an important piece of information from the point of view of determining whether charge injection into Ti02 is favorable. 

Since the energy of electrons in a material is specified by the Fermi level, ep, the flow of electrons across an interface must likewise depend on the relative Fermi levels of the materials in contact. Redox properties are therefore predicted to be a function of the Fermi energy and one anticipates a simple relationship between the Fermi level and redox potential. In fact, the Fermi level is the same as the chemical potential of an electron. Clearly when dealing with charged particles, the local energy levels e are increased by qV, where q is the charge on the particle and V is the local electrostatic potential. The e, should therefore be replaced by e,- + qV and so... 

BChl a in solution ( 660 + 10 mV).66 ENDOR/TRIPLE further showed that the spin is completely localized on one BChl half (Fig. 3). The pigment analysis showed that the RC contains 3 BChl and 3 BPh, whereas 4 BChl and two BPh are found in wild type. On the basis of earlier work5467 it is therefore assumed that in these mutants, a BChl-BPh heterodimer is the primary donor. Since BPh has a higher redox potential than BChl,66 only the BChl half is oxidized and carries the spin and positive charge. This situation is energetically less favorable than delocalization in a dimeric species, which leads to the observed increase of P 865. For the heterodimer mutants, a lower quantum yield and reduced ET rates have been observed. The experiments clearly show that dimer formation alone significantly lowers and, thereby, adjusts the donor s oxidation potential.68... 

Beyond the primary coordination sphere, electrostatic interactions serve to modulate the redox potential set by the heme type and axial ligand. Local charges as well as solvent-exposure of heme have been correlated with redox potential in artificial and natural heme proteins. Since 1978, Stellwagen observed an inverse dependence between the redox potential and the exposure of heme to aqueous solvent [28]. On the other hand, the influence of charged residues near the heme group has been analyzed in artificial proteins for example, the role of negatively charged residues... 

It is a major surprise (98) to find a bis(Met)heme with such a low redox potential (Table II) and a possible explanation for this is that the heme is located in a region of the protein with a relatively high local negative charge density arising from carboxylic acid groups. 

A few years ago Bard and his group developed the technique called scanning electrochemical microscopy (SECM) which makes possible a spatial analysi,s of charge transfer processes [9]. In this method an additional tip electrode of a diameter of about 2 pm is used as well as the three other electrodes (semiconductor, counter and reference electrode). Assuming that a redox system is reduced at the semiconductor, then the reduced species can be re-oxidized at the tip electrode, the latter being polarized positively with respect to the redox potential. The corresponding tip current / [ is proportional to the local concentration of the product formed at the semiconductor surface and therefore also to the corresponding local semiconductor current, provided... 

The strength of the electrostatic bond between adsorbate and adsorbent varies according to factors such as pH, temperature, redox potential, salt concentration, and charge density. Adsorbed ions will be displaced and readsorbed according to the local conditions and their movement will thus be slowed or stopped. Some ionic species such as chloride tend not to undergo ion exchange and in this case their movement through the landfill will be dependent upon other factors. The unreactive nature of the chloride ion means that it is often used as a marker of the front of a landfill leachate pollution plume. 

The redox potential and the halide potential thus determine the silver activity, and so all defect concentrations are fixed at the phase boundaries [47]. All local transport coefficients are, in turn, determined by the defect concentrations. Therefore, all the prerequisites necessary for a quantitative treatment of the developing process are satisfied, provided that the spatial distribution and size distribution of the nuclei of the latent image are known, or that plausible assumptions regarding these distributions can be made, and provided that electrical double layer and space charge effects can be neglected. 

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