Potential activity dependence

At each interface the interfacial potential will depend upon the chemical potentials of the species involved in the equilibrium. Thus at the Zn/Zn electrode there will be a tendency for zinc ions in the lattice to lose electrons and to pass across the interface and form hydrated ions in solution this tendency is given by the chemical potential of zinc which for pure zinc will be a constant. Similarly, there will be a tendency for hydrated Zn ions in solution to lose their hydration sheaths, to gain electrons and to enter the lattice of the metal this tendency is given by the chemical potential of the Zn ions, which is related to their activity. (See equation 20.155.) Thermodynamically... 

As already stated, the indicator electrode of a cell is one whose potential is dependent upon the activity (and therefore the concentration) of a particular... 

Figure 18 shows the dependence of the activation barrier for film nucleation on the electrode potential. The activation barrier, which at the equilibrium film-formation potential E, depends only on the surface tension and electric field, is seen to decrease with increasing anodic potential, and an overpotential of a few tenths of a volt is required for the activation energy to decrease to the order of kBT. However, for some metals such as iron,30,31 in the passivation process metal dissolution takes place simultaneously with film formation, and kinetic factors such as the rate of metal dissolution and the accumulation of ions in the diffusion layer of the electrolyte on the metal surface have to be taken into account, requiring a more refined treatment. 

It is sometimes said that this electrode is reversible with respect to the anion. This claim must be examined in more detail. An electrode potential that depends on anion activity still constitutes no evidence that the anions are direct reactants. Two reaction mechanisms are possible at this electrode, a direct transfer of chloride ions across the interface in accordance with Eq. (3.34) or the combination of the electrode reaction... 

Often, H+ or OH ions are involved in the electrode reactions, and the electrode potential then depends on the concentration of these ions (or solution pH). Because of the dissociation equilibrium of water, the activities of these ions are interrelated as fle+floH = = 1-27 X 10 moH/L. For this reason these reactions can be for-... 

Can this demand for a significant number of metal active sites be further quantified by a general expression in terms of cathode potential demand The answer is, in principle, yes, although the dependence of the relative populations of metal surface sites and oxidized surface sites on cathode potential could depend on (H20)/r-oh d somewhat different way, depending on the degree to which the... 

The potential of an electrode measured relative to a standard, usually the SHE. It is a measure of the driving force of the electrode reaction and is temperature and activity dependent (p. 230). By convention, the half-cell reaction must be written as a reduction and the potential designated positive if the reduction proceeds spontaneously with respect to the SHE, otherwise it is negative. If the sign of the potential is reversed, it must be referred to as an oxidation potential. 

Activity Dependence of Electrode Potentials - The Nernst Equation... 

Even after this initial reduction, the enzyme from A. vinosum remains inactive. When performed at 2°C and pH 6, the redox potential in such an enzyme solution can be lowered to —350 mV without any increase in activity also no EPR signals of nickel-based unpaired spins are detectable. So the active site can shuttle between Nij y and the one-electron reduced state Nif y-S (S stands for EPR silent), without activation of the enzyme. A proton accompanies the electron (apparendy a strict charge compensation is obligatory), as the midpoint potential is dependent on the pH (—60 mV per pH unit). Although the changes in the EPR spectra suggest just a reduction of nickel in both cases, the ETIR spectra reflect clear changes at the Fe site for the ready enzyme, but not for the unready one (Fig. 7.6). 

Furthermore, since the cell growth arrest is often linked to cell death. The annexin V staining positive cell or the amount of DNA fragmentation assessed by TUNEL and FACS analysis has been interpreted as indicative of apoptosis. The HDACI-induced apoptosis can also be determined by Western blotting of target proteins, detection of mitochondrial membrane potentials, activation of caspases and their substrate cleavages in a dose- and time-dependent manner. 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

Platinum electrodes are widely used as an inert electrode in redox reactions because the metal is most stable in aqueous and nonaqueous solutions in the absence of complexing agents, as well as because of its electrocatalytic activity. The inertness of the metal does not mean that no surface layers are formed. The true doublelayer (ideal polarized electrode) behavior is limited to ca. 200-300 mV potential interval depending on the crystal structure and the actual state of the metal surface, while at low and high potentials, hydrogen and oxygen adsorption (oxide formation) respectively, occur. 

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