Galvani potential difference

Thus Pi, surface potential jump X, the chemical potential p, and the Galvani potential difference between two phases A0 = are not. While jl, is defined, there is a practical dif-... 

In addition to the case of a metal in contact with its ions in solution there are other cases in which a Galvani potential difference between two phases may be found. One case is the innnersion of an inert electrode, such as platinum metal, into an electrolyte solution containing a substance S that can exist m either an oxidized or reduced fomi tlirough the loss or gain of electrons from the electrode. In the sunplest case, we have... 

Once current passes tluough the interface, the Galvani potential difference will differ from that expected from the Nemst equation above the magnitude of the difference is temied the overpotential, which is defined heiiristically as... 

Of fundamental importance in understanding the electrochemistry of ion-selective membranes and also of biomembranes is the research in the field of voltammetry at ITIES mainly pioneered by Koryta and coworkers 99 101 . Koryta also demonstrated convincingly that a treatment like corroding metal electrodes is possible 102). For the latter, the description in the form of an Evans-diagram is most appropriate Fig. 4 shows schematically some mixed potentials, which are likely to arise at cation-selective membranes if interfering ions disturb an ideal Nernstian behavior82. Here, the vertical axis describes the galvani potential differences (absolute po-... 

The standard Galvani potential difference can be related to the standard electrode potentials and oxi/rediCo) oii hydrogen scale in water or organic... 

Assuming that the two space-charge regions are separated by a layer of solvent molecules (inner layer or mixed solvent layer), the Galvani potential difference can be expressed as the sum of three contributions ... 

FIGURE 32.4 Potential dependence of the interfacial tension J and the capacity C for the interface between solutions of 5mM tetrabutylammonium tetraphenylborate in 1,2-dichloroethane and lOOmM LiCl in water. The potential scale E represents the Galvani potential difference relative to the standard ion transfer potential for tetraethylammonium ion, cP o EA+ = 0.02 V. 

However, as stated above, the partition coefficients measured by the shake-flask method or by potenhometric titration can be influenced by the potenhal difference between the two phases, and are therefore apparent values which depend on the experimental condihons (phase volume ratio, nature and concentrahons of all ions in the solutions). In particular, it has been shown that the difference between the apparent and the standard log Pi depends on the phase volume raho and that this relationship itself depends on the lipophilicity of the ion [80]. In theory, the most relevant case for in vivo extrapolation is when V /V 1 as it corresponds to the phase ratio encountered by a drug as it distributes within the body. The measurement of apparent log Pi values does not allow to differentiate between ion-pairing effect and partihoning of the ions due to the Galvani potential difference, and it has been shown that the apparent lipophilicity of a number of quaternary ion drugs is not due to ion-pair partitioning as inihally thought [80]. 

Bouchard, G., Carrupt, P. A., Testa, B., Gobry, V., Girault, H. H. The apparent lipophilicity of quaternary ammonium ions is influenced by Galvani potential difference, not ion-pairing a cyclic voltammetry study. Pharm. Res. 2001, 18, 702-708. 

From the thermodynamic point of view, the occurrence of a heterogeneous ET event at a liquid-liquid interface is determined not only by the relation of redox potentials between reactants in each phase, but also by the Galvani potential difference. Let us consider the general reaction,... 

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