Standard transfer potential

As mentioned above, the distribution of the various species in the two adjacent phases changes during a potential sweep which induces the transfer of an ion I across the interface when the potential approaches its standard transfer potential. This flux of charges across the interface leads to a measurable current which is recorded as a function of the applied potential. Such curves are called voltammograms and a typical example for the transfer of pilocarpine [229] is shown in Fig. 6, illustrating that cyclic voltammograms produced by reversible ion transfer reactions are similar to those obtained for electron transfer reactions at a metal-electrolyte solution interface. 

Experimentally, Ao j/2 is generally considered equal to the midpeak potential, and is then directly deduced from the voltammograms. This does not generate experimental errors, since an ion of known standard transfer potential (for instance, tetramethyl ammonium, TMA+) must be used as an internal reference to transpose the experimental potential scale (noted E) to the absolute Galvani potential scale, so that is obtained by ... 

The ion activities employed in Eq. (5) should be those established after equilibration of the two phases It is generally assumed that the equilibrium distribution values can be approximated by the initial ionic concentrations. The validity of this approximation is dependent on the standard transfer potentials of the counterions, and in practical cases it will only hold over a restricted range ( 0.1— 0.2 V [19]). The variance of such a system means that its Galvani potential difference is defined, over the above working range, through the ratio of activities of X. 

System of Ions with Large Difference in Standard Transfer Potential... 

We now consider a system (System II) of two salts, one of which is strongly soluble in water (NaCl) and the other (TBATPB) in NB. The standard transfer potential of all ions in the system is strongly negative or strongly positive. 

As shown in Fig. 3 the curve Q versus potential has a wide plateau (from —0.125 to -I- 0.2 V) whereas the Q values are close to zero. With this change in potential, only a negligible amount of charge (0.00076 F) is transferred through the interface, which behaves like an ideal polarization interface. The potential window for voltammetric measurement is wide. On the other hand, the equilibrium potential is sensitive to the presence of ions that have standard transfer potentials within the window. Therefore, system II cannot be used as a reference electrode. 

Table 3 indicates that, in equilibrium, depending on the standard transfer potential, the amount of ions transferred from one phase to another follow the order Cl < TPB < Na+ < TBA +. ... 

In the system of KCl and TBATPB, the presence of ions, that can be easily transferred through the interface (standard transfer potential of the ion is close to zero) can strongly influence the galvani potential and the distribution equilibrium as well. Let us consider the system ... 

The calculation can be carried out for different solvent systems in conditions that the standard transfer potential of all components and the equilibrium constants of all interactions (if they occur) are given. 

From Table 7 and Fig. 19 it is indicated that the only difference between this system and the water/nitrobenzene system is an expansion of the potential scale because of the larger difference in standard transfer potential between ions present in the system. 

From the basic parameters initial concentration of ions, their standard transfer potential, distribution coefficients for neutral components, equilibrium constants of reactions taking place in the system, volume of phases, and temperature, a unique general problem for the Galvani potential difference and distribution concentration of all components was established. A numerical solution to the problem with the help of computer program EXTRA.FIFIl provided a good means for quantitative investigation of the liquid-liquid interface. It is also useful for the study of liquid-liquid extraction, electroextraction, voltammetry at interface of two immiscible electrolyte solutions (ITIES) [15,18], liquid-liquid membrane ion-selective electrodes, biomembrane transport, and other fields of science and engineering. 

Gibbs transfer encryy or standard transfer potential of every component (basic constants tabled as mentioned in section II.C.I)... 

The proportion of the Galvani potential difference occurring in the aqueous phase is termed a. At equilibrium, at the standard transfer potential, we have... 

It should be noted that in order to apply this refined model in general it is necessary to know the potential drop across the inner layer. We can recover Eq. (56) if we make the assumption that a is independent of A

following equation valid at equihbrium at the formal standard transfer potential ... 

Equation (17.3.2.7) is analogous to the Nemst equation for electrode potential, and is called the standard transfer potential. 

Standard Transfer Potential and Apparent Ion-Transfer Rate Constant... 

Of course, from a voltammetric viewpoint, assisted-ion-transfer reactions are characterized by a shift of apparent standard transfer potentials to lower values. From a thermodynamic viewpoint, and in the case of a 1 1 reaction between a cation and a ligand, we can consider the partition coefficient of the ion I+, of the ligand L, and of the complex IL+ as shown in Figure 1.16. 

An alternative method is to deposit an aqueous drop on a solid electrode such as a silver-silver chloride electrode or a platinum electrode and immerse it in an organic solvent [291]. This method provides a fast and convenient method to measure standard transfer potentials, for example, of ionized drug molecules. It differs from the three-phase junction discussed in the following text, in the sense that the electrode is in contact with one phase only, namely, the aqueous one. In this case, we have two interfaces in series. 

The standard transfer potential and, consequently, the standard Gibbs energy of transfer, by measurement of the halfwave potential by cyclic voltammetry... 

This condition should also be fulfilled for the half wave, or more precisely the midpeak potential, to be equal to the standard transfer potential (as defined by Eq. (38), assuming further the equality of the diffusion coefficients of the reactants and products. 

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