Voltammetry at ITIES

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 previous chapters dealt with ISE systems at zero current, i.e. at equilibrium or steady-state. The properties of the interface between two immiscible electrolyte solutions (ITIES), described in sections 2.4 and 2.5, will now be used to describe a dynamic method based on the passage of electrical current across ITIES. Voltammetry at ITIES (for a survey see [3, 8, 9, 10, 11, 12,18]) is an inverse analogue of potentiometry with liquid-membrane ISEs and thus forms a suitable conclusion to this book. 

Besides determination of data of theoretical interest such as standard Gibbs transfer energies, distribution coefficients and stability constants, voltammetry at ITIES has found the following applications ... 

The concept of assisted ion transfer is, of course, applicable to proton-transfer reactions assisted by the presence of an acid or base, hydrophilic or lipophilic. As pioneered by Kontturi and Murtomaki [165], voltammetry at ITIES has proved to be an excellent method to measure the log values of protonated or deprotonated molecules. Indeed, for therapeutic molecules, the logF values, which are related the Gibbs energy of transfer as shown by Equations 1.11 and 1.12, provide an important physical parameter to assay the toxicity of a molecule. If a molecule is lipophilic, that is, logF >2, it is potentially toxic. In fact, with the concept of ionic distribution diagrams (vide infra) it is even possible to measure the logF values of the neutral associated bases. The application of voltammetry at ITIES to the study of therapeutic molecules has been one of the success stories of electrochemistry at liquid-liquid interfaces. The field has been reviewed over the years [166,167] and very recently by Gulaboski et al. [168]. 

Voltammetry at Micro-ITIES Biao Liu and Michael V. Mirkin... 

The voltammograms at the microhole-supported ITIES were analyzed using the Tomes criterion [34], which predicts ii3/4 — iii/4l = 56.4/n mV (where n is the number of electrons transferred and E- i and 1/4 refer to the three-quarter and one-quarter potentials, respectively) for a reversible ET reaction. An attempt was made to use the deviations from the reversible behavior to estimate kinetic parameters using the method previously developed for UMEs [21,27]. However, the shape of measured voltammograms was imperfect, and the slope of the semilogarithmic plot observed was much lower than expected from the theory. It was concluded that voltammetry at micro-ITIES is not suitable for ET kinetic measurements because of insufficient accuracy and repeatability [16]. Those experiments may have been affected by reactions involving the supporting electrolytes, ion transfers, and interfacial precipitation. It is also possible that the data was at variance with the Butler-Volmer model because the overall reaction rate was only weakly potential-dependent [35] and/or limited by the precursor complex formation at the interface [33b]. 

It should finally be mentioned that cyclic voltammetry at the ITIES allows for a precise discrimination between compounds of different charges, since the position of the peak currents depends directly on lipophilicity, since their intensity varies with Zj [Eq. 

Polarography and Voltammetry (DC, AC, SW, pulse methods for each) Amperometry Chronopotentiometry, Polarography and Voltammetry at the interface between two immiscible electrolyte solutions (ITIES)... 

Cyclic voltammetry has been used mainly for the determination of the standard ion-transfer potential Aq (or the standard Gibbs energy of ion transfer A ttx °), and e ion diffusion coefficient. The Figure shows an example of the cyclic voltammogram for the Cs+ ion-transfer reaction at ITIES in the electrochemical cell... 

Figure 6.8.1 Schematic diagram for the apparatus for cyclic voltammetry at the ITIES between water and nitrobenzene. Ref a and Ref p are reference electrodes, Wk a and Wk p are metal working electrodes.

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. 

A number of other methods (chronopotentiometry [24], polarography with dropping electrolyte electrode [25], faradaic impedance measurement [26], current scan voltammetry [27] etc.) were also applied to the study of electrolysis at ITIES. 

In this communication our attempts to exploit the electrolysis at ITIES for analytical purposes are described. As examples the determination of acetylcholine, tetraethylammonium, calcium, barium and strontium cations by differential pulse stripping voltammetry (DPSV) at the hanging electrolyte drop electrode (HEDE) will be presented. 

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