Mixed ion-transfer potentials

4 NON-EQUILIBRIUM POTENTIOMETRIC RESPONSES 7.4.1 Mixed ion-transfer potentials 

ISEs are operated under open circuit condition, where no significant current flows. When a Nemstian response is obtained, only one ion is exchanged significantly between the membrane and sample phases so that there is no net current carried by the ion at the interface. In contrast, when co-extraction of the analyte and its aqueous counter ion (Donnan exclusion failure) or exchange of the analyte and its interfering co-ion occurs at the interface, the partial current carried by each ion, / , may become significant The partial currents, however, cancel each other so that the total current at the interface, /, is zero 

The non-equilibrium ion-exchange processes can cause deviation of the potentiometric responses from those predicted by the Nikolsky-Eisenman equation and the equilibrium phase boundary potential model. 

The non-equilibrium effects on potentiometric responses can be described using the concept of mixed ion-transfer potential (47). When ion transfers at a sample solution/mem-brane interface are fast enough, a local equilibrium at the interface is always achieved. The salt-extraction and ion-exchange processes, however, induce concentration polarization of the ions near the interface so that the potential is determined by the interfacial ion concentration as 

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The concept of this mixed ion-transfer potential was recently extended to quantify the non-equilibrium responses of neutral-ionophore-based ISEs (48). Figure 7.11 shows the... 

Non-equilibrium processes at the sample/membrane interface and across the bulk membrane bias the selectivity and detection limits of the electrodes. Elimination of these nonequilibrium effects by operating the electrodes under complete equilibrium conditions will be of both practical and fundamental significance. While non-equilibrium responses are useful for potentiometric polyion-selective electrodes, it is not obvious whether potentiometry based on mixed ion-transfer potentials is a better transduction mechanism than amperome-try/voltammetry based on selective polyion transfer (65, 66). Ion-transfer electrochemistry at polarized liquid/liquid interfaces is introduced in Chapter 17 of this handbook. 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

In 2005, an interesting critical summary of the current knowledge on capacitance measurements and potential distribution, together with a new model, was published by Monroe et al. [64]. In this work, the good old Verwey-Niessen theory was extended to allow ionic penetration at the interface. With this adaptation, several features could be accounted for, such as asymmetry and shifts of the capacitance minimnm, that could not be described by the classical Gouy-Chapman or Verwey-Niessen theories. Gibbs energies of ion transfer were used as input parameters to describe ionic penetration into the mixed-solvent interfacial layer, and experimental data were successfully reproduced. 

As the cell is discharged, Zn2+ ions are produced at the anode while Cu2+ ions are used up at the cathode. To maintain electrical neutrality, SO4- ions must migrate through the porous membrane,dd which serves to keep the two solutions from mixing. The result of this migration is a potential difference across the membrane. This junction potential works in opposition to the cell voltage E and affects the value obtained. Junction potentials are usually small, and in some cases, corrections can be made to E if the transference numbers of the ions are known as a function of concentration.ee It is difficult to accurately make these corrections, and, if possible, cells with transference should be avoided when using cell measurements to obtain thermodynamic data. 

The investigations of interfacial phenomena of immiscible electrolyte solutions are very important from the theoretical point of view. They provide convenient approaches to the determination of various physciochemical parameters, such as transfer and solvation energy of ions, partition and diffusion coefficients, as well as interfacial potentials [1-7,12-17]. Of course, it should be remembered that at equilibrium, either in the presence or absence of an electrolyte, the solvents forming the discussed system are saturated in each other. Therefore, these two phases, in a sense, constitute two mixed solvents. 

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