Immiscible electrolyte solutions

Interfaces between two immiscible solutions with dissolved electrolytes, which are most interesting to workers in several disciplines, cover theoretical physical electrochemistry and analytical applications for sensor design. These interfaces are used in interpretation of processes that occur in biological membranes and in biological systems. The interface between two immiscible electrolyte solutions was studied for the first time at least 100 years ago by Nemst (I), who performed the experiments that provide the theoretical basis for current potentiometric and voltammetric studies of interfaces. In 1963, Blank and Feig (2) suggested that an interface between two immiscible liquids could be used as a model (at least as a crude approximation) for... 

Figure 1. Comparison of the interface between an electronically conductive electrode and a solution reduction of Fe3+) (A) and the interface between two immiscible solutions of electrolytes (ITIES) during current flow in a closed electric circuit [transport of picrate (Pi ) from nonaqueous phase (n) to water (w)] (B). (Reproduced from reference 4. Copyright 1990 American Chemical...

The interface between two immiscible solutions (e.g. water and nitrobenzene) containing dissolved species is a site of an electric potential. By measuring this potential difference at the aqueous elec-trolyte/solid electrolyte phase boundary, the phenomena taking place at the interface between two immiscible solutions or the membranes of ion-selective electrode have been studied. Changing the composition of the solutions in contact can alter this potential or applied current can alter the composition of the solutions. Thus, judicious choice of applied potential or current can be used to study the structure of the interface. Since the interface is ul-trathin (< cl nm), it cannot be observed directly. It can be, however, investigated by electrochemical or optical methods [14,... 

The first major observation of ionic current across the interface between two immiscible solutions was reported by Nernst and Riesenfeld," who in 1902, studied the transport of colored electrolytes across water-phenol-water concentration cells. However, it was only in 1974 that Gavach et al applied what we could call... 

Koryta, J. Electrolysis at the interface of two Immiscible Electrolyte Solutions and its Analytical Aspects, in Ion-Selective Electrodes. 3rd. Symposium held at Matrafured, Hungary 1980, ed. Pundor, E., Elsevier, Amsterdam—Oxford—New York 1981... 

The electrolyte salt must be processed to recover the ionic plutonium orginally added to the cell. This can be done by aqueous chemistry, typically by dissolution in a dilute sodium hydroxide solution with recovery of the contained plutonium as Pu(OH)3, or by pyrochemical techniques. The usual pyrochemical method is to contact the molten electrolyte salt with molten calcium, thereby reducing any PUCI3 to plutonium metal which is immiscible in the salt phase. The extraction crucible is maintained above the melting point of the contained salts to permit any fine droplets of plutonium in the salt to coalesce with the pool of metal formed beneath the salt phase. If the original ER electrolyte salt was eutectic NaCl-KCl a third "black salt" phase will be formed between the stripped electrolyte salt and the solidified metal button. This dark-blue phase can contain 10 wt. % of the plutonium originally present in the electrolyte salt plutonium in this phase can be recovered by an additional calcium extraction stepO ). 

The system of distinctions and terminology of the thermodynamic and electric potentials introduced by Lange is still very useful and recommended for describing all electrified phases and interphases. Therefore these potentials can be assigned to metal/solution (M/s), as well as the liquid/liquid boundaries created at the interfaces of two immiscible electrolyte solutions water (w) and an organic solvent (s). 

XL VOLTAIC CELLS WITH INTERFACES OF IMMISCIBLE ELECTROLYTE SOLUTIONS... 

For interfaces between liquid electrolytes, we can distinguish three cases (1) interfaces between similar electrolytes, (2) interfaces between dissimilar but miscible electrolytes, and (3) interfaces between immiscible electrolytes. In the first case the two electrolytes have the same solvent (medium), but they differ in the nature and/or concentration of solutes. In the second case the interface separates dissimilar media (e.g., solutions in water and ethanol). An example for the third case is a system consisting of salt solutions in water and nitrobenzene. The interface between immiscible dissimilar liquid electrolytes is discussed in more detail in Chapter 32. 

A special case of interfaces between electrolytes are those involving membranes. A membrane is a thin, ion-conducting interlayer (most often solid but sometimes also a solution in an immiscible electrolyte) separating two similar liquid phases and exhibiting selectivity (Fig. 5.1). Nonselective interlayers, interlayers uniformly permeable for all components, are called diaphragms. Completely selective membranes (i.e., membranes that are permeable for some and impermeable for other substances) are called permselective membranes. 

Glass electrodes are used for the analysis of hydrogen ions various other types of ion-selective electrodes are used for the other ions. Electrodes with ion-selective solvent membranes have become very popular. These electrodes are made in the form of thin glass capillaries (about 1 rm in diameter), which in the lower part contain a small volume of a liquid that is immiscible with water the remainder of the capillary is filled with electrolyte solution (e.g., 3M KCl). 

An interface between two immiscible electrolyte solutions (ITIES) is formed between two liqnid solvents of a low mutual miscibility (typically, <1% by weight), each containing an electrolyte. One of these solvents is usually water and the other one is a polar organic solvent of a moderate or high relative dielectric constant (permittivity). The latter requirement is a condition for at least partial dissociation of dissolved electrolyte(s) into ions, which thus can ensure the electric conductivity of the liquid phase. A list of the solvents commonly used in electrochemical measurements at ITIES is given in Table 32.1. 

ITIES interface between two immiscible electrolyte solutions... 

Liquid surfaces and liquid-liquid interfaces are very common and have tremendous significance in the real world. Especially important are the interfaces between two immiscible liquid electrolyte solutions (acronym ITIES), which occur in tissues and cells of all living organisms. The usual presence of aqueous electrolyte solution as one phase of ITIES is the main reason for the electrochemical nature of such interfaces. 

The interface separating two immiscible electrolyte solutions, e.g., one aqueous and the other based on a polar organic solvent, may be reversible with respect to one or many ions simultaneously, and also to electrons. Works by Nernst constitute a fundamental contribution to the electrochemical analysis of the phase equilibrium between two immiscible electrolyte solutions [1-3]. According to these works, in the above system electrical potentials originate from the difference of distribution coefficients of ions of the electrolyte present in the both phases. 

Le Hung presented a general theoretical approach for calculating the Galvani potential Ajyj at the interface of two immiscible electrolyte solutions, e.g., aqueous (w) and organic solvent (s) [25]. Le Hung s approach allows the calculation of when the initial concentration (Cj), activity coefficients (j/,) and standard energies of transfer of ions (AjG ) are known in both solutions. 

The above equation allows the calculation of Galvani potentials at the interfaces of immiscible electrolyte solutions in the presence of any number of ions with any valence, also including the cases of association or complexing in one of the phases. Makrlik [26] described the cases of association and formation of complexes with participation of one of the ions but in both phases. In a later work [27] Le Hung extended his approach and also considered any mutual interaction of ions and molecules present in both phases. Buck and Vanysek performed the detailed analysis of various practical cases, including membrane equilibria, of multi-ion distribution potential equations [28,29]. 

Measurement of electrical potential differences requires a complete electrical circuit, i.e., the electrochemical cell. An electrochemical galvanic cell consisting of all conducting phases, and among them at least one interface separating two immiscible electrolyte solutions is called for short a liquid galvanic cell. In contrast, the system composed of con-... 

In modern investigations of the electrochemical properties of immiscible electrolyte solutions mixed cells are used, i.e., cells containing one interface, e.g., that under investigation - Nernstian or polarizable, and a second reference interface of the Haber types (Scheme 7 or 10). 

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