Interfacial potentials without electrolyte transport

Potentiometric methods of electroanalysis (see Chapter 7 of this handbook) depend on the ability of a membrane material to transport either cations or anions selectively. This selective behaviour results in an imbalance of concentrations on either side of the membrane which, in turn, leads to the estabhshment of a measurable potential difference across the membrane. In the simplest possible analysis, if we consider our membrane to be infinitely thin, or alternatively to have some kind of electrolyte boundary that is infinitely thin, then the equality of electrochemical potential, /I, in either of the solution phases, a and P, implies that 

the potential difference established across the monbrane depends on the ratio of the activities of the ions in phases a and P  

The establishment of such interfacial potentials is readily envisaged for cases where the net transport of an electrolyte is prevented because one of its constituents cannot partition. What is perhaps less obvious is that such potentials arise continually within solution phases, even where there is no physical separation into distinct phases. These so-called liquid junction potentials or diffusion potentials play an important role in electrochemical experiments, but because there is no well-defined phase boundary, they are intrinsically more difficult to measure. This chapter discusses how these potentials arise, how they may be calculated, what quantities are associated with them, and how they may be minimised. Finally, interfaces between electrolytes (i.e. those interfaces between immiscible electrolyte solutions (ITIES)) and the application of some of the concepts developed earlier in the chapter to non-standard electrolyte systems, such as polymer electrolytes and room-temperature ionic liquids, will be discussed. 

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Solid-solution electrodes therefore provide a field of complementary scientific interest to solid electrolytes and have at least equal potential application in battery systems. Apart from the presence of rapid ion transport, the extra keys to the utility of solid-solution electrodes lie in the interfacial aspects of ions being able to pass directly between the electrolyte phase and the electrode without any change in crystal structure of the electrode or the necessity to form a new compound by electrocrystallization (compare the electrode... 

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