Chemical exchange of halide ions

Note that in this equation all of the sample-independent emf contributions are included into one constant potential term (E ). In practice, ISEs obey this equation when the ion-selective membrane interacts reversibly with the specific ion of interest that is, a rapid ion transfer must occur between the sample and the sensing membrane (high exchange current density). On the other hand, this partitioning process must have no substantial effect on the chemical composition of the ion-selective membrane (that is, ai (mem) must remain constant to obtain Nernstian response). Silver halide precipitate membranes, for example, can be used as ion-selective sensing materials as long as the surface composition remains intact and does not become fouled by other silver precipitates. 

Evidence foi the anionic complex PuCP is the precipitation of complex halides such as Cs2PuClg from concentrated HCl (aq). The ability of Pu(IV) to form stable nitrate complexes provides the basis for the Purex and ion-exchange (qv) process used in the chemical processing of Pu (107). Pu(VI) is similar to Pu(IV) in its abihty to form complex ions. Detailed reviews of complex ion formation by aqueous plutonium are available (23,94,105). 

The liquid-membrane electrode is another important type of ion-selective electrode. The internal filling solution contains a source of the ion under investigation, i.e. one for which the ion exchanger is specific, while also containing a halide ion to allow the reference electrode to function. The physico-chemical behaviour of the ISE is very similar to that of the fluoride electrode, except that ise and the selectivity are dictated by the porosity of a membrane rather than by movement through a solid-state crystal. 

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

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