Selectivity of liquid membrane ion-selective electrodes

The current status of describing the selectivity of liquid membrane ion-selective electrodes (ISEs) is reviewed. For ISEs that show Nernstian response toward all measured ions, the mixed ion response can often be described rigorously by a newly developed model that establishes required Nikolskii coefficients for a given target application. For ions of different charge, these obtained values are significantly different than predicted using the traditional Nikolskii-Eisenman equation. In contrast, for ISEs that show non-Nernstian responses toward highly discriminated ions, two different procedures are recommended, depending on the scope of the experiment. Selectivities obtained with the so-called matched potential method yield information about the apparent selectivity behavior of ISEs in real-world samples, irrespective of theoretical expectations, but can, for the very same reason, generally not be used to predict the empirical selectivity for different analytical situations. On the other hand, experimental procedures are presented that allow for the determination of fundamental Nikolskii coefficients of such membranes, i.e., Nernstian slopes can now often be obtained by carefully adjusting the experimental conditions. Such values more closely reflect the ion extraction behavior of the membrane and can be used for characterization and optimization purposes. Each of the procedures has its validity and the respective values should preferably be reported simultaneously.