Mechanism of transport in carrier-based ion-selective electrodes


Inert, passive, overplasticized poly(vinyl chloride) membranes loaded naturally or artificially with trapped, hydrophobic sites, and with added carriers (neutral or charged) show the 'closed circuit' carrier mechanism when perturbed by a.c. and d.c. voltages. These solvent polymeric membranes behave like homogeneous phases, as observed by impedance spectroscopy. The interfacial ion exchange processes have been analysed and data favour direct aqueous ion hopping to surface carriers in the membrane phase, subject to electroneutrality constraints. In potentiometry, carriers serve as selective reagents, establish the interfacial space charge and potential differences, but only show carrying properties when perturbed. Transport then occurs, in most cases, by motion of ion-carrier complexes, not by ion hopping. Exceptions are pH membranes where carriers are fixed in space and protons hop. Effects of hydrophobic additives can be analysed to interpret different cases recently observed for charged carriers. New results cover the interpretation of monotonic I-t curves (with decreasing slope) and monotonic I-t curves with a shoulder (decreasing- increasing-decreasing slope). Ohmic, non-ohmic and limiting current characters of I-V curves are emphasized to distinguish characteristics of fixed from mobile, trapped, site behaviour.