Calculated effects of membrane transport on the long-term response behavior of polymeric membrane ion-selective electrodes

Zero-current ion transport through polymeric membranes of ion-selective ele ctrodes (ISEs) alters the composition of the inner solution of these sensor s. This causes long-term drifts of the electromotive force that are relevan t for miniaturized ISEs as well as for most solid-contacted ISEs in which a thin aqueous layer is formed between the membrane and the metal electrode during conditioning. By a similar mechanism, the composition of the sample can be significantly altered during the measurement if the sample volume is small relative to the volumes of the membrane and the inner solution. This paper provides a formal model that describes the response behavior to be e xpected for such cases.