OFF–ON stirring potentiometry with solvent polymeric membrane ion-selective electrodes: A theory-guided approach to foreign ions

Abstract

The response of ion-selective electrodes (ISEs) towards a foreign ion is comprehensively investigated as a function of the nature and concentration of the ion, and of the mass transport conditions in the outer solution. Notably, a striking transient potential drift is evidenced by sheer activation of stirring in an initially quiescent solution of the foreign ion, both in the absence and presence of the primary ion. This phenomenon is rationalized in terms of the ion exchange at the outer membrane side coupled to the mass transfer of ions. This has been modelled through a novel theoretical treatment of the corresponding time-dependent mass transport problem, leading to simple closed-form expressions for the membrane potential and for the interfacial ion concentrations in the unstirring and stirring periods. The theoretical predictions are corroborated experimentally by monitoring the potential of a nitrate ISE exposed to ions of very different affinity towards the membrane (tetraphenylborate, perchlorate and chloride) before and after switching on the agitation of the solution. Such OFF-ON stirring program enables the study of foreign ions, even simultaneously with the determination of the primary species, by the joint analysis of the potentials before and after the activation of the stirring.