APPLICABILITY OF THE PHASE-BOUNDARY POTENTIAL MODEL TO THE MECHANISTIC UNDERSTANDING OF SOLVENT POLYMERIC MEMBRANE-BASED ION-SELECTIVE ELECTRODES

Recent experimental evidence suggests that the equilibrium partitioning of sample ions at the sample/membrane interface is the main parameter governing the potentiometric response of polymeric membrane-based ion-selective electrodes (ISEs). It is shown that the response of a neutral-carrier-based H+-selective electrode can be fully predicted on the basis of equilibrium concentrations measured optically within a thin organic him having the same composition as the ISE membrane. Consequently, using this simple and powerful phase boundary potential model together with mass balances and observed complex formation constants, the response and selectivity of various ISE membranes may be described. In this contribution, the most recent applications of the model are reviewed including: (1) a novel and general selectivity description, which is related to the so-called matched potential method and clearly shows the limitations of the extended Nicolsky-Eisenman equation if ions of different charge are considered; (2) the measuring range of neutral-carrier-based H+-selective ISEs, showing that previous experimental findings can now be explained by theory; (3) prediction of the influence of anionic and cationic sites on the selectivity of charged-carrier-based ISEs demonstrating that such ISE membranes need the incorporation of sites of the same charge type as the analyte ion to induce optimum potentiometric selectivity; and (4) the determination of the concentration of anionic impurities in poly(vinyl chloride) by measuring the influence of the anionic site concentration on the divalent/monovalent ion selectivity of a neutral-carrier-based ISE.