ISE SELECTIVITY AND ION-EXCHANGE KINETICS

Abstract

The complex AC-impedance spectra of several ion-selective membranes (valinomycin-based potassium-selective PVC, aliquat 3 36 S PVC, and LaF3-single crystal) have been measured with high reproducibility in the range of 0,01 Hz to 65 kHz. The measuring conditions have been optimized by a special membrane pretreatment and by using an impedance matching circuit, which prevented the typical distortion by stray capacitances. In all cases studied, it was found that the second semicircle of the complex impedance plane diagram at medium frequencies - normally attributed to a charge transfer resistance parallel to the double layer capacitance - correlated directly with the potentiometric selectivity of the ISE membrane under study. By applying the normal Butler-Volmer formalism for electrode kinetics, standard ion-exchange current densities could be extrapolated in a straightforward manner from the concentration dependence of the charge transfer resistances. However, in order to distinguish the obtained data from the corresponding data of metallic electrodes with only one diffuse double layer, the term “apparent exchange current densities” is favored. The quotient of the apparent ion exchange current density (proportional to the reciprocal charge transfer resistance) of an interfering ion exchange current densities (=equilibrium condition) with the net charges transported across that interfaces by the different ions before this point is reached.