Apparent ion-exchange current densities at valinomycin-based potassium ion-selective PVC membranes obtained with an AC-impedance method

Abstract

The complex ac-impedance spectra of thin (< 70 μm) valinomycin-based potassium-selective PVC membranes symmetrically bathed in KCl, NaCl and LiCl solutions of different concentrations, have been measured with high resolution from 0.01 Hz to 10 kHz. With the exception of those KCl solutions > 10 −4 M, all impedance spectra obtained showed a more or less pronounced and stable second semicircle at medium frequencies, which could be identified as the double-layer capacitance in parallel to a charge-transfer resistance of the corresponding ion equilibrating across the interface PVC membrane/aqueous solution. Using the formalism originally derived for redox reactions at metallic electrodes, apparent standard ion-exchange current densities and consistent charge-transfer coefficients could be obtained with high reproducibility. The apparent standard exchange current density of K + at a valinomycin-based PVC membrane with tetraphenylborate as an additive, lies in the range of about 25 mA cm −2 which is the highest value reported so far for these membranes. A strong correlation was found between the potentiometricall y observed ion selectivity expressed by the selectivity coefficient used in an extended Nernst equation and the apparent exchange current densities of the corresponding ions.