Charge transfer resistance at the interface between valinomycin-based potassium-selective membranes and KCl aqueous solutions

Potassium-selective electrodes (K-ISEs) with membranes containing neutral ionophore valinomycin and potassium tetrakis(p-chlorophenyl)borate as ion-exchanger, in different concentrations, were studied by the electrochemical impedance spectroscopy (EIS). When ISEs are in contact with KCl solutions with concentrations of 10⁻⁴ M and higher, only a high-frequency semicircle is present in the respective Nyquist plots. Nyquist plots obtained with ISEs in contact with KCl solutions with concentrations below 10⁻⁴ M contain also well-resolved low-frequency semicircles. A regular dependence of the values of R_LF - the low-frequency resistance on the concentration of KCl in solution allowed ascribing R_LF to the charge transfer resistance at the membrane/solution interface. The exchange current densities estimated from the R_LF values are significantly larger than small currents flowing through ISEs in their analytical applications under non-zero current modes. This suggests that the interfacial electrochemical equilibrium is not violated by the current flow, in favor of using the Nernst equation for the interpretation of the data obtained by, e.g., the constant potential coulometry method.