Electrochemical Impedance Spectroscopy of Ion-Selective Membranes: Artifacts in Two-, Three-, and Four-Electrode Measurements.

Electrochemical impedance spectroscopy is frequently used to characterize, optimize, and monitor ion-selective membranes. However, because of the relatively high resistance of ion-selective membranes, their impedance spectra often contain artifacts that can cause misinterpretation. While in the high-frequency range artifacts are often readily identifiable by the occurrence of inductive features or negative resistances, artifacts are easy to overlook in the low-frequency range, where telltale characteristics are typically missing. Some artifacts can be avoided by the use of two-electrode cells, but this experimental design makes it hard to distinguish the impedance of the ion-selective membrane from that of the measuring electrodes. This work shows that experimental data can be analyzed accurately with the use of models that account for the capacitive leakage present in the reference channels of the impedance spectrometer. To test these models, valinomycin-doped K+-selective membranes were studied by electrochemical impedance spectroscopy with two-, three-, and four-electrode cells, using several measuring electrodes with low to high impedances. The models were found to correctly predict experimental data and provide an intuitive understanding of the cause of the impedance artifacts. This understanding can be applied to design electrochemical impedance spectroscopy experiments of ion-selective membranes with three- and four-electrode cells that minimize artifacts.