Evaluation of parasitic elements contributing to experimental cell impedance: Impedance measurements at interfaces between two immiscible electrolyte solutions

This work analyses the parasitic elements contributing to the impedance of a four-electrode cell employed in measurements at interfaces of two immiscible electrolyte solutions or membranes. The cell is modelled by an equivalent circuit, which accounts for the finite impedances of the reference and counter electrodes, as well as for various parasitic couplings. The model allows prediction of the conditions under which the impedance plots exhibit the high-frequency capacitive and/or inductive artifacts. An analytical expression is derived, which enables the evaluation of the parasitic elements from experimental impedance data. As a limiting case, the model describes also the behaviour of a three-electrode cell. Its validity is tested by simulating the impedance of a water/o-nitrophenyl octyl ether interface in the range of frequencies 1-10(6) Hz. Experimental data are reproduced with an excellent precision up to a frequency of ca. 20 kHz, above which only a qualitative agreement is seen. In spite of the remarkably high frequency dispersion observed, the correction of the low-frequency experimental impedance for the parasitic contributions has little effect on its resolution in terms of the interfacial capacitance, charge-transfer resistance or the Warburg impedance.