Equivalent Circuit Models for Soils and Aqueous Solutions Under 2-Terminal Test Configuration

Numerous circuit models have been proposed to represent the electrode–electrolyte interface (EEI) impedance and bulk medium impedance of conducting media. Following a review, two suitable models are constructed to represent the behavior of conduction in electrolytes and soils, respectively. Both models incorporate a constant phase element in parallel with an apparent Faradaic resistance, which is found to reproduce the EEI behavior accurately. For the electrolyte model, a single parallel R-C branch is added to represent the impedance of the bulk medium, whereas for the soil model, an equivalent ladder network of R-C branches is found to be suitable. Experimentally obtained electrolyte and soil impedance data based on 2-terminal impedance spectroscopy over a frequency range of 10 mHz to 10 MHz with variable current density are compared with values obtained from the models and where model parameters are determined by a curve fitting routine. The effects of electrolyte concentration, soil moisture, and electrode material are analyzed, and the models help to illustrate clearly how the EEI effect dominates at low frequencies while the intrinsic characteristics of the test medium prevails at high frequencies. The models are extended to account for soil-electrolyte impedance dependence on current density, which is most evident at low frequencies. The extent of the impedance plateau region is described by limiting upper and lower frequencies.