Electric impedance of aqueous KCl and NaCl solutions: Salt concentration dependence on components of the equivalent electric circuit

In this work, we report an investigation of electrical impedance behavior of dilute aqueous solutions of potassium chloride KCl(aq) and sodium chloride NaCl(aq) (0.1 to 0.7 mMol/L) in the frequencies ranging from 10.0 mHz to 10.0 MHz. The real and imaginary parts of the complex electrical impedance are obtained by using the Electrical Impedance Spectroscopy (EIS) technique. The amplitude of the AC applied voltage is 20.0 mV. The complex electrical impedance results of the solutions are modeled by using an equivalent electrical circuit with a good agreement with the experimental data. This proposed circuit is composed by three sections connected in series. The first one is composed by a parallel RC associated with bulk effects, the second section is composed by a capacitor in parallel to a constant phase element (CPE) associated with the electric double-layer and the last one section composed by a resistance in series with an inductor associated with resistances of the electrical contacts and with parasites inductances of the connected cables. The components of the equivalent circuit are investigated as a function of salt concentration. As additional results, the electrical conductivity and complex permittivity of the samples are calculated and the electrical properties of the solutions are investigated as a function of kind of cations. For both K+ and Na+ cations, the bulk effects are well described by Debye model. However, the electric double-layer shows a higher resistance associated with cation Na+ than that associated to cation K+. Even for very low concentrations of salt, for both salts, it was observed an inverse relationship between impedance and salt concentration.