The behaviour of the mobile charges in a electrolyte is described by an over-damped harmonic oscillator model (ODO) with natural resonant frequency omega(o), charge q and mass m. This model is used as a tool to explain the impedance data at low frequencies, because in this range the electrode polarisation effect plays a dominant role. Here we propose a simple extension of this model by including an extra interaction of a single charge with its image in order to account for other surfaces effects. It is shown that the extended mechanical model has an analogous equivalent circuit that contains a constant phase element (CPE). Experimental data on saline solutions for various concentrations are used to validate the model. (C) 2016 Elsevier B.V. All rights reserved.