The role of the electrodes on the electrical response of a medium containing ions is investigated. The analysis is performed in the linear limit, where the variation of the bulk density of ions due to the presence of the external field is negligible with respect to the ionic bulk density in thermodynamical equilibrium. The case of fully dissociated impurities is considered. The non-ideal character of the electrodes is described by means of a linear dependence of the density of ionic current on the surface electric field, similar to the law of Ohm valid for metallic conductors. In this framework, it is shown that in the low frequency region, the electrical response of the medium strongly depends on the electrical properties of the interface electrode-medium. On the contrary, in the high frequency region, it is independent of the nature of the electrodes. Approximated formulae in the limit of small and large frequency are reported for the real and imaginary parts of the impedance and of the admittance. The peculiar frequencies of the spectra, related to the maxima and minima of the imaginary parts of the impedance and of the admittance, are determined. The importance of the reported theoretical results on the experimental characterization of liquid media is discussed. (C) 2012 Elsevier BV. All rights reserved.