Impedance of mixed conductors contacted with electrodes which are blocking for ions and reversible for electrons (or vice versa) is modelled phenomenologically. A closed form approximation is found which is valid in a very broad materials parameter window. The approximation is verified by the exact solution of Nernst-Planck-Poisson set of equations. The closed form results show that at low frequencies electrostatic as well as chemical relaxation occur. In particular, the closed form expression describes the transition from the 45 degrees Warburg impedance to a 90 degrees rise in the impedance spectra. In the former case the low frequency relaxation time bears information about the chemical diffusion coefficient while in the latter example information about the electrode capacitance is contained.