  The derivation of Warburg's impedance presented in several books and
scientific papers is reconsidered. It has been obtained by assuming that the
total electric current across the sample is just due to the diffusion, and that
the external potential applied to the electrode is responsible for an increase
of the bulk density of charge described by Nernst's model. We show that these
assumptions are not correct, and hence the proposed derivations questionable. A
correct determination of the electrochemical impedance of a cell of an
insulating material where are injected external charges of a given sign, when
the diffusion and the displacement currents are taken into account, does not
predict, in the high frequency region, for the real and imaginary parts of the
impedance, the trends predicted by Warburg's impedance in the Nernstian
approximation. The presented model can be generalized to the case of asymmetric
cell, assuming boundary conditions physically sound.
