The Grahame and Grahame-Parsons concepts for the double layer structure are used for modelling the impedance behaviour of a metal electrode in contact with a solution of its salt, i.e. in the absence of a base electrolyte. The Lorenz-Salie concept of partial charge transfer is also used. Provided that the faradaic reaction is sufficiently slow, the apparent double-layer capacity, i.e. the capacity describing the impedance behaviour of the electrode at high frequencies, is equal to the thermodynamic capacity even in the case of strong ionic adsorption. A sufficiently fast faradaic reaction changes the structure of the electrode impedance. In this case the apparent double-layer capacity can have a different value and sign from those of the thermodynamic electrode capacity. This is also the case for a completely reversible electrode.