The semiconductor-like behavior of many passive films has been characterized by measurements of electrochemical impedance spectroscopy (EIS). The flat band potential and carrier density are then obtained from plots of 1/C2 (C = capacitance) versus potential, and by assuming that the Mott-Schottky approximation is valid. Our study focuses on a significant shortcoming of the Mott-Schottky approximation as applied to passive films. Namely, the equivalent circuit for most films consists, in part, of a constant phase element (CPE), and not a true capacitance. Because of the CPE, analysis indicates that the capacitance associated with the passive film can only be obtained from the impedance measured at one particular frequency, f*. A perusal of the literature indicates that in most investigations impedance measurements have not been conducted at f*. Consequently, the reported values of flatband potentials and carrier densities are incorrect. Our results show that in most cases the error in the estimate of flatband potential is negligible but that the error in the estimate of carrier density is often large.