A theory for copper electropolishing in concentrated phosphoric acid is compared to ac-impedance and electrohydrodynamic (EHD)-impedance spectra. A water-acceptor mechanism is hypothesized. Agreement between experiment and theory is good. Curve fits of the EHD-impedance data estimate that the Schmidt number of the system is Sc approximate to 3 x 10(6) at room temperature. At temperatures near 90 degrees C, Sc approximate to 7 x 10(4). The plausibility of these high values of Sc is discussed. These estimates of the Schmidt number are obtained without any assumptions about the bulk concentration of the acceptor. With a known viscosity and diffusion coefficient, the Levich analysis of the variation of the limiting current with rotation speed is subsequently used to estimate the bulk concentration of acceptor. The value obtained is in agreement with the known water content in phosphoric acid which indicates that water is the acceptor species.