A simple approach is proposed to predict the magnitude of charge transfer between a polarized transition metal electrode and a chemisorbed molecule in an electrolytic solution. The approach is based on the equalization of the chemical potentials, as defined in density-functional theory (DFT). The system is modeled by a metal cluster, a molecule, and a set of point charges (counter ions) and the properties evaluated using methods derived from DFT; specifically, we study here the chemisorption of acrylonitrile on nickel. This approach allows us to point out the various factors determining the charge transfer. The results show a linear increase in the amount of charge transferred to the chemisorbed molecule with the polarization of the metal electrode.