Potentials of the outer Helmholtz plane, phi(2), at mercury as function of electrode potential and temperature are calculated on the basis of the Gouy-Chapman theory using Grahame’s data for surface charge and capacitance in aqueous NaF solutions. An empirical equation for calculating phi(2) at negative rational potential is presented. Grahame’s original data of surface charge and capacitance are summarized in ASCII files on floppy disk.