A general-purpose equation has been proposed for the dependence of conditional standard potential, E*, on the effective radius of the solvent cation and on the ionic momentum of the potential-determining element. It is found that the application of E* markedly increases the accuracy and confidence of equilibrium potential estimates, since the E* unlike standard potentials, E0, may be directly and precisely measured (the reproducibility of the E* independent measurements is within ± 0.01 v). The proposed equation is tested by the measured data on E* for Mg2+/Mg in different salt solvents. This equation makes it possible to estimate the temperature dependence of E* in any melt from that calculated for one solvent.