The Free Energy of Hydration of Gaseous Ions, and the Absolute Potential of the Normal Calomel Electrode

Abstract

The free energies of hydration of the alkali and halide ions are found to agree reasonably well with the simple expression of Born (—ΔF=(1–1/D)Ne2/2re) for solution of charged spheres in a dielectric medium, provided the crystal radii are suitably modified so as to correspond to the radii of the cavities in the dielectric medium. The results show that the dielectric constant of water remains large even in the intense field next to the ion. The entropies of hydration are also found to be consistent with these radii. Because of the simplicity of this calculation, the resulting free energies of solution of individual ions are considered to be a priori the most probable and are used to calculate a value of —0.50 volt for the absolute potential of the calomel half-cell.