Relative and absolute electrochemical quantities. Components of the potential difference across the electrode/solution interface

Abstract

The absolute value of the standard potential of the hydrogen electrode is calculated on the basis of a value of –263.1 kcal mol^–1 for [graphic ommitted], the standard chemical free energy of hydration of the proton. This quantity, in turn, is evaluated from the experimental value of [graphic ommitted], the standard real free energy of hydration, as measured by Randles, and the value of 0.13 V for χ^H₂O, the surface potential of water at the air/solution interface. The reliability of the derivation of χ^H₂O is much increased by using, as a basis, a value for g_(Hg)^H₂O(dipole), the surface potential of water at the mercury/solution interface, derived from indirect experimental measurements. The values of g_(M)^H₂O(dipole) on a number of other metals are correlated to the enthalpy of formation of oxides MO and the resulting linear relationship interpreted in terms of different interaction of water with different metal surfaces. The significance of ε_abs^°(H⁺/H₂), the absolute potential of the standard hydrogen electrode, and its relation to Δ^M_Sϕ, the actual potential difference across the electrode/solution boundary (operative electrode potential) are discussed. An attempt to calculate Δ^M_Sϕ for alkali metals is made on the basis of chemical potentials of electrons as calculated from the theory of metals. Incorrect conclusions in previous works on the significance of µ^M_e, the chemical potential of electrons, are shown and discussed. Implications are extended to the field of potentials of zero charge.