The concept of the absolute half-cell potential is discussed from a simple thermodynamic point of view and defined as -ΔG/ne for the half-cell reaction, treating the electron like other individual chemical species. With the standard reference state of the electron taken as the free electron at rest at infinity, the absolute half-cell potential is simply the Fermi level of the electrons in the half-cell lead wire. This potential can be simply measured with a Kelvin probe by determining the vacuum potential level over the electrolyte relative to the Fermi level of the half-cell lead. Since the probe cannot determine potentials absolutely, it must be calibrated using a material whose work function is known. Using this procedure we obtain for the standard hydrogen half-cell, $E_{\mathrm{abs}\left(\mathrm{SHE}\right)=4.456\mathrm{}\pm 0.025}$ eV, assuming our Hg as used in calibration has a work function of 4.495 eV.

• Received 6 April 1987

DOI:https://doi.org/10.1103/PhysRevA.36.1396