Those species which by virtue of their electron distribution between the solid and liquid phase (or by their equilibrium with electrons in the solid) determine the difference in Galvani potential between these phases. This definition requires that adsorbed p.d.
ions are part of the adsorbent and belong to the category of surface ions.
PAC, 1991, 63, 895. 'Nomenclature, symbols, definitions and measurements for electrified interfaces in aqueous dispersions of solids (Recommendations 1991)' on page 897 (https://doi.org/10.1351/pac199163060895)