specifically absorbing ion

Ions which possess a chemical affinity for the surface in addition to the Coulomb interaction, where chemical is a collective adjective, embracing all interactions other than purely Coulombic. Examples are van der Waals or hydrophobic bonding, pi-electron exchange and complex formation. Specifically adsorbing ions can adsorb on an initially uncharged surface and hence provide it with a charge. The term specifically adsorbed applies to the sorption of all other ions having an affinity to the surface in addition to the purely Coulombic contribution.
See also: indifferent absorbing ion
PAC, 1991, 63, 895. 'Nomenclature, symbols, definitions and measurements for electrified interfaces in aqueous dispersions of solids (Recommendations 1991)' on page 899 (https://doi.org/10.1351/pac199163060895)