DONNAN EXCLUSION FAILURE OF NEUTRAL IONOPHORE-BASED ION-SELECTIVE ELECTRODES STUDIED BY OPTICAL 2ND-HARMONIC GENERATION

Optical second harmonic generation (SHG) at the liquid-liquid interfac es of ionophore-free and neutral ionophore-incorporated membranes has been measured in the presence and absence of ionic sites in order to b etter understand the influence of ionic sites on the potentiometric re sponses of liquid membrane ion-selective electrodes. Quick and lasting EMF and SHG responses were observed upon increasing the primary ion a ctivity in aqueous solutions in contact with membranes with ionic site s. For membranes without ionic sites, transient EMF responses due to e xtraction of primary ion salts occurred. They were accompanied by very similar transient changes in the SHG intensity. No EMF and SHG respon ses to the primary ion activity were, however, observed for site-free membranes in equilibrium with the aqueous phase, The extent of charge separation at the interfaces of such membranes, as observed by SHG fro m primary ions or primary ion complexes, does not depend on the primar y ion activity in the aqueous phase. This explains why membranes free of ionic sites do not exhibit EMF responses and confirms that ionic si tes are a necessity for counterion-independent primary ion responses o f potentiometric sensors based on neutral carriers.