IONIC ADDITIVES FOR ION-SELECTIVE ELECTRODES BASED ON ELECTRICALLY CHARGED CARRIERS

The influence of ionic sites on the behavior of charged carrier-based ion-selective liquid membrane electrodes is described by theory and experiments for cation- and for anion-selective electrodes. The cation exchanger potassium tetrakis[3,5-bis-(trifluoromethyl)phenyl]borate proved to be a beneficial additive for nitrite-selectve electrodes. On the other hand, the anion exchanger tridodecylmethylammonium chloride improved the potentiometric properties of cation-selective electrodes. By the incorporation of a defined amount of these sites, the selectivity was enhanced, the emf functions became theoretical, the electrode resistance was lowered, and the long-term stability improved. The optimal molar ratio of additive to ionophore was in the range of 0.3-0.6 for the ionophores studied. The theory and the experiments show clearly that ionic sites should be used not only with neutral ionophores but also with charged ones. ISE membranes without ionic additives should be avoided, since otherwise inherent ionic impurities could have a decisive influence os the response characteristics.