CO-ION INTERFERENCE FOR ION-SELECTIVE ELECTRODES BASED ON CHARGED ANDNEUTRAL IONOPHORES - A COMPARISON

Interferences from co-ions, i.e., ions with a charge sign opposite to that of the measured ion, limit the upper detection limit of ion-selec tive electrodes (ISEs) based on electrically neutral or charged ionoph ores. A theoretical description based on phase boundary equilibria is used here to predict the co-ion interference (Donnan failure) of both types of ISEs quantitatively and to compare the extent of co-ion inter ference in these two cases. It is shown that, for practically relevant ionophore-site ratios, Donnan failure for ISEs based on electrically charged and neutral ionophore occurs at very similar co-ion concentrat ions if the charged and the neutral ionophores form complexes of equal stabilities and stoichiometries. With the exception of the selective complexation of the primary ion and charged ionophores, association of two or more ions such as between ionic sites and complexes has only a limited effect on co-ion interference. If relevant at all, ion pairin g is predicted to affect upper detection limits more unfavorably in th e case of neutral than in the case of charged ionophore-based ISEs. Ex perimental determination of co-ion interference for ISEs with five dif ferent charged and two electrically neutral H+ ionophores shows that c o-ion interference increases as the stability of the H+ complexes incr eases and confirms that co-ion interferences for charged and neutral i onophore-based ISEs indeed occur to an equal extent if their H+ comple xes have the same stabilities. These results corroborate the high pote ntiality of charged ionophore-based ISEs.