CARRIER MECHANISM OF ACIDIC IONOPHORES IN SOLVENT POLYMERIC MEMBRANE ION-SELECTIVE ELECTRODES

The behavior of cation-selective solvent polymeric membrane electrodes based on acidic ionophores is investigated by studying the selectivity coefficients as a function of cationic or anionic additive concentration. This technique allows discrimination between neutral and charged carrier-related mechanisms, resulting in the following finding. Ionophores with a carboxylic add group act as neutral carriers and sulfonic acid derivatives behave as charged carriers irrespective of the plasticizer used. The Ca2+-selective organophosphate ionophore examined (bis[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate) acts in low dielectric constant plasticizers in a mixed mode where the analyte ion forms two different complexes in the membrane phase-one with the charged form of the carrier and the other with the protonated form (neutral ionophore). From these results it can be seen that when developing novel ion-selective electrodes based on acidic ionophores, both cationic and anionic additives as well as plasticizers of high and low dielectric constant should be tested since completely different selectivity behavior could be obtained due to the charged and neutral forms of the carrier in the organic phase. The addition of potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate to the calcium-selective membrane phase based on the organophosphate ligand mentioned above, the plasticizer dioctyl phenylphosphonate, and poly(vinyl chloride) improves the potentiometric properties of the electrodes with respect to membrane resistance and anion interference.