Zirconium(IV)-salophens as fluoride-selective ionophores in polymeric membrane electrodes

The feasibility of using Zr(IV)-salophens as ionophores to prepare anion-selective polymeric membrane electrodes is examined. It is shown that electrodes formulated with these compounds exhibit greatly enhanced selectivity towards fluoride anion (as compared to a classical quaternary ammonium anion-exchanger) when introduced into plasticized PVC films containing lipophilic anionic site additives. Electrodes constructed with optimal membrane compositions exhibit the following selectivity pattern: F− > ClO4− > SCN− > NO3− > Br− > Cl−. For these electrodes, near-Nernstian and rapid potentiometric fluoride responses are observed. It is shown that the optimal response to fluoride occurs when the sample solution is buffered in the range of pH 4.5-6.0. At present, the response lifetime of the proposed electrodes is limited to 14 days due to fast deterioration of fluoride response with time. This phenomenon is ascribed to the decomposition of the ionophore within the polymeric membrane phase of the electrode in contact with the aqueous test solution.