Gallium(III)-Schiff Base Complexes as Novel Ionophores for Fluoride Selective Polymeric Membrane Electrodes

Gallium(III)‐Schiff base complexes are examined as potential ionophores for the development of fluoride ion‐selective polymeric membrane electrodes. To probe the effect of ionophore structure, several different salen and salophen compounds have been synthesized and tested in plasticized poly(vinyl chloride) membranes (H21–5). A salophen ligand structure highly substituted with t‐butyl groups (H21) was found to be optimal with respect to membrane compatibility, and its corresponding Ga(III) complex yields membrane electrodes with greatly enhanced potentiometric selectivity for fluoride ion over a host of other anions. Addition of lipophilic anionic sites into the membrane phase provides optimized response and selectivity, indicating that the Ga1 ionophore functions as a charge carrier. The use of dioctylsebacate as the membrane plasticizer helps reduce any dimer‐monomer equilibria in the membrane phase, yielding sensors with rapid and fully reversible EMF response. Optimal potentiometric detection of fluoride is achieved in solutions buffered to relatively low pH values (pH 3.0), to reduce anionic response to hydroxide. The use of gallium(III)‐Schiff base complexes is shown to offer a promising research direction to ultimately achieve polymeric membrane fluoride sensors that exhibit potentiometric response equivalent to the classical lanthanum fluoride‐based solid state type membrane electrode.