Role of plasticizers on the characteristics of poly(vinyl chloride)-membrane lithium-selective electrodes based on phenanthroline derivatives

Abstract

Ion selectivities of PVC-membrane lithium-selective electrodes based on 2,9-dibutyl-1,10-phenanthroline were tested for various plasticizers which were used as solvent mediators to incorporate the carrier into the membrane. Selectivity coefficients were measured by several methods, including separate and mixed solutions methods. It was confirmed that differences in the method gave only small differences in the measured selectivity coefficients. Among the plasticizers used, o-nitrophenyl phenyl ether gave the best lithium selectivity towards other alkali and alkaline earth metal cations. The selectivity attained was log k_Li,Na=–3.3 (2000 times preference for Li⁺ over Na⁺), which proves to be the best lithium-selective electrode so far reported in the literature. The impedance measurement was carried out for two kinds of membranes with different plasticizers, to find out the mechanism of the improved lithium selectivity. It was found that the membrane resistance has only slight correlation with the improved lithium selectivity. The high rate of cation–neutral carrier complexation (1:2 ratio) was inferred to be the significant factor in determining high lithium selectivities. The role of plasticizers as a medium of complexation was demonstrated.