The application of germanium(IV)-porphyrins as fluoride-selective ionophores for polymeric membrane electrodes

Abstract

In this work, the possibility of using Ge(IV)-porphyrins as anion-selective ionophores for potentiometric measurements was tested. Conducted experiments shown that polymeric membranes doped with these compounds exhibit high selectivity towards fluoride anion. Ge(IV)-tetraphenylporphyrin was initially used as a ionophore, however rapid crystallization of this compound in the membrane phase was observed. To mitigate this problem, more lipophilic derivative with tert-butyl groups was synthesized and doped into polymeric membranes. Ge(IV)-porphyrins were shown to work according to charged carrier mechanism, requiring 10 to 20mol% of borate additive for optimal selectivity. The anti-Hofmeister selectivity pattern: F−>SCN−>ClO4−>Br−>NO3−>Cl− was observed for membranes containing Ge(IV)-porphyrin and anionic additive. Electrodes with these membranes responded to changes in fluoride concentration in fast and near-Nernstian fashion. Strong interaction between fluoride ion and tested ionophores resulted in the inversion of the F− calibration curve at high concentration, which was especially pronounced for electrodes with membranes containing higher amounts of borate additive.