Organotin compounds: An ionophore system for fluoride ion recognition

Ion-selective properties were established for membrane electrodes prepared by using organotin compounds of type (LCNRSnF2)n, (R = n-Bu (I), = Ph (II)) and (LCNSnF3)n (III) (LCN = C6H4(CH2NMe2)-2). Electrodes formulated with the optimized membranes containing the organotin compounds I–III as ionophores and sodium tetraphenylborate (10–30%) exhibited high selectivity for fluoride over other anions. An electrode prepared with ionophore II using dibutyl phthalate as the plasticizer and 15% sodium tetraphenylborate (NaTPB) as anion additive, possesses the best potentiometric response characteristics. It shows a detection limit of 7.9 × 10−7 M with a slope of 62.7 mV decade−1 of activity in buffer solutions of pH 5.5. The interference from other anions is suppressed under this optimized measurement conditions. An entirely non-Hofmeister selectivity sequence (F− > CH3COO− > Cl− > I− ∼ Br− >ClO4− > NO2− > NO3− > SCN−) with remarkable preference towards fluoride is obtained. The influence on the electrode performances by anion additive was studied, and the possible response mechanism was investigated by UV–vis spectra. The electrode has been used for direct determination of fluoride in drinking mineral water with satisfactory results.