Development of highly selective potentiometric thorium(iv) ion-selective electrode: exploration supported with optical and DFT analysis

Abstract

A new liquid membrane based on poly(vinyl chloride) composed of dibutyl(8-hydroxyquinolin-2-yl)methylphosphonate (L) as a neutral ion carrier, sodium tetraphenylborate (NaTPB) as a lipophilic salt dissolved in dioctyl phthalate (DOP) as a plasticizer-cum-membrane solvent was developed for the selective determination of Th⁴⁺. The best electrode response for Th⁴⁺ is achieved with the membrane composition of PVC : DOP : L : NaTPhB (33% : 59% : 4% : 4%, w/w). The electrode response follows a super-Nernstian response with a slope 31.2 ± 0.4 mV per decade at 25 °C over a wide concentration range of Th⁴⁺ from 1 × 10⁻⁷ to 1 × 10⁻¹ M. The electrode has a response time of 5 s and detection limit of 1 × 10⁻⁸ M, working in the pH range of 4 to 6.5, with a service lifetime of about three months. The selectivity coefficient was determined by the match potential method (MPM) and the results show good selectivity towards Th⁴⁺ over a wide variety of selected metal ions (from s, d and f-block metals). The binding mechanism was investigated by FTIR, NMR (¹H, ³¹P) and mass analysis complemented by density functional theoretical results.