Application of N,N′-bis(2-quinolinecarboxamide)-1,2-benzene as an ionophore in the construction of a novel polymeric membrane sensor for the selective monitoring of the holmium(III) concentration

It was found that N,N′-bis(2-quinolinecarboxamide)-1,2-benzene (H2BQB) containing PVC membranes show a good Ho(III) selectivity in comparison to most common first and second main group elements (representative), transition, and lanthanide ions. Hence we studied its application as a neutral ionophore, for preparing an Ho(III) PVC-membrane sensor. The primarily studied PVC-based membranes contained sodium tetraphenyl borate (NaTPB) as an anionic additive, and were tested by using different solvent mediators such as benzyl acetate (BA), nitrobenzene (NB), and o-nitrophenyloctyl ether (NPOE). The optimum ratio of H2BQB:PVC:NaTPB:BA was found to be 6.0:30.0:4.0:60.0. The sensor has a Nernstian response of 19.5 ± 0.3 mV/decade of Ho(III) activity, and a linear range from 2.0 × 10−5 to 1.0 × 10−2 M. The mentioned sensor can be used in a pH range of 2.5–10.5 and has a detection limit (DL) of 5.0 × 10−6 M. Another advantage of the sensor is its excellent Ho(III)-selectivity with regard to most common metal ions, and especially, all lanthanide ions. To investigate the analytical applicability of the sensor, it was successfully used as an indicator electrode for titration of 25 ml of a 1.0 × 10−4 M Ho(III) solution with a 1.0 × 10−2 M EDTA solution, and vice versa.