A coated graphite thorium-ion selective potentiometric sensor based on a calix[4]arene bearing phosphoryl groups

5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis(diphenylphosphinoylmethoxy)calix[4]arene (1) has been used for the preparation of a graphite coated thorium ion-selective electrode (Th4+-ISE). The plasticized PVC membrane containing 30% PVC, 58% ortho-nitrophenyloctylether (NPOE), 4% sodium tetraphenylborate (NaTPB) and 8% ionophore was directly coated on a graphite rod. This sensor gave good Nernstian responses with a slope of 15.5 +/- 0.1 mV/decade over a concentration range of 1 x 10(-5) - 1 x 10(-3) M of thorium ions with a limit of detection of 7.9 x 10(-6) M. The dynamic response time of the electrode to achieve a steady potential was found to be about 15 seconds. The potential of the prepared sensor was independent of the pH variation in the range 2.3-4.0. The selectivity relative to several mono-, di- and tri-valent metal ions, i.e. Li+, Na+, K+, Ag+, NH4+, Sr2+, Mn2+, Fe2+, Co2+ Ni2+, Cu2+, Zn2+, Cd2+, La3+, Sm3+, Dy3+, Er3+ and Y3+ was examined. This electrode can be used for 6 months without any considerable divergences in the potential response. The sensor was successfully used as an indicator electrode for the potentiometric titration of a thorium solution using a standard solution of EDTA.