1139 J. Indian Chem. Soc., Vol. 93, October 2016, pp. 1139-1145 Fabrication of a new Pr 3+ nanocomposite carbon paste electrode based on bis(salicylaldehyde)thiocarbohydrazone as a material sensing Sare Moshirian Farahi a , Hassan Ali Zamani* a , Fateme Joz-Yarmohammadi a , Mohammad Reza Abedi b and Hossein Behmadi a a Department of Applied Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran E-mail : haszamani@yahoo.com b Department of Applied Chemistry, Quchan Branch, Islamic Azad University, Quchan, Iran Manuscript received online 12 March 2016, accepted 10 August 2016 Abstract : The fabrication and electrochemical behavior of nanocomposite CP-electrode modified with MWCNT/nanosilica was investigated for bis(salicylaldehyde)thiocarbohydrazone (STCH) as a new ionophore in the presence of various alkaline, lanthanide and transition metal ions. Pursuant obtained results the best electrode behavior was belonged to Pr 3+ -carbon paste electrode (PrCPE) with composition of STCH : 3%, binder (paraffin oil) : 30%, modifier (MWCNT : 2%, NS : 0.3%), and graphite powder : 64.7%. This PrCPE displayed a Nernstian response by the slope of 19.6 ± 0.2 mV decade –1 in linear range 1.0 × 10 –8 to 1.0 × 10 –2 mol L –1 with a lower detection limit 8.5 × 10 –9 mol L –1 . The pH range of this new praseodymium sensor was gained 2.1 to 8.9 with a response time about 6 s. To assessment the response of new prepared electrode toward different cations the matched potential method ( K MPM ) was applied. Obtained results show good selectivity for this PrCPE with respect to a number of lanthanide and transition metal ions. It was success- fully used as an indicator electrode in potentiometric titration of Pr 3+ ions with EDTA and determination of Pr 3+ content in several blends of ions. Keywords : Praseodymium, ion selective electrode, bis(salicylaldehyde)thiocarbohydrazone, nanocomposite carbon paste electrode, potentiometry sensor. Introduction Potentiometric techniques based on ion selective elec- trodes are regularly used as acceptable analytical method between several methods which have been employed to determine the praseodymium ion in aqueous solutions 1–7 . In the meantime, the carbon paste electrodes (CPEs) are one of the subgroup of ion selective sensors which tender renewable surface, stable response, and low ohmic resis- tance electrodes 8–10 . Among the lanthanide series praseodymium is the sec- ond element with atomic number 59. It has a several implementations in various fields such as use in color TV, glasses, and fluorescent lamps. It is also used to add in fiber optic cables as a doping agent. Praseodymium could be entered in environment and gradually accumu- late in soils and water by many various sources such as petrol-producing industries, throwing away the household equipment, etc. which cause to augment the concentra- tions of Pr 3+ in humans body, animals and soil particles 11 . Lately, some ISEs for lanthanide ions such as praseody- mium were reported by us and other researchers 12–58 . In the present work, nanocomposite-CP based on bis(salicylaldehyde)thiocarbohydrazone (STCH) (Fig. 1) as the active material, NS (nanosilica), MWCNTs (multi- walled carbon nanotubes) and paraffin oil has been devel- oped to applied for monitoring of Pr +3 ion. Fig. 1. Structure of STCH. Experimental EMF measurements : A potentiometric cell involving the Pr 3+ -CP sensor as indicating electrode, an Ag/AgCl electrode purchased