Influence of different plasticizers on the response of chemical sensors based on polymeric membranes for nitrate ion determination

Abstract

Seven types of ion-selective chemical sensors based on poly(vinyl chloride) (PVC) as polymeric membrane, for determination of nitrate ion, have been prepared by using 3-nitro-o-xylene (NOX), tributylphosphate (TBP), bis-(2-ethylhexyl) sebacate (DOS), o-nitrophenyloctyl ether (o-NPOE), tris-2-ethylhexylphosphate (TEHP), dioctylphthalate (DOP) and dibutylphthalate (DBP) as plasticizers and tetradecil ammonium nitrate TDAN) as ionophore. The response of each electrode was determined in relation to the lipophilicity of the plasticizer used, its rate of exudation to the aqueous phase, polarity, viscosity and its dielectric constant (e,). The experimental data was validated by calibrating the constructed electrodes against a reference Ag/AgCl electrode. The selectivity coefficient (K-AB(Pot)) of the electrodes was determined by applying the standard mixed solutions method. No correlation between K-AB(Pot) and epsilon(r) of the plasticizer was found. ATR-FIF-IR technique was used in order to evaluate the life-time of the polymeric membranes ion-selective electrodes (ISEs). The life-time of the electrodes was found to be limited fundamentally by the loss of the sensor and not due to the loss of the plasticizer. All the constructed ISEs showed Nernstian slopes with values of the practical detection limit (PDL) and lower limit of linear response (LLLR) in the range 10(-6) to 10(-5) mol/dm(3). The electrodes constructed by using o-NPOE and/or DBP showed the longest life-time, this being longer than 6 months. The principal interfering anions were found to be CIO3-, BrO3-, Br-, Cl- and Bz(-) (benzoate). The selectivity loss of the sensors was found to be due to the loss of the ionophore by exudation. No correlation between the calculated values for K-AB(Pot) of the electrodes and the dielectric constants of the plasticizers was observed.