A new approach for lowering the detection limit of a pethidine ion-selective electrode is presented. A coated graphite (CGE) and carbon paste (CPE) electrodes for pethidine ions based on pethidine-phosphotungstate (PD-PT) as ion-pair complex are described. The sensors exhibit a Nernstian slope of 58.1 and 54.2 mVdecade(-1) for pethidine ion over a wide concentration range from 2.6 × 10(-7) to 1.0 × 10(-2) M and 2.1 × 10(-6) to 1.0 × 10(-2) M with a detection limit of 1.8 × 10(-7) M and 7.3 × 10(-7) M for pethidine coated graphite (PD-CGE) and pethidine carbon paste electrode (PD-CPE), respectively. These sensors exhibited a fast response time (about 5-8 s) and good stability. The standard electrode potentials, E(o) , were determined at different temperatures and used to calculate the isothermal temperature coefficient (dE(o) /dT) of the PD-CGE and PD-CPE, which was 0.0062 and 0.0071 V/ °C, respectively. Selectivity coefficients, determined by matched potential method (MPM) and separate solution method (SSM), showed high selectivity for pethidine hydrochloride (PDCl) over a large number of inorganic cations, organic cations, sugars, urine components, and some common drug excipients. The sensors were applied for determination of PDCl in ampoule and in spiked urine samples using potentiometric determination, standard addition and the calibration curve methods. The results obtained were satisfactory with excellent percentage recovery comparable and sometimes better than those obtained by other routine methods for the assay.

A simple, precise, rapid and low-cost potentiometric method for pethidine determination in pharmaceuticals and urine is proposed. A chemically modified carbon paste electrode (CMCPE) for pethidine hydrochloride (PDCl) based on pethidine-phosphotungstate (PD-PT) as ion-pair complex was prepared and fully character-ized in terms of composition, usable pH range, response time and temperature. The pethidine electrode showed Nernstain responses in the concentration range 2.1 × 10–6-1.0 × 10–2 M with a detection limit of 7.3 × 10-7 and usable within the pH range 3.5-6.6. This sensor exhibited a fast response time (about 5-8 s), good stability. The value (dE/dt) of the electrode was found to be 0.00071 V/?C, which indicates fairly high ther-mal stability. Selectivity coefficients determined by matched potential method (MPM) and separate solution method (SSM) showed high selectivity for PDCl with respect to a large number of inorganic cations, organic cations, sugars and some common drug excipients. The sensor could be used successfully in the estimation of PDCl in ampoules and in spiked urine samples

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