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The construction and electrochemical response characteristics of poly(vinylchloride) (PVC) membrane selective electrodes for the determination of distigmine (Ds) are described. The sensing membrane comprised an ion-pair based on distigmine phosphomolybdate (Ds-PM), distigmine phosphotungstate (Ds-PT), distigmine silicomolybdate (Ds-SM), distigmine silicotungstate (Ds-ST), distigmine tetraphenylborate (Ds-TPB), and distigmine reineckate (Ds-Rein) in a plasticized PVC matrix with dioctylphthalate (DOP). The influence of membrane composition on the electrodes' response was studied. The electrodes showed a fast, stable and Nernstian response over a wide distigmine concentration range 5.0 · 10 À7 -1 · 10 À2 mol L À1 with a slope of $30.5 ± 1.0 mV dec À1 . The response is independent of the pH of test solution within the range 3.8-10.5. The life span of the electrodes extends to at least 2 months without any considerable divergence in potential and has a fast response time of <15 s. The electrodes showed good selectivity towards distigmine with respect to large numbers of ions in batch and FIA systems. The electrodes have been applied to the determination of distigmine in pure solution, pharmaceutical compound and human urine. The dissolution profile for Ubretid tablets (5 mg/tablet) was studied.

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Experimental ApparatusPotentiometric measurements in a batch mode were carried out at 25 ± 1˚C using a Jenway 3010 digital pH/mV meter with a surfactant-CMCPE sensor in conjunction with an Ag/AgCl reference electrode. A Techne circulator thermostat, Model C-100 (Cambridge, England), was used to control the temperature A comparative study was made between developed chemically modified carbon paste electrodes and PVC membrane electrodes for the cationic surfactant cetyltrimethylammonium bromide (CTAB). The carbon paste electrode modified with cetyltrimethylammonium-tetrachloropalladate(II) (CTA-TClP) provides a more sensitive and stable device than that shown by electrodes with an inner reference solution. The best performance was obtained by an electrode based on the paste containing 3.6 wt% CTA-TCIP, 1.8 wt% ethylhexadecyldimethylammonium bromide, 37.6 wt% graphite and 57 wt% tricresyl phosphate. The sensor exhibited a Nernstian response for CTAB over a wide concentration range of 3.5 × 10 -7 to 1.0 × 10 -3 M with a detection limit of 2.0 × 10 -7 M between pH 2.7 and 8.2 with a fast response time of ≤15 s. The electrode showed excellent selectivity for CTAB over a large number of ions. Interferences caused by common cationic surfactants have been investigated in simulated mixtures containing high concentration levels of interfering surfactants, and the sensor was found to be tolerant against these compounds.

Three novel chemically modified Carbon Paste Sensors (CMCPs) were proposed for the determination of nafronyl potentiometric in bulk, pharmaceutical dosage form; human plasma/urine. The sensors were based on an ion-pair associates of nafronyl silicotungstic acid (Nf-St) (sensor 1), nafronyl silicomolybdic acid (Nf-SM) (sensor 2), a mixture of (Nf-St)+(Nf-SM) (sensor 3). The modified sensors showed Nernstian slopes ranging from 58.5 ± 0.5-60.7 ± 0.5 mV over the concentration ranged from 1.0x10 -7 -1.0x10 -2 M and pH 2.0-6.0 with detection limit 0.1 nM. The sensors exhibited good selectivity for nafronyl with respect to inorganic/organic cations, sugars and amino acids. The calibration curve, standard addition and potentiometric titration methods were applied for the determination of nafronyl ion in its bulk powder, pharmaceutical dosage form, and human fluids plasma/urine taken from a healthy volunteer and for the monitoring Praxilene tablets in vitro dissolution rates. Sensor 3 had been the best sensitivity so was successfully used for the determination the solubility products of ion-pair associates. The results were excellent and satisfactory recovery comparable to those obtained with the British Pharmacopoeia.