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Two potentiometric sensors responsive to sildenafil citrate (SC) drug (the active component of viagra) are described, characterized, compared and used for drug assessment. The sensors are based on the use of the ion-association complexes of (SC) cation with tungstophosphate (TP) and reineckate (Re) anions as electroactive materials in plasticized poly(vinyl chloride) membranes. The sensors demonstrate fast near-Nernstian response for SC over the concentration ranges 1.0Â10 )2 -7.9Â10 )7 and 1.0Â10 )2 -1.0Â10 )6 M with detection limits of 0.53 and 0.67 lg ml )1 over pH 3-6 for TP and Re based membrane sensors, respectively. The sensors display good selectivity for SC drug over many nitrogeneous compounds, some inorganic cations and excipients and diluents commonly used in drug formulations. Validation of the assay methods with both sensors by measuring the lower detection limit, range, accuracy, precision, repeatability and between-day-variability reveals good performance characteristics confirming applicability for continuous determination of SC in pharmaceutical formulations and in spiked human serum. A membrane incorporating SC-TP complex in a tubular flow detector is used in a two channel flow injection set up for continuous monitoring of the drug at a frequency of 25-30 samples h )1 . The results obtained with drugs containing 50-100 mg SC tablet )1 show a mean standard deviation of ±2% of the nominal which agree fairly well with data obtained by spectrophotometry.

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Coated-wire (CW) and tubular (Tu) type membrane sensors for creatinine are developed. These consist of creatinine tungstophosphate(CTP), creatinine molybdophosphate (CMP) and creatinine picrolonate (CPC) ion-pair complexes as electroactive materials dispersed in plasticized poly(vinyl chloride) matrix membranes. Electrochemical evaluation of these sensors under static (batch) mode of operation reveals near-Nernstian response with slopes of 62.9, 58.1, and 55.2 mV decade À1 over the concentration range 1 Â 10 À2 -5.0 Â 10 À6, 1 Â 10 À2 -7.5 Â 10 À5, and 1 Â 10. The lower detection limits are 0.39, 3.49, and 2.20 mg mL À1 creatinine with CTP, CMP and CPC membrane based sensors plasticized with o-NPOE, respectively. Tubular and coated wire CTP membrane sensors are incorporated in flow-through cells and used as detectors for flow injection analysis (FIA) of creatinine. The intrinsic characteristics of the detectors under hydrodynamic mode of operation in a low dispersion manifold are determined and compared with data obtained under static mode of operation. With 10 À2 mol L À1 phosphate buffer of pH 4.5 as a carrier solution, the tubular and coated wire CTP detectors exhibit rapid response of 58.9 and 50.7 mV decade À1 over the concentration range 1 Â 10 À2 -1 Â 10 À5 mol L À1 and detection limits of 0.39 mg mL À1 and 0.85 mg mL À1, respectively. Validation of the assay methods with the proposed sensors by measuring the lower detection limit, range, accuracy, precision, repeatability and between-day-variability reveals good performance characteristics confirming applicability for continuous determination of creatinine. The sensors are used for determining creatinine in human blood serum at an input rate of 40 samples per hour. No interferences are caused by creatine, most common anions, cations and organic species normally present in biological fluids. The results favorably compare with data obtained using the standard spectrophotometric method.