Continuous potentiometric monitoring of viagra (sildenafil) in pharmaceutical preparations using novel membrane sensors

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 1.0×10−2–7.9×10−7 and 1.0×10−2–1.0×10−6 M with detection limits of 0.53 and 0.67 μg 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.