A poly(vinylchloride) membrane electrode for determination of phenytoin in pharmaceutical formulations.

The construction and general performance characteristics of an ion-selective membrane electrode sensitive to the drug phenytoin, based on its ion-pair complex with the quaternary ammonium cation, tricaprylmethylammonium, in a PVC matrix are described. The electrode shows near-Nernstian response over a 10(-1)-10(-4) mol l(-1) range and a detection limit of 1.5 x 10(-5) mol l(-1). The selectivity of the electrode to a number of inorganic and organic anions is reported. OH(-) interference, in the linear range of the calibration curve, is negligible up to pH 11.0. The standard addition method is used to determine phenytoin in pharmaceutical formulations, such as tablets and capsules, with good results. The method is rapid and simple, and does not require prior sample pretreatment.