Nuclear Magnetic Resonance Analysis of Phenytoin and Sodium Phenytoin in Solid Dosage Forms

A rapid and specific nuclear magnetic resonance (NMR) spectroscopic method was developed for determining phenytoin and its sodium salt in capsules and tablets. Acetamide was used as the internal standard and 0.5% sodium deuteroxide in deuterium oxide served as the NMR solvent. The concentration of drug per unit dose was calculated from the integration values for the resonance signals of phenytoin at about 7.40 ppm and of the internal standard at about 1.97 ppm. The average recovery value of phenytoin added to synthetic samples, in concentrations ranging from 84 to 122 mg, was 99.9 ± 0.2% (SD) with a coefficient of variation of 0.2%. The method using commercial products gave results comparable to those obtained by the titrimetric and gravimetric methods of USP XX. Excipients of tablets and capsules such as sucrose and lactose did not interfere with the determinations. The proposed method was found suitable for measuring the content uniformity of capsules and tablets, and offered a positive means of identification on phenytoin in these dosage forms.