Nuclear Magnetic Resonance Analysis of Pharmaceuticals. IX. Methenamine and Methenamine Mandelate in Tablets

An NMR procedure is described by which methenamine and mandelic acid are simultaneously determined in methenamine and methenamine mandelate tablets. Maleic acid was chosen as the internal standard and dimethylformamide-acetone-acetonitrile (10+25+65) as the solvent. The solvent system was selected to resolve the problems of solubility of methenamine and its salt with mandelic acid, overlapping of the resonance signals of the components, and potential decomposition of methenamine. Known standard and commercial preparations were analyzed and the results were compared to those of official USP and NF procedures. The NMR technique, when applied to the determination of methenamine and methenamine mandelate in tablets, is rapid, simple, and specific and can provide an assay with an accuracy of 1–2%.