Colorimetric determination of catecholamines by 2,3,5-triphenyltetrazolium chloride.

A convenient spectrophotometric method was developed for the determination of epinephrine, levarterenol, isoproterenol, and methyldopa by reduction of 2,3,5-triphenyltetrazolium chloride and subsequent measurement of the formazan at 485 nm. With absolute alcohol as the solvent, maximum color absorption was attained in 30 min at 25 degrees in the presence of 0.1 N KOH. Evidence is provided to account for the reduction of the tetrazolium salt at the expense of the epinephrine catechol moiety. In addition to the considerably high values of the molar absorptivities of the chromogen formed, ideal adherence of the color absorption to the Beer-Lambert law permitted a sensitive microdetermination of these catecholamines in both pure forms and pharmaceutical formulations. The tetrazolium interaction was selective. No interference was encountered from common catecholamine antioxidants, adjuvants, or noncatechol degradation products.