COLORIMETRIC DETERMINATION OF CAPTOPRIL IN DOSAGE FORMS

Two simple and sensitive colorimetric methods were developed for the estimation of captopril in pure form or in its tablet formulations. The two methods are based on the reactivity of the thiol group. In the first method promethazine hydrochloride when reacted with N-bromophthalimide in 50% w/v o-phosphoric acid solution is oxidised to promethazine free radical which absorbs at 516 nm. The red free radical is readily reduced quantitatively by captopril to the colorless promethazine. The addition of captopril to red solution of promethazine free radical result in a decrease in absorbance in direct proportion to the quantity of captopril, this forms the basis for the quantitative estimation of microgram quantities of captopril. The second method involves the use of molybdophosphoric acid (MPA), which is a heteropoly acid of molybdenum (VI) as oxidising agent for the determination of captopril. When captopril is added to aqueous solution of MPA, heating in a boiling water bath for 30 min. a blue color of molybdenum (IV) was developed which absorbs at 685 nm. The developed color is stable over 24 hrs. The molecular ratios of the reactants and the experimental conditions leading to maximum color development were studied. Beer's law was obeyed in the concentration range 2-12 ugml-l for the two methods. The proposed methods were applied to the determination of captopril tablets and the results demonstrated that the methods are equally accurate, precise and reproducible as the official USP XXII method.