Stability of captopril in tap water

The stability of captopril in tap water was studied. Twenty 25-mg captopril tablets were crushed separately, added to 25 mL of tap water in individual volumetric flasks, and shaken vigorously. Five flasks each were incubated at 25, 50, and 75 degrees C in a shaking water bath and refrigerated at 5 degrees C. Samples were taken from each flask immediately after dissolving the drug and at intervals up to 28 days for 5 and 25 degrees C, 15 days for 50 degrees C, and 16 days for 75 degrees C. Captopril concentrations were analyzed by high-performance liquid chromatography. First-order rate constants were calculated for each temperature setting, and the Arrhenius plot was applied to estimate the shelf life of captopril at 5 degrees C. More than 90% of the initial concentration of captopril remained after 28 days at 5 degrees C. Captopril concentration in the samples stored at 75, 50, and 25 degrees C decreased to 90% of the initial concentration at calculated times (mean +/- S.E.) of 2.1 +/- 0.1, 3.6 +/- 0.4, and 11.8 +/- 1.2 days, respectively. The estimated time required for the concentration of a 1-mg/mL solution of captopril stored at 5 degrees C to decrease to 90% of initial concentration was 27 days. The shelf life of a solution of captopril 1 mg/mL in tap water stored at 5 degrees C was 27 days.