Argentimetric assay of captopril in bulk drug and in tablets

Three simple, selective and cost-effective procedures for the determination of captopril in bulk drug and in tablets are described. All the procedures make use of silver nitrate as a reagent and involve titrimetry and spectrophotometry as measurement techniques. In titrimetry (Method A), the aqueous solution of the drug is titrated directly with the standard silver nitrate solution to a potassium chromate end-point. In one spectrophotometric method (Method B), the sample solution is treated with excess of silver nitrate and a known amount of methyl orange and the increase in absorbance at 520 nm, caused by a decrease in pH due to release of nitric acid, is measured and related to drug concentration. The other spectrophotometric method (Method C) involves the addition of a measured excess of silver nitrate to the sample solution followed by the determination of residual silver ion by an ion-associate complex formation reaction involving eosin and 1,10-phenanthroline. The decrease in absorbance at 550 nm, which corresponds to Ag+ reacted with the drug, is measured and is found to be linearly related to drug concentration. All experimental variables involved in the methods were investigated and optimized. Stoichiometry of the reaction that forms the basis for titrimetry is found. Method A is applicable in the range of 1.0-20.0 mg of drug while methods B and C can be conveniently used in the concentration ranges of 2.5-50.0 and 0.25-4.0 mu g ml(-1), respectively. Several optical characteristics such as molar absorptivity, Sandell sensitivity, limits of detection and quantification, and correlation coefficient were calculated. The methods were applied to the analysis of tablets containing captopril. Statistical treatment of the results indicates that the procedures are precise and accurate. The excipients used as additives in tablets did not interfere in the proposed procedures as revealed by the recovery studies