Simple and Sensitive Titrimetric and Spectrophotometric Determination of Enalapril Maleate in Pharmaceuticals using Permanganate

Two titrimetric and two spectrophotometric methods are described for determination of enalapril maleate (ENP) in pure drug as well as in tablets. In titrimetry, ENP was quantified by either direct oxidation of ENP content with potassium permanganate in H2SO4 medium (method A) or oxidation of ENP by a known excess of potassium permanganate in H2SO4 medium followed by determination of unreacted permanganate by titration with ferrous ammonium sulphate (method B). In both the methods, the reaction stiochiometry is found to be 1:2 (ENP: KMnO4) and the methods are applicable over the 1.0-10.0 mg. In spectrophotometry, ENP was quantified based on the reduction of potassium permanganate by ENP either in neutral medium (method C, lambda max at 340 nm) or in H2SO4 medium (method D, lambda max at 550 nm) over the concentration ranges, 2.0-12.0 μg mL^-1 and 7.0-70.0 μg mL^-1 by method C and method D, respectively. The calculated molar absorptivities are 1.8 x 104 and 3.8 x 103 L mol^-1 cm^-1 for method C and method D, respectively with corresponding Sandell sensitivity values of 0.028 and 0.115 μg cm^-2. The limits of detection (LOD) and quantification (LOQ) have also been reported. The interference due to common excipients present in the formulations in method A was successfully overcome by extraction with acetone. The methods were successfully applied to the determination of ENP in tablets and the results were statistically compared with those of a reference method by applying the Student’s t-test and F-test. The accuracy and validity of the methods were ascertained by recovery studies via standard addition technique.