Optimized and validated spectrophotometric methods for the determination of lisinopril in pharmaceutical formulations using ninhydrin and ascorbic acid

Two simple and sensitive spectrophotometric methods are described for the determination of lisinopril in pure form and pharmaceutical preparations. The first method is based on the reaction of the drug with ninhydrin in N,N′-dimethylformamide (DMF) medium at room temperature which is followed spectrophotometrically by measuring the increase in absorbance at 595 nm as a function of time. The initial-rate, rate-constant and fixed-time (at 10 minutes) procedures were used for constructing the calibration graphs to determine the concentration of the drug. The initial-rate and fixed-time procedures showed a linear response over the concentration range 10-50 μg mL^-1 whereas rate-constant procedure was applicable in the range 10-40 μg mL ^-1. In the second method, the drug reacts with ascorbic acid in DMF medium resulting in the formation of a coloured product, which absorbed maximally at 530 nm. Beer's law is obeyed in the concentration 5-50 μg mL^-1 of lisinopril with molar absorptivity of 4.548×10 ³ L mol^-1 cm^-1. The variables affecting the development of the colour are optimized and the developed methods are validated statistically and through recovery studies. The proposed methods have been successfully applied to the determination of lisinopril in commercial tablets.