Spectrophotometric and Atomic Absorption Determination of Ramipril, Enalapril Maleate and Fosinopril through Ternary Complex Formation with Molybdenum (V)-Thiocyanate (Mo(V)-SCN)

Three different sensitive and accurate spectroscopic procedures were developed for the determination of three angiotensin-converting enzyme inhibitors, namely, ramipril, enalapril maleate and fosinopril. The first two spectrophotometric (extractive and non-extractive) procedures were based on ternary complex formation with molybdenum(V) thiocyanate. The formed complex can be determined by extraction with chloroform measured at lambda(max) 517nm Beer's law seas obeyed in the concentration range from (10-90 mu g ml(-1)) for ramipril and fosinopril and (4-36 mu g ml(-1)) for enalapril maleate with molar absorptivity 1.2 x 10(4), 2 x 10(4) and 3.4 x 10(4) l mol(-1) cm(-1), respectively, or by direct measurement after addition of benzalkonium chloride as surfactant and measuring the formed ternary complex at lambda(max) 545 nm with a linear relationship in the concentration range from (8-72 mu g ml(-1)), (3-27 mu g ml(-1)) and (8-72 mu g ml(-1)) for ramipril, enalapril maleate and fosinopril with molar absorptivity 1.5 x 10(4), 5 x 10(4) and 2.1 x 10(4) l mol(-1) cm(-1), respectively. The third procedure is atomic absorption measurement through the quantitative determination of molybdenum content of the complex. These methods hold their accuracy and precision well when applied to the determination of ramipril, enalapril maleate and fosinopril in their dosage forms.