Spectrophotometric determination of enalapril through ternary complex formation

Enalapril is an angiotensin-converting enzyme inhibitor widely used in the treatment of hypertension and heart failure.

To design and validate a simple and sensitive spectrophotometric method. This method is based on a ternary complex formation between enalapril, copper (II), and bromothymol blue, in a slightly alkaline environment, complex that is extractable in chloroform, with an absorption maximum at 426 nm.

To a volume of 1 milliliter of standard enalapril drug solution in concentration range 200/500 microg/ml, two milliliters of Cu (II) chloride solution 0.2 % followed by 2 ml bromothymol blue solution 0.01% and 0.5 ml of pH 8 buffer solution (Britton-Robinson) were added. The complex was two times extracted with 2 x 1.5 ml chloroform. Ten minutes after extraction, absorbance was measured at 426 nm against blank.

Our spectrophotometric method was validated by determining linearity, limit of detection and quantification, system and method precision, and method accuracy. The method showed a good linearity in the range 200/500 microg/ml (correlation coefficient r = 0.9993). Limit of detection (LD) was 9.907 microg/ml and limit of quantification (LQ) 33.024 microg/ml. Precision showed an RSD = 1.83%, and accuracy with mean recovery is 102.6% in the range 98.8/103.9.

The ternary complex formed under the above-mentioned condition and measured spectrophotometrically can be regarded as an ion-association complex between the metal-drug cation and bromothymol blue anion. The experimental results demonstrated a good sensitivity.