Individual and simultaneous determination of ephedrine and phenylephrine by use of kinetic methodology

Kinetic methodology was for the first time used to develop a simple photometric method for the determination for ephedrine and phenylephrine using the stopped-flow mixing technique. The method involves oxidation of these compounds and kinetic monitoring of the reactions of the aldehydes formed with 2-methylbenzothiazoline-2-one hydrazone in the presence of iron (III) chloride to give the corresponding coloured formazans. Ephedrine and phenylephrine can be determined simultaneously by applying the proportional-equation method to the reaction rate and absorbance amplitude of the kinetic curves obtained for each analyte. The calibration graph was linear over the range 0.7–70.0 g mL^–1 ephedrine and 1.2–70.0 g mL^–1 phenylephrine. The proposed method was applied to the determination of ephedrine in pharmaceutical samples with a mean recovery of 103.2%. Synthetic mixtures of ephedrine and phenylephrine in ratios between 3:1 and 1:5 were satisfactorily resolved.