Macromolecular Oxidative–Coupling Strategy for Colorimetric Determination of Tadalafil in Pharmaceutical Formulations

A simple and sensitive colorimetric method has been developed for the determination of tadalafil based on a macromolecular oxidative–coupling reaction. In the first step, tadalafil was oxidized using potassium permanganate (KMnO 4 ) as an oxidizing reagent; this was followed by a coupling with a crystal violet dye, which resulted in a blue solution with the highest absorption at the wavelength 590 nm in a basic medium. The factors affecting the reaction have been studied; they included: type and volume of oxidation agent, dye type and volume, base's type and volume, oxidation time, addition sequence, temperature, and coupling time. The best conditions were reached for the formation of the colored complex. The method showed a good linearity within a concentration range between 45 and 50 µg/mL with a determination coefficient ( R 2 ) = 0.9995, while Sandell's index reached 0.163934 µg/cm 2 and molar absorptivity was 2375.34 L/mol/cm. The limit of detection was also determined to be 0.3825 µg/mL, and the limit of quantification was 1.1591 µg/mL. As for accuracy, the recovery percentages (Rec%) ranged between 96.72% and 102.87%, while the values of relative standard deviation (RSD%) ranged between 0.2808 and 0.0923, confirming that this method is of high sensitivity and accuracy and was successfully applied for the determination of tadalafil within its pharmaceutical formulations.