Development of a Spectrophotometric Method to Determine Minoxidil by Diazotization-Coupling Reaction with Sulfanilic Acid

A new spectrophotometric method has been developed to accurately quantify minoxidil using diazotized sulfanilic acid as a reagent. This method involves the formation of a yellow azo dye with a maximum absorption at 400 nm. It demonstrates excellent linearity with a Beer's law range of 5-40 μg/ml and a high determination coefficient (R² = 0.9993). The method has a detection limit of 0.012 μg/ml and a quantitation limit of 0.040 μg/ml, with precision ranges (RSD%) from 0.16% to 1.59%. The stoichiometry of the reaction is 1:1, and the method has been successfully applied to determine minoxidil in tablet form.