Simultaneous Determination of Sulfamethoxazole and Trimethoprim Based on Different Reaction Rates Using H-Point Standard Addition Method

Abstract

A simple, selective, cost-effective, and reliable kinetic spectrophotometric method has been developed for the simultaneous quantification of sulfamethoxazole (SMX) and trimethoprim (TMP) in pharmaceutical formulations. The method is based on the differential oxidation reaction rates of SMX and TMP with potassium permanganate (KMnO₄) in an alkaline medium (NaOH), resulting in the formation of green potassium manganate (K₂MnO₄), which exhibits a maximum absorbance at 610 nm. Kinetic studies of the oxidation reaction were conducted, and simultaneous determination of SMX and TMP was achieved using the H-point standard addition method. Absorbance measurements were recorded at 610 nm after 120 and 300 seconds following the addition of TMP. The method demonstrated high accuracy and precision, with percentage errors of 1.04% to -2.24% for SMX and 3.01% to -2.33% for TMP. Repeatability, assessed over five consecutive determinations of 10 µg/mL SMX and 4 µg/mL TMP, yielded relative standard deviations of 1.39% and 0.58%, respectively. The detection limits were 0.7 µg/mL for SMX and 0.3 µg/mL for TMP. The method was successfully applied to the analysis of various tablet formulations containing both active pharmaceutical ingredients.