Spectrophotometric determination of pyrimethamine (PYM) in pharmaceutical formulation using 1,2-naphthoquinone-4-sulfonate (NQS)

A simple and sensitive spectrophotometric method for the quantitative analysis of pyrimethamine (PYM) in pharmaceutical formulations has been described. The method is based on the formation of colored product between PYM and 1,2-naphthoquinone-4-sulfonate (NQS) at 60 °C. The reaction is followed spectrophotometrically by measuring the increase in absorbance at 483 nm as a function of time. Under the optimized reaction condition, Beer’s law correlation for the absorbance (A) with PYM concentration (C) was obeyed in the range 12–40 μg mL−1 the regression equation for the calibration data was A = 0.704 + 0.0132C, with correlation coefficient (0.996). The molar absorptivity (ε) was 5.8 × 103 L mol−1 cm−1. The limits of detection and quantification were 3.25 and 10.83 μg mL−1, respectively. The proposed method was successfully applied to the determination of PYM in pharmaceutical tablets with good accuracy and precision; the percentage for PYM was 101.4 ± 1.47% for batch 20 and 100.4 ± 0.51% for batch 13. The results were in good agreement with those obtained with the official high performance liquid chromatography (HPLC) method.