Spectrophotometric determination of sulfanilic acid and sulfonamides in pharmaceutical samples with potassium 1,2-naphthoquinone-4-sulfonate

Abstract

An accurate and simple method is proposed for the determination of sulfanilic acid in the presence of sulfonamides. This method is based on measuring the intensity of the red colour that develops when sulfanilic acid is allowed to react with potassium 1,2-naphthoquinone-4-sulphonate (NS) in a chloroaceticchloroacetate buffer at pH 3,4. Colour development reaches completion after 2 h, allowing sulfanilic acid to be quantified spectrophotometrically at 470 nm (ε = 4.7 × 103 L mol−1 cm−1). The main product causing colour formation, potassium 1,2-naphthoquinone-4-(N-aminophenylen-4-sulphonate) (NSSA), was isolated and characterized. When samples also contain sulfonamides an extraction into chloroform must be performed. Sulfanilic acid in binary mixtures with sulfanilamide, sulfacetamide, or sulfathiazole can be determined either by direct measurement of the aqueous phase after extraction at 470 nm or by subtracting from the absorbance of the aqueous phase before extraction the absorbance of sulfonamide as determined by measuring the extracted chloroform phase at 345 nm. Sulfadiazine, sulfamethoxipyridazine, and sulfamethoxazole interferences are prevented by their extraction into chloroform at pH 7.2; these species cannot be determined. The effects of pH, reagent concentration, time, and temperature on colour formation were investigated. In all cases the standard addition method gave more accurate results. The method was applied to several pharmaceutical samples. Keywords: spectrophotometry, sulfanilic acid, sulfonamides, potassium 1,2-napthoquinone-4-sulphonate.