Rapid colorimetric assay of trimethoprim and sulfamethoxazole in pharmaceuticals.

A method is described for the direct colorimetric determination of trimethoprim and sulfamethoxazole in pharmaceutical preparations, without prior separation. Estimation of trimethoprim is based on its ion-pair formation with bromophenol blue and subsequent measurement of absorbance of the ion-pair at 418 nm. Estimation of sulfamethoxazole is possible without removal of trimethoprim by solvent extraction.