Determination of Sulfamethoxazole and Trimethoprim in Pharmaceuticals by Visible and UV Spectrophotometry

Abstract

A method for the simultaneous determination of sulfamethoxazole (SMX) and trimethoprim (TMP), based on a direct determination of SMX after diazotization and coupling with 2-naphthol by visible spectrophotometry and an indirect determination of TMP in the UV region by difference was developed. By the suggested procedure, it was possible to analyze SMX and TMP in pharmaceutical preparations without separating from each other or from the excipients. Primarily a portion of the sample was diluted to have an absorbance of 0.4-0.7 at 271 nm, which corresponds to both SMX and TMP. By adding sodium nitrite, HCl and 2-napthol to 1 ml of the diluted solution, only SMX is diazotized and coupled with 2-naphthol forming a colored product with ? max at 482 nm where the quantity of SMX could be obtained directly from a calibration curve. By plotting another calibration curve for SMX at 271 nm, the influence of SMX on the total absorbance of a mixture of SMX and TMP at 271 nm could be deduced. The absorbance corresponding to TMP only was obtained by difference, which was transferred to quantity using a calibration curve at 271 nm. This procedure was applied successfully for the analysis of SMX and TMP in pharmaceutical preparations without prior separation and with acceptable errors.