The optimum pH for the derivative spectrophotometric determination of co-trimoxazole in binary mixtures.

BACKGROUND: Although the experimental assessment of co-trimoxazole by use of derivative spectrophotmetry underscores the usefulness of this method due to its relative simplicity with which it can be carried out over the official United States Pharmacopoeia (USP) high pressure liquid chromatography (hplc) methods for this drug, suitable optimum conditions ought to be refined for its universal acceptability. OBJECTIVE: The objective of the present work was to obtain the optimum pH level for the UV assessment of co-trimoxazole. METHODS: The aqueous solutions of the individual drugs and their binary mixtures were buffered with Sodium Acetate-Acetic Acid buffer in the pH ranges 2-7 and scanned on zero order and on first-order derivative at the wave length between 200- 300 nm. RESULTS: At the same drug solution concentrations, spectral shifts occurred with change in pH, especially between the wavelengths 200 and 240 nm, only seeming to converge from approximate wavelength 260 nm onwards. Absorbance fluctuations were also observed at the same drug concentrations in the pH range 2 to 3.5 and 5 to 7 when the solutions were scanned, even at the wavelength where the spectra seem to converge. However there were no absorbance differences between pH 4 and 5. CONCLUSION: The UV spectrophotometric method is dependent on the optimum pH and this has been found to range from 4 to 5.

Spectrophotometric determination of sulphamethoxazole and trimethoprim (co-trimoxazole) in binary mixtures and in tablets.

BACKGROUND: The formulation of sulphamethoxazole (S) and trimethoprin (T) (CO-TRIMOXAZOLE) in a combination mixture is very good pharmacologically since it enhances the efficacy of the individual drugs. However in this combination, difficulties in analysis on ordinary UV spectrophotometry are introduced because the two components give overlapping spectral bands on zero-order. The United States Pharmacopoea (USP)-recommended HPLC analytical method is quite expensive. OBJECTIVE: The objective of the present work was to assess whether derivative spectrophotometry could be used to circumvent the overlapping spectral bands of the components and hence use it for routine analysis of the drug. STUDY DESIGN: Experimental. METHODS: The aqueous solution of the individual drugs and their binary mixtures were scanned on zero order and on first derivative at the wave length between 200- 300 nm and at the pH of 4.5. RESULTS: The zero-order spectra of the compounds were completely overlapping. However the first-derivative scan offered better separation and hence T was determined from the absorbance at 237.6 nm with negligible contribution from S (since at this point it was reading zero). Likewise S was determined at a wavelength of 259 nm when T was reading zero. The linear calibration graphs were obtained for 4-25 microg x ml(-1) of S and for 4-20 microg x ml(-1) of T. CONCLUSION: The method is rapid, simple and can be applied successfully to assay a mixture of the two drugs in pharmaceutical preparations.