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- 300nm
Results: At the same drug solution concentrations, spectral shifts occurred with change in pH, especially between the wavelengths 200 and 240nm, only seeming to converge from approximate wavelength 260nm 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.

Keywords
hypsochromic shift, auxochromes, sulphamethoxazole, trimethoprim, spectroscopy, pH, UV, pyridines