Simultaneous quantitative determination of metronidazole and nalidixic acid in tablets by difference spectroscopy

A difference spectrophotometric procedure has been developed for simultaneous determination of metronidazole (MDZ) and nalidixic acid (NA) in tablets. The method comprised the measurement of the absorbance of a solution of the tablet extract in 0.1 M NaOH relative to that of an equimolar solution in 0.1 M HCl at 292 nm for NA and 325 nm for MDZ. The presence of identical isosbestic points for pure drug solutions and tablet extracts indicated the non-interference of excipients in the absorption at these wavelengths. Compliance with Beer's law was observed in the concentration ranges 5-25 mu g ml(-1) for MDZ and 15-35 mu g ml(-1) for NA at these wavelengths.