Short communication.

A simple and rapid calorimetric method for the determination of 10 sulphonamides as single entities and sulphamethoxazole in combination with trimethoprim without prior separation was developed. The method is based on the reaction of sulphonamide with phenothiazine and N-bromosuccinimide at pH 6 to produce a blue coloured product after acidification. The chromogen for all the sulphonamides was measured at 605 nm. The effect of several variables on colour development (concentration of phenothiazine and N-bromosuccinimide, time, pH) were established. Beer's Law was obeyed for all the drugs. The method was successfully applied to the analysis of single component sulphonamide tablets and ophthalmic solutions, with average results of labelled claim of 96.7 +/- 0.95 to 100.67 +/- 1.2. A good correlation was observed between molar absorptivities and pK(a) values of the sulphonamides (r = 0.9005).