Influence of pH and Temperature on Stability of Sulfamethoxazole Alone and in Combination with Trimethoprim (Co Trimoxazole)

Abstract

The present research was under taken to assess the stability of medicinal agent, sulfamethoxazole (pure and in combination with trimethoprim) to the prevailing physical factors like temperature and pH. A pure sulfamethoxazole and a commercially available brand of co-trimoxazole were included in this study for comparison. The stability study was carried out by storing sulphamethoxazole and co-trimoxazole at four different temperature i.e., 25, 37, 50 and 60 ºC and different pH i.e., 3, 4, 5, 6, 7, 8, 9 and 10. Drug contents were determined at 0, 6, 24, 48 and 72 h intervals by non-aqueous acid-base titration method that was developed in our laboratory. The percentage of drug remaining was plotted against time for each temperature and pH. The degradation processes were assumed to follow pseudo-first order kinetics and rate constant (k) were calculated. The heat of activation (DHa) at particular pH was calculated by Arrhenius plot. It was observed that sulfamethoxazole in combination with trimethoprim (co-trimexazole) is more stable at higher temperature and lower pH. Degradation pattern of co-trimoxazole is steeper than sulphmethoxazole, but its overall stability pattern is better than sulphamethoxazole alone.