Free radical formation in salicylic acid and heating parameters ñ application of epr, uv-vis, tga and colorimetry examination to optimize thermal sterilization

Abstract

Salicylic acid heated at different temperatures and times was examined by an X-band (9.3 GHz) EPR spectroscopy, UV-Vis spectrophotometry, TGA and colorimetry test to optimize its thermal sterilization process. Free radical formation (~1018 spin/g) during thermal sterilization of salicylic acid according to the pharmaceutical norms at temperature 120OC and time of 120 min were compared with those for heating at the new tested temperatures and times: 130OC and 60 min, and 140OC and 30 min. It was obtained that the relatively lower free radical concentrations characterized salicylic acid heated at temperatures (times): 120OC (120 min), and 130OC (60 min.), than at temperature (time) 140OC (30 min). So treatment at temperature 120OC during 120 min, and temperature 130OC during 60 min, were recommended as the optimal for thermal sterilization of salicylic acid. Salicylic acid should not be sterilized at temperature 140OC for 30 min, because of the highest free radical formation. Free radical systems of thermally treated salicylic acid revealed complex character. Fast spin-lattice relaxation processes existed in heated salicylic acid. Strong dipolar interactions characterized all the heated salicylic acid samples. EPR spectroscopy, UV-Vis spectrophotometry, thermogravimetry, and color measurement may be helpful besides microbiological analysis to optimize thermal sterilization conditions of salicylic acid.