Revisiting the thermal behavior of dipyrone

Although some thermoanalytical studies regarding thermal analysis of dipyrone sodium monohydrate had already been presented in literature, results are incomplete once no evolved gas analysis has been presented. In addition, the inorganic residue should also be better characterized regarding the presence of sodium sulfate or sodium hydrogen sulfite, once the amount of residue varies depending on experimental conditions. In this work, the thermal degradation of dipyrone sodium monohydrate was revisited regarding evolved gases, intermediates produced during heating and inorganic residue. Thus, dipyrone sodium monohydrate was analyzed using TG/DTG/DTA, DSC, hot-stage microscopy, TG–FTIR and HPLC–MS looking for proposing a thermal behavior mechanism for this important pharmaceutical. Moreover, inorganic residue was investigated by XRD and FTIR. The evolved gases were detected and identified in the following sequence: water, sulfur dioxide, methylamine, isocyanic acid, benzene, ammonia and carbon dioxide. Regarding the inorganic fraction, intermediates were collected at 500 and 1000 °C and characterized as Na2SO4 in dry air atmosphere. From these data obtained by combining results from thermoanalytical techniques with those from HPLC–MS, FTIR and XRD, a mechanism for the thermal behavior of dipyrone sodium monohydrate was proposed.