Comparative evolved gas analyses on thermal degradation of thiourea by coupled TG-FTIR and TG/DTA-MS instruments

Identification and monitoring of gaseous species released during thermal decomposition of pure thiourea, (NH₂)₂C=S in argon, helium and air atmosphere have been carried out by both online coupled TG-FTIR and simultaneous TG/DTA-MS apparatuses manufactured by TA Instruments (USA). In both inert atmospheres and air between 182 and 240°C the main gaseous products of thiourea are ammonia (NH₃) and carbon disulfide (CS₂), whilst in flowing air sulphur dioxide (SO₂) and carbonyl sulphide (COS) as gas phase oxidation products of CS₂, and in addition hydrogen cyanide (HCN) also occur, which are detected by both FTIR spectroscopic and mass spectrometric EGA methods. Some evolution of isothiocyanic acid (HNCS) and cyanamide (NH₂CN) vapours have also observed mainly by EGA-FTIR, and largely depending on the experimental conditions. HNCS is hardly identified by mass spectrometry. Any evolution of H₂S has not been detected at any stage of thiourea degradation by either of the two methods. The exothermic heat effect of gas phase oxidation process of CS₂ partially compensates the endothermicity of the corresponding degradation step producing CS₂.