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DSC investigation of the thermal behaviour of (NH4)2SO4, NH4HSO4 and NH4NH2SO3
Authors:Ilona Konkoly Thege
Affiliation:Institute of Inorganic and Analytical Chemistry, L. Eötvös University, P.O. Box 123, H-1443 Budapest Hungary
Abstract:Gaseous products evolved from (NH4)2SO4, NH4HSO4 and NH4NH2SO3 during successive heating and cooling cycles were flushed with inert gas into analyzer Dräger tubes hooked tightly to the terminal port of the DSC cell base. This simple procedure allowed the starting temperature of the decomposition to be determined and the amount of the individual gases in the mixture to be identified and even estimated. NH4NH2SO3 at 523 K in humid air produced HNH2SO3 initially and, on further cycling, (NH4)2SO4 and NH4HSO4 also appeared. The ΔHf values for NH4HSO4 were (kJ mole?1): in an airtight sample holder 12.67, in a dry argon atmosphere 11.93, and in a static air atmosphere 10.92. Endothermic peaks for (NH4)2SO4 and 498 and 411 K represented the incongruent melting point and the polymorphic transition of (NH4)2SO4·NH4HSO4. After the first heating in air to 530 K, (NH4)2SO4 and NH4HSO4 exhibited closely similar cyclic DSC curves. The endothermic peaks at about 393–420 K may be assigned to different combinations of (NH4)2SO4 and NH4HSO4.
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