2005•physica status solidi (a)Requires access
On the high‐temperature phase transitions of NH4HSO4
J.E. Diosa, M. Fernández, R.A. Vargas, E. Torijano, B.‐E. Mellander
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Abstract
Abstract Modulated differential scanning calorimetry (MDSC), thermogravimetric analysis (TGA) and X‐ray diffraction (XRD) were used to study the phase behaviour of NH4HSO4 (AHS) above room temperature. For powder AHS measured during successive thermal cycles, the MDSC curves show an endothermic peak at 147 °C on heating scans and an exothermic peak at about 115 °C on subsequent cooling runs. However, after the second thermal cycle, a second endothermic peak starts to appear at about 135 °C on heating runs whose intensity increases on subsequent cycles. The thermogravimetric measurements on powder AHS show weight loss above 135 °C. X‐ray powder diffraction data for AHS heated to 140 °C during 24 hours, show the diffraction pattern for (NH4)2S2O7. We discuss the endothermic effect at 135 °C in terms of a partial dehydration setting in at reaction sites distributed on the surface of the crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)