Phase transitions of KHSO4 above room temperature

Modulated differential scanning calorimetry (MDSC), thermogravimetric analysis (TGA) and X-ray diffraction were used to study the phase transitions in single crystals of KHSO4 (KHS) above room temperature. Our results show three phase transitions, on heating a freshly prepared sample: the first one at approximate to 120 degrees C (exothermic and metastable) accompanied by small weight loss (less than 0.25wt%), the second one at approximate to 178 degrees C (endothermic) and the third one at approximate to 212 degrees C (endothermic, melting point). The latter two phase transitions are also accompanied by weight loss and its rate depends on the surface of the sample. We discuss the first transition in terms of absorbed water molecules weakly bound to the surface that are released at 120 degrees C on heating a fresh crystal from room temperature. The second transition at 178 degrees C is discussed in terms of a partial polymerization setting in at reaction sites distributed on the surface of the crystals followed by the final decomposition above 212 degrees C.