Calorimetric investigation of the concentration dependent enthalpy change around semicongruent melting CaCl2·6H2O

In this study, the impact of a deviation from the stoichiometrically correct hydrate concentration of a salt hydrate on the melting enthalpy is investigated exemplarily via 10 samples of CaCl2 + H2O around semicongruent melting CaCl2·6H2O by using differential scanning calorimetry (DSC). While the melting enthalpy of CaCl2·6H2O is determined to be (185 ± 19) J g−1, a trend of decreasing melting enthalpy with increasing deviation in the CaCl2 concentration from CaCl2·6H2O is observed. Three out of the 10 samples of CaCl2 + H2O were measured via DSC and T‐History. The determined enthalpy changes agree within the standard deviation of the DSC measurements. However, the value of the enthalpy change determined for consecutive heating cycles of the same sample via T-History is more stable than for consecutive heating cycles measured via DSC. This observation is attributed to the different sample volume of DSC and T-History and indicates a volume dependent behaviour of semicongruent melting CaCl2·6H2O. Statistical investigations into the high randomness of measured values for the melting enthalpy of consecutive heating cycles of CaCl2·6H2O were conducted via DSC. These reveal a strong negative correlation between the crystallisation temperature of CaCl2·4H2O and the melting enthalpy determined for the succeeding heating cycle. By adding 1 wt% of Sr(OH)2·8H2O as nucleating agent for CaCl2·6H2O, the crystallisation of CaCl2·4H2O in the DSC measurements is suppressed and reproducible values for the melting enthalpy of consecutive heating cycles are obtained.