Thermodynamics Research Center / ThermoML | Thermochimica Acta

ABSTRACT

In this study, the impact of a deviation from the stoichiometrically correct hydrate concentration of as alt 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.