Thermodynamic properties of ammonium magnesium sulfate hexahydrate (NH4)2Mg(SO4)2·6H2O

Heat capacity of (NH4)2Mg(SO4)2·6H2O (CAS: 7785-18-4) was measured by low-temperature vacuum adiabatic calorimetry (AC) in the temperature range from 8 to 320 K. A linear combination of Einstein functions was applied to approximate obtained data on the heat capacity. Heat content and entropy of (NH4)2Mg(SO4)2·6H2O in the temperature range from 0 to 320 K were calculated from these data. The molar enthalpy of (NH4)2Mg(SO4)2·6H2O dissolution in water was measured at 298.15 K by solution calorimetry (SC). Standard entropy, enthalpy of formation and Gibbs energy formation of (NH4)2Mg(SO4)2·6H2O at 298.15 K were calculated from these results and literature data. Melting point of (NH4)2Mg(SO4)2·6H2O was evaluated at external pressure 104 kPa by differential scanning calorimetry (DSC). The reversible phase transition of (NH4)2Mg(SO4)2·6H2O crystal aggregates was recorded using three independent methods: DSC, AC and dielectric permittivity measurements. It was shown that this transition is absent for (NH4)2Mg(SO4)2·6H2O having smaller crystal size obtained by grinding of the crystal aggregates in an agate mortar.