Thermodynamic properties and phase transitions of salt hydrates between 270 and 400 K I. NH4Al(SO4)2 · 12H2O, KAl(SO4)2 · 12H2O, Al2(SO4)3 · 17H2O, ZnSO4 · 7H2O, Na2SO4 · 10H2O, and Na2S2O3 · 5H2O

Heat capacities and phase-transition enthalpies have been measured for five sulphate hydrates and sodium thiosulphate pentahydrate by adiabatic-shield calorimetry between 270 and 400 K. Enthalpies and entropies have been derived for the exact stoichiometries and tabulated for selected temperatures. The enthalpy of fusion of NH4Al(SO4)2 · 12H2O at 367.13 K is (122.0 ± 0.9) kJ · mol−1. The enthalpy of transition of KAl(SO4)2 · 12H2O to KAl(SO4)2 · 3H2O plus aqueous solution at 358.99 K is (95.2 ± 1.2) kJ · mol−1. Including the enthalpy of dissolution of 3-hydrate, the total enthalpy of transition of KAl(SO4)2 · 12H2O to an aqueous solution in the range 358.99 to 386 K is (112.7 ± 1.5) kJ · mol−1. Al2(SO4)3 · 17H2O melted in the range 350 to 381 K with enthalpy of fusion (120.5 ± 1.0) kJ · mol−1. The gradual onset of the phase transition is probably due to release of zeolitically bound water. The enthalpy of transition of ZnSO4 · 7H2O to ZnSO4 · 6H2O plus aqueous solution at 311.3 K is (16.5 ± 0.5) kJ · mol−1. The temperature of the peritectic transition of Na2SO4 · 10H2O to Na2SO4 plus aqueous solution is (305.533 ± 0.002) K, and the phase-transition enthalpy (78.04 ± 0.3) kJ · mol−1. The enthalpy of transition of Na2S2O3 · 5H2O to Na2S2O3 · 2H2O plus aqueous solution at 321.31 K is (48.8 ± 0.6) kJ · mol−1. Including the enthalpy of dissolution of 2-hydrate, the total transition enthalpy of Na2S2O3 · 5H2O to an aqueous solution in the range 321 to 335 K is (51.8 ± 0.4) kJ · mol−1. Sulphate and thiosulphate hydrates may be of interest as energy-storage materials because of their high enthalpies of fusion, if supercooling and segregation of unwanted solid phases can be prevented.