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