Thermochemical studies on the langbeinite-type double-sulfate salts, (NH4)2M2(SO4)3

The molar enthalpies of dissolution of (NH4)2SO4:2MSO4 and the double salts (NH4)2M2(SO4)3 (where M=Mn2+ or Cd2+) in double-distilled water were determined by solution–reaction calorimetry measurements at 298.2 K. From the values obtained together with auxiliary data and by using appropriate thermochemical cycles, the standard molar enthalpies of formation of (NH4)2M2(SO4)3 were hence derived: ΔfHmθ[(NH4)2Cd2(SO4)3(s)]=−3031.74±0.08 kJ mol−1ΔfHmθ[(NH4)2Mn2(SO4)3(s)]=−3250.16±0.09 kJ mol−1