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

The molar enthalpies of dissolution of (NH4)(2)SO4:2MSO(4) and the double s alts (NH4)(2)M-2(SO4)(3) (where M = Mn2+ or Cd2+) in double-distilled water were deter-mined by solution-reaction calorimetry measurements at 298.2 K. From the values obtained together with auxiliary data and by using appropr iate thermochemical cycles, the standard molar enthalpies of formation of ( NH4)(2)M-2(SO4)(3) were hence derived: Delta fH(m)(theta)[(,NH4)(2)Cd-2(SO4)(3) (s)] -3031.74 +/- 0.08 KJ mol(-1) Delta H-f(m)theta[(NH4)(2)Mn-2(SO4)(3) (s)] = -3250.16 +/- 0.09 kJ mol(-1) (C) 2001 Elsevier Science B.V. All rights reserved.