Heat of solution of polyhalite and its analogues at T = 298.15 K

Calorimetric measurements have been performed to determine the heat of dissolution of polyhalite K2SO4 · MgSO4 · 2CaSO4 · 2H2O and its analogues K2SO4 · MSO4 · 2CaSO4 · 2H2O (M = Mn, Co, Ni, Cu, and Zn) at T = 298.15 K. The dissolution experiments were carried out in NaClO4 solution with varying concentrations (0.5 to 2.0) mol kg−1. All polyhalites dissolve exothermically. Exothermicity increases with concentration of NaClO4. An extrapolation to infinite dilution was done using the SIT model.Within the limits of experimental uncertainty, the enthalpies of dissolution for the triple salts K2MgCa2(SO4)4 · 2H2O with M = Mg, Mn, Ni, and Zn coincide. The value for the cobalt salt is noticeably less exothermic. Dissolution enthalpy of leightonite K2CuCa2(SO4)4 · 2H2O, which does not crystallize in the polyhalite structure, deviates considerably within the series.