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

Abstract

Calorimetric measurements have been performed to determine the heat of dissolution of polyhalite K 2SO 4 · MgSO 4 · 2CaSO 4 · 2H 2O and its analogues K 2SO 4 · MSO 4 · 2CaSO 4 · 2H 2O (M = Mn, Co, Ni, Cu, and Zn) at T = 298.15 K. The dissolution experiments were carried out in NaClO 4 solution with varying concentrations (0.5 to 2.0) mol kg −1. All polyhalites dissolve exothermically. Exothermicity increases with concentration of NaClO 4. 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 K 2MgCa 2(SO 4) 4 · 2H 2O with M = Mg, Mn, Ni, and Zn coincide. The value for the cobalt salt is noticeably less exothermic. Dissolution enthalpy of leightonite K 2CuCa 2(SO 4) 4 · 2H 2O, which does not crystallize in the polyhalite structure, deviates considerably within the series.