Solubility of jarosite at 4-35 degrees C

The solubility of jarosite (KFe3(SO4)(2)(OH)(6)) was studied in a series of dissolution experiments. The experiments were conducted at 4-35 degrees C and at pH values between 1.5 and 3.0 using a synthetic jarosite with a composition very close to ideal. The solids were left in the reaction vessel for up to 6 months. Equilibrium was established in the experiments after approximately 3 to 4 months. The log K-sp for the jarosite dissolution reaction KFe3(SO4)(2)(OH)(6) + 6H(+) approximate to K+ + 3Fe(3+) + 2SO(4)(2-) + 6H(2)O at 25 degrees C is determined to be -11.0 +/- 0.3. From this measured solubility product, the free energy of formation, Delta G(f,298)degrees is calculated to be -3309.8 +/- 1.7 kT mol(-1). Based on the temperature dependence of the solubity product, the enthalpy of reaction at 25 degrees C, Delta H-r,298(o), is -45 +/- 5 kJ mol(-1), the entropy of reaction, Delta S-r,298(o), is -350 +/- 40 J mol(-1) K-1, and the heat capacity of the reaction, Delta C-pr, over the temperature range of the experiments is determined to be -2.1 +/- 0.2 kJ mol(-1) K-1. The rate of the dissolution reaction can be described by a first-order model.