Thermodynamic modelling of aqueous Fe(II) sulfate solutions

The H2O–FeSO4 system has been modelled using the Pitzer activity coefficient approach over a temperature interval of −2–220 °C and in concentrations from pure water to the solubility limit of ferrous sulfate hydrates, the maximum FeSO4 molality being 3.58 mol/kg at the FeSO4⋅7H2O(s) to FeSO4⋅H2O(s) transition temperature of 56.5 °C. The thermodynamic properties of FeSO4 hydrates were refined and FeSO4⋅4H2O(s) was found to be a metastable phase in the whole temperature range. The model in this work presents all the experimental data available, including the solubilities, mean activity coefficients, activities of water, enthalpy and heat capacity of solution, and hydrate dissociation pressure, with good accuracy and consistently up to 220 °C, but the model has limitations at temperatures higher than 100 °C due to lack of experimental data.