ISOPIESTIC DETERMINATION OF THE OSMOTIC COEFFICIENT OF CONCENTRATED ACIDIC FERRIC SULFATE AQUEOUS SOLUTIONS AT 298.15 AND 323.15 K

The oxidation of sulfide minerals can produce acid brines rich in iron(III) and sulfate, with total concentrations reaching 1 and 4 mol×L^-1, respectively. The relationships between the activities and concentrations of dissolved species in these concentrated, acidic solutions are unknown. The activity coefficients of iron(III), sulfate, and hydrogen ions can be retrieved from the analysis of the osmotic coefficients of concentrated {yH₂SO₄ + (1-y)Fe₂(SO₄)₃}_(aq) solutions, where y is the solute mole fraction of H₂SO₄. These data can then be used to model aqueous solutions in equilibrium with highly soluble iron(III) sulfate minerals. We have measured the osmotic coefficients of concentrated {yH₂SO₄ + (1-y)Fe₂(SO₄)₃}_(aq) solutions using the isopiestic method. The osmotic coefficients of the reference standards were calculated using the models of Archer (J. Phys. Chem. Ref. Data 21: 793-829) for NaCl and Clegg et al. (J. Chem Soc. Faraday Trans. 90: 1875-1894) for H₂SO₄. Measurements have been made for 34 different values of y ranging from 0.74948 to 0.94682 at 298.15 K and for 18 different values of y, over the same range, at 323.15 K. At 298.15 K, the total molal concentration (m_T or [H₂SO₄] + [Fe₂(SO₄)₃]) increased from 0.42221 to 6.71642 mol×kg^-1 and the stoichiometric ionic strength (I_S) from 1.94671 to 25.88862 mol×kg^-1. At 323.15 K, the m_T ranged from 0.8124305 to 7.236699 mol×kg^-1 and the I_S from 3.99018 to 28.40216 mol×kg^-1. The stoichiometric osmotic coefficients (f_S) of the test solutions ranged from 0.47324 to 0.75499 at 298.15 K and from 0.48413 to 0.69933 at 323.15 K. Values of f_S converged as m_T increased and the solutions became increasingly saturated with respect to ferric sulfate phases. This behavior is to be expected: the solute-solvent interactions of the test solutions become similar as the concentrations increase. The data provided by this study can be modeled using Pitzer’s equations to obtain a general expression of the {yH₂SO₄ + (1-y)Fe₂(SO₄)₃}_(aq) solution osmotic coefficient as a function of the iron(III), sulfate, and hydrogen ion concentrations. An equation for the activity coefficient for each one of the ionic species in this system can be developed during the fitting and modeling process.