# 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 {*y*H_{2}SO_{4} + (1-*y*)Fe_{2}(SO_{4})_{3}}_{(aq)} solutions, where *y* is the
solute mole fraction of H_{2}SO_{4}. 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 {*y*H_{2}SO_{4} + (1-*y*)Fe_{2}(SO_{4})_{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 H_{2}SO_{4}.
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 [H_{2}SO_{4}] + [Fe_{2}(SO_{4})_{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 {*y*H_{2}SO_{4} + (1-*y*)Fe_{2}(SO_{4})_{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.