Isopiestic determination of the osmotic and activity coefficients of {zH₂SO₄ + (1 - Z)MgSO₄} (aq) at T = 298.15 K. II. Results for z = (0.43040, 0.28758, and 0.14399) and analysis with Pitzer's model

Isopiestic vapor pressure measurements were made for {zH₂SO₄ + (1 - z)MgSO₄}(aq) solutions with H₂SO₄ molality fractions of z = (0.43040, 0.28758, and 0.14399) at the temperature 298.15 K. Measurements extend from total molalities of m_T = (0.29999, 0.32948, and 0.36485) mol·kg^-1 to m_T = (4.8474₀, 4.2843₃, and 3.9620₆)mol·kg^-1, respectively. These highest molalities correspond to the greatest degrees of supersaturation that could be achieved during isothermal removal of solvent. The corresponding water activity ranges are 0.9924 ≥ a_w ≥ 0.7469, 0.9924 ≥ a_w ≥ 0.7983, and 0.9924 ≥ a_w ≥ 0.8291 for z = (0.43040, 0.28758, and 0.14399), respectively. Isopiestic results for this system with z = (0.85811, 0.71539, and 0.57353) were reported previously (J. Chem. Thermodynamics 1997, 29, 533-555). The combined results to m_T ≤ 3.5 mol·kg^-1 are represented essentially to experimental accuracy with Pitzer's model, using ionic strength-dependent third virial coefficients for the constituent binary solutions. A second set of parameters is also presented for the more extended molality range of m_T ≤ 6.0 mol·kg^-1, where this model gives a slightly less accurate representation of the experimental information. The nature and extent of ionic association in MgSO₄(aq) is discussed, but it was not possible to select a value of the association constant owing to large discrepancies among the various studies.