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

Joseph A. Rard, Simon L. Clegg

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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 mT = (0.29999, 0.32948, and 0.36485) mol·kg-1 to mT = (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 ≥ aw ≥ 0.7469, 0.9924 ≥ aw ≥ 0.7983, and 0.9924 ≥ aw ≥ 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 mT ≤ 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 mT ≤ 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.

Original languageEnglish
Pages (from-to)399-429
Number of pages31
JournalJournal of Chemical Thermodynamics
Issue number3
Publication statusPublished - Mar 1999


  • Aqueous solutions
  • Isopiestic measurements
  • Magnesium sulfate
  • Pitzer's model
  • Sulfuric acid

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