Water activity in the ternary system LiCl–SrCl2–H2O and its sub-binary systems has been
elaborately
measured by the isopiestic method. The measured water activities were
used to justify the reliability of solubility isotherms reported in
literature by correlating them with two thermodynamic models, that
is, the extended Pitzer model and the Pitzer–Simonson–Clegg
model. It was found that the extended Pitzer model cannot correlate
consistently the water activities measured and either set of the solubility
isotherms reported in literature for this concerned system, no matter
how its parameters were adjusted. However, the Pitzer–Simonson–Clegg
model can correlate consistently our measured water activities and
the solubility isotherms reported by the literature (Kydynov et al. Issled. Obl. Khim. Tekhnol. Miner. Solei Okislov
1965, 146–150), which should be more reliable than solubility
data reported in other references.
Utilizing improvements in experimental equipment, analytical methods and the initial material, the solubility isotherms of the ternary system MgCl 2 −MgSO 4 −H 2 O were determined in detail at T = (323.15 and 348.15) K using an isothermal method of solid−liquid equilibrium. The results indicate that the solid phases MgSO 4 •nH 2 O (s) (n = 6, 1) and MgCl 2 •6H 2 O (s) are stable and MgSO 4 •nH 2 O (s) (n = 5, 4) are metastable at 323.15 K, which contradicts the results of a previous experimental study 1 in which the phase MgSO 4 •4H 2 O (s) was reported as stable. The liquidus of the four solid phases MgSO 4 •nH 2 O (s) (n = 6, 4, 1) and MgCl 2 •6H 2 O (s) were measured at 348.15 K in this work. The findings indicate that the phases MgSO 4 •H 2 O (s) and MgCl 2 • 6H 2 O (s) are stable and MgSO 4 •6H 2 O (s) and MgSO 4 •4H 2 O (s) are metastable at 348.15 K. Remarkable differences between this work and the literature solubility data for the phase MgSO 4 •H 2 O (s) at 348.15 K are observed. A Pitzer−Simonson−Clegg thermodynamic model was chosen to simulate the properties of the binary systems and to correlate the solubility isotherms of the ternary system at 298.15 K in our previous study and 323.15 K and 348.15 K in this work. Good agreement has been found between the calculated and experimental results. Applying the model parameters and solubility isotherms in the ternary system measured in this work, we obtained the solubility product parameters ln K and chemical potentials for the solid phases MgSO 4 •nH 2 O (s) (n = 6, 5, 4, 1) over a wider temperature range than those for the binary system MgSO 4 −H 2 O.
Solubility isotherms of the ternary system (LiCl + NH4Cl + H2O) were elaborately determined at T = (273.15, 298.15, and 323.15) K by an isothermal method. In the equilibrium phase diagram, there are two solubility branches at 273.15 K, corresponding to the solid phase LiCl·2H2O and NH4Cl. The invariant point composition at 273.15 K is w = 0.401 for LiCl, w = 0.024 for NH4Cl, and w = 0.575 for H2O. However, there are three solubility branches at (298.15 and 323.15) K, corresponding to the solid phase LiCl·H2O, NH4Cl, and a new found solid solution phase (NH4Cl)
x
(LiCl·H2O)1−x
. A Pitzer−Simonson−Clegg thermodynamic model was selected to represent the thermodynamic properties of this system. Thermodynamic consistence between our solubility data and water activity from other research groups shows that these concerned experimental data are reliable.
Water
activity for binary KCl–H2O, NaCl–H2O and MgCl2–H2O, as well as two
ternary NaCl–MgCl2–H2O and KCl–MgCl2–H2O, systems has been measured by using
an isopiestic method at 323.15 K. The isopiestic results obtained
show that the isopiestic composition lines of the NaCl–MgCl2–H2O system was found to obey the Zdanovskii
rule, whereas the KCl–MgCl2–H2O system was observed to deviate slightly. The experimental water
activities determined were applied to regress the parameters of the
Pitzer model with a good agreement. The model with new parameters
is validated by comparing water activity predictions with those given
in the literature and not used in the parametrization process and
calculating the solubility of the NaCl–MgCl2–H2O and KCl–MgCl2–H2O systems
at various temperatures with the comparison of literature values.
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