Calculation of the free Gibbs energy of phase transitions using
solubility data. 1. The system Na2SO4-Na2Se04-H2O
at 15 °C: Stable and metastable
Chr. Balarew
Institute of General and Inorganic Chemistry, Bulgarian
Academy of Sciences, Acad. G. Bontchev Str., Bl.11, 1113 Sofia, Bulgaria
Abstract: The solubility isotherms of the systems Na2SO4
·10H2O–Na2SeO4·10H2O–H2O
and Na2SO4·7H2O–Na2SeO4
·7H2O–H2O have been investigated at 15 °C. It
is established that discontinuous series of mixed crystals are formed
in both systems. Two methods of calculation of the free Gibbs energy
of phase transition at the interruption point of the solubility diagrams
are used: (i) on the basis of the composition of the two mixed crystal
types, which are in equilibrium with the saturated solution in the eutonic
point of the system, and (ii) from the distribution coefficients between
each of the mixed crystal phases and their saturated solution, using
both experimentally obtained values and calculated distribution coefficients
for ideal isomorphic mixing. The data found by the two methods exhibit
very good agreement. It is established that the free Gibbs energy of
the phase transition in the case of decahydrates is lower than in the
case of heptahydrates. This fact is considered as a confirmation of
the buffering action of water molecules with respect to the distortion
of the crystal structures of the pure salts provoked by the formation
of mixed crystals.
* Lectures presented at the 10th International Symposium
on Solubility Phenomena, Varna, Bulgaria, 22 -26 July 2002. Other
lectures are published in this issue, pp. 1785-1920.
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