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Termodynamika roztworów nasyconych a rozpuszczalność

Buchowski H.

Wiadomości Chemiczne

2000

[Z] 54, 9-10

889-913

The thermodynamic information given by solubility is very poor as the activity coefficient of the solute A can be calculated only. To obtain more complete information a method of precise measurements of vapour pressure over saturated solutios has been elaborated. The activity of the solvent B can be derived from vapour pressure data and the excess Gibbs' energy along the liquids can be determined with no adjustable parameters [10] as shown in Fig. 4. Theoretical equation for the activity of solvent in saturated solutions as well as for the solubility have been deduced. The activity and the solubility depend on two constants l and b. The methods of determination of the constants from vapour pressure and solubility data are given. The knowledge of l and b permits the calcuation of excess functions of mixing in saturated solutions by Eqs. The analysis of excess functions gives evidence of the existence of a short-range order, i.e. a correlation of molecular orientations in the liquid A. The destruction of the correlation by mixing with the an inert solvent involves positive contributions to the excess enthalpy as well as to the excess entropy. The effect decreases the solubility. Sometimes both contributions cancel out and so called ideal solubility is observed. But the saturated solutions are pseudoideal only: excess Gibbs' energy is very small but excess enthalpy and excess entropy can be large. The thermodynamic properties of systems conforming to Schröder or Malesiński ideal solubility equations are discussed in detail. The test of thermodynamic cosistency of solubility data is formulated.