Determination of the activity of a molecular solute in saturated solution

Fredrik L. Nordström, Åke C. Rasmuson

Research output: Contribution to journalArticlepeer-review

Abstract

Prediction of the solubility of a solid molecular compound in a solvent, as well as, estimation of the solution activity coefficient from experimental solubility data both require estimation of the activity of the solute in the saturated solution. The activity of the solute in the saturated solution is often defined using the pure melt at the same temperature as the thermodynamic reference. In chemical engineering literature also the activity of the solid is usually defined on the same reference state. However, far below the melting temperature, the properties of this reference state cannot be determined experimentally, and different simplifications and approximations are normally adopted. In the present work, a novel method is presented to determine the activity of the solute in the saturated solution (=ideal solubility) and the heat capacity difference between the pure supercooled melt and solid. The approach is based on rigorous thermodynamics, using standard experimental thermodynamic data at the melting temperature of the pure compound and solubility measurements in different solvents at various temperatures. The method is illustrated using data for ortho-, meta-, and para-hydroxybenzoic acid, salicylamide and paracetamol. The results show that complete neglect of the heat capacity terms may lead to estimations of the activity that are incorrect by a factor of 12. Other commonly used simplifications may lead to estimations that are only one-third of the correct value.

Original languageEnglish
Pages (from-to)1684-1692
Number of pages9
JournalJournal of Chemical Thermodynamics
Volume40
Issue number12
DOIs
Publication statusPublished - Dec 2008
Externally publishedYes

Keywords

  • Activity of the solute
  • Heat capacity
  • Heat of fusion
  • Ideal solubility
  • Solid-state activity
  • Supercooled liquid heat capacity

Fingerprint

Dive into the research topics of 'Determination of the activity of a molecular solute in saturated solution'. Together they form a unique fingerprint.

Cite this