The Gibbs-Helmholtz (G-H) equation connects vapor-liquid equilibrium (VLE) and calorimetric data. If experimental measurements for the heat of solution are not available, they may be estimated through the G-H equation. If both VLE and heat-of-solution data are available, their mutual thermodynamic consistency can be evaluated through the G-H equation, to develop the most accurate and reliable model. This analysis can be particularly useful for chemical-absorption systems such as the capture of CO2 using aqueous solutions of MEA and ammonia where regeneration energies are of significance.
In this work, the G-H equation is first re-derived to unambiguously establish its rigorous and general form for systems where phase equilibrium is accompanied by chemical reactions in the liquid. Next, uncertainties of heats of solution estimated by the G-H equation from VLE data are analyzed and estimated. Finally, the G-H equation is used to evaluate the consistency between VLE and calorimetric data for two systems (CO2 absorption in aqueous solutions of MEA and in ammonia). These cases suggest that the G-H equation provides a more powerful data-analysis technique than may have been previously recognized.