Distributions of Hydrochloric Acid between Water and Organic Solutions of Tri-n-octylphosphine Oxide: Thermodynamic Modeling

Trin-octylphosphine oxide (TOPO) is a widely used extractant because of its high extractive ability. However, there is no systematic research on the thermodynamics of TOPO/n-dodecane in the separation of hydrochloric acid (HCl) from aqueous solution. In this study, the liquid–liquid equilibrium (LLE) system (water + n-dodecane + TOPO + HCl) was investigated. Both the equimolar series and slope methods were used to determine the composition of the complex formed in the equilibrated organic phase. The form of the water molecules in the equilibrated organic phase was first investigated by the thermodynamic method. The thermodynamic model was established with the Pitzer equation for aqueous phase and both Margules and organic Pitzer equations for the organic phase. Two chemical equilibrium constants and their corresponding interaction parameters were regressed from experimental LLE data. The correlated results were in good agreement with the experimental data. Furthermore, this model can also be used to predict the organic phase composition for this system. This confirmed that the thermodynamic model chosen was suitable for the extraction system.