Solid-Liquid Phase Equilibrium in the Na2CO3+Na2SO4+H2O System at 298.15 K and 101 kPa by Isothermal Titration Microcalorimetry

Phase diagrams of aqueous saline systems constitute the cornerstone of the brine purification industry. Although ternary phase diagrams may be studied using many experimental methods, the developed isothermal titration micro-calorimetry (ITC) approach is simple, generic, and can be extended to the investigation of phase diagrams of multi-salt aqueous solutions and other related systems. In this study, the phase equilibrium of one complex ternary system, Na2CO3+Na2SO4+H2O, with several hydrated salts and a double salt, was determined using ITC with the aid of X-ray dif-fraction, differential scanning calorimetry, and thermogravimetry. The isobaric and isothermal titration methods can not only determine the boundaries of different phase regions by changes in the slope of the heat evolved vs. time plot or in the slope of the observed heat vs. solvent concentration plot, but also provide additional information regarding the hy-dration heat of ions and hydrates, dissolution heat, and dilution heat. The use of ITC can be extended to complex systems after optimization of data processing.