Determination of the eutectic solubility lines of the ternary system NaHCO₃-Na₂CO₃-H₂O

A crystallizer was built and a procedure developed to accurately measure the eutectic solubility lines where ice and salt coexist in equilibrium with the solution, for potential application of Eutectic Freeze Crystallization. The eutectic solubility lines of the ternary system NaHCO₃-Na₂CO₃-H₂O were determined experimentally and calculated with the extended UNIQUAC model. The extended UNIQUAC model describes the experimental data quite well. Anhydrous NaHCO₃ and Na₂CO₃·10H₂O were the only two types of crystals present in equilibrium with ice crystals in the ternary system. At the quadruple point NaHCO₃ and Na₂CO₃·10H₂O are in equilibrium with a solution of about 4.34wt% of Na₂CO₃ and 4.77wt% of NaHCO₃ at -3.32°C. The anhydrous NaHCO₃ crystals were needle shaped with lengths between 5 and 10μm, that were agglomerated into particles of about 100-300μm, while the Na₂CO₃·10H₂O crystals were hexagonally shaped with sizes between 100 and 500μm.