Determination and graphics expression of ice-salt eutectic points of the multicomponent electrolyte solutions

The eutectic point data of the ternary system Na+//Cl−,SO42−−H2O, the quaternary system Na+//Cl−,SO42−,NO3−−H2O, and the quinary system Na+,K+//Cl−,SO42−,NO3−−H2O were studied by cooling curves method. Based on the experimental data, the eutectic temperature-solution composition diagrams and projection diagrams were constructed. At the eutectic temperatures, these systems were found to belong to simple types without the formation of the double salts. For the ternary system Na+//Cl−,SO42−−H2O, the eutectic temperature-solution composition diagram could be plotted in the form of a trigonal prism, which exhibited the existence of three eutectic curves of single salt and ice or two salts, and one eutectic point of two salts and ice. For the quaternary homo-ion system Na+//Cl−,SO42−,NO3−−H2O, the eutectic temperature-solution composition diagram could be drawn as a trigonal prism, which showed three eutectic regions of single salt and ice, three eutectic curves of two salts and ice, and one eutectic point of three salts and ice. Furthermore, for the quinary system Na+,K+//Cl−,SO42−,NO3−−H2O saturated with NaCl·2H2O, the relationship between the eutectic temperature and the solution composition could be described based on the solvent-less and Cl-less diagram. This diagram exhibited the presence of four eutectic regions of two salts and ice, five eutectic curves of three salts and ice, and two eutectic points of four salts and ice, and the eutectic temperature was found to be −27.45, −24.75 °C, respectively. The eutectic region of ice-NaCl·2H2O-NaNO3 was the smallest; however, that of the ice-NaCl·2H2O-Na2SO4·10H2O was the largest. The results of this study can offer fundamental data to promote the research and application of crystallization processes at low temperature and further theoretical exploration.