Effect of solvent and temperature on the phase equilibrium behavior of hydroxylamine sulfate in water-ethanol mixed solvents: Solubility and ternary liquid-liquid equilibrium diagram with liquid-liquid equilibrium

The thermodynamic properties of hydroxylamine sulfate (HAS) in binary mixed solvents of water-ethanol were investigated in this work. Experimental results indicated that a temperature above 323.15 K will bring about the occurrence of liquid-liquid phase separation in the water-ethanol-HAS system. Solubility of HAS in water and ethanol-water binary mixed solvents was experimentally determined from 283.15 to 323.15 K under atmospheric pressure (101.3 kPa) by employing a gravimetric method. As the mass fraction of water in the mixed solvent increase, there is an increase in the solubility of HAS. In addition, the solubility of HAS is positively dependent on the temperature. Furthermore, the modified van't Hoff model, Apelblat model, van't Hoff - Jouyban - Acree model, and Apelblat - Jouyban - Acree model were used to correlate the experimental solubility of HAS, and the Apelblat model showed the best agreement. Besides, the van't Hoff equation was utilized to evaluate the thermodynamic parameters of HAS in binary solvents, calculation results confirm that these dissolving processes are endothermic and entropy-driving. Ternary phase diagrams with liquid-liquid equilibrium at 324.15, 328.15 and 333.15 K were constructed, and the boundaries of each phase area of the ternary phase diagram were verified at 333.15 K. As for the water-ethanol-HAS system, the temperature has an influence on the phase zone of the ternary phase diagram by affecting solid-liquid phase equilibrium. However, the temperature has no effect on the liquid-liquid phase equilibrium. The nucleation process of oiling-out crystallization of HAS was determined to follow a two-step mechanism: the liquid-liquid phase separation is generated at first, and then nucleation occurred and crystals subsequently grow in the aqueous phase. (C) 2022 Published by Elsevier B.V.

Automated summary

This study investigates the thermodynamic properties of hydroxylamine sulfate (HAS) in binary mixed solvents of water-ethanol. The experimental results show that liquid-liquid phase separation occurs within a certain temperature range and the solubility of HAS is influenced by the mass fraction of water and temperature in the mixed solvent. Various models were used to correlate the experimental solubility, and the Apelblat model showed the best agreement. Ternary phase diagrams were constructed to study the phase equilibrium, and the influence of temperature on the phase zone was observed in the water-ethanol-HAS system. The nucleation process of oiling-out crystallization of HAS follows a two-step mechanism.