采用等温溶解平衡法测定N,N′-二(2-羟丙基)哌嗪(HPP)?Na2SO4?H2O三元体系在273.15和298.15 K下的相平衡数据，采用湿渣法测定其平衡固相数据，绘制等温相图。用改进的单组分电解质Pitzer方程计算该体系中Na2SO4和Na2SO4?10H2O的溶解平衡常数，并对相平衡数据进行理论计算。结果表明，273.15 K时存在3个结晶区，298.15 K时存在4个结晶区。HPP的存在降低了Na2SO4和Na2SO4?10H2O的相转变温度，使298.15 K下的相图中存在Na2SO4的结晶区域，且273.15和298.15 K的相图中不存在纯HPP的结晶区域。理论计算与实验数据的均方根偏差不高于0.0290，表明相平衡数据计算值与实验值较吻合，证实了改进的单组分电解质Pitzer方程适用于该体系计算。
The solid?liquid equilibrium data of N,N′-bis-(2-hydroxypropyl)-piperazine (HPP)?sodium sulphate?water ternary system were determined by isothermal method at 273.15 and 298.15 K under ambient pressure by wet-residue method. The components of invariant points were determined by XRD and FT-IR analysis. Two isothermal phase diagrams were plotted according to the equilibrium data. There were four crystalline regions in the course of crystallization (pure Na2SO4?10H2O, pure Na2SO4, mixed Na2SO4?10H2O and Na2SO4, mixed Na2SO4 and HPP, respectively) at 298.15 K. Meanwhile, there were three crystallization regions (pure Na2SO4?10H2O, mixed Na2SO4?10H2O and HPP, mixed Na2SO4?10H2O, Na2SO4, HPP, respectively) at 273.15 K. With the decrease of temperature, the crystallization regions of pure Na2SO4?10H2O increased and the solubility of sodium sulfate decreased obviously. The presence of HPP decreased the phase transition temperature between Na2SO4 and Na2SO4?10H2O, resulting in the existence of pure Na2SO4 crystalline region at 298.15 K. But there was not crystallization zone of pure Na2SO4?10H2O at 273.15 K. Moreover, the existence of pure HPP crystalline region had not been found at 273.15 and 298.15 K, that meant if the concentration of HPP was lower than the solubility of HPP in water, HPP would not crystallize in the frozen crystallization process. Furthermore, the modified Pitzer model of single component electrolyte was used to correlate the phase equilibrium data of the system. By means of the solubility data, the Pitzer parameters and solution equilibrium constants of Na2SO4 and Na2SO4?10H2O in the system were calculated by multiple linear regression. Using the Pitzer model to calculate the theoretical data of phase equilibrium, the relative root mean square deviation between the calculated data and the experimental data was no more than 0.0290, which proved that the modified single component electrolyte Pitzer equation can be applied to the calculation of this system.
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