Zhang Qian, Li Mingyuan, Kong Jing, Chen Yanrong, Liu Jie
School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, Shandong 252059, P. R. China.
ACS Omega. 2022 Jul 13;7(29):25811-25821. doi: 10.1021/acsomega.2c03339. eCollection 2022 Jul 26.
Isothermal titration microcalorimetry (ITC) is a common and powerful tool in thermodynamics and related fields, and the connection between the solution behavior and raw titration curve is complex, important, and worth studying, so we try to discuss this problem using a complex solid-solid-liquid diagram. Although there are many experimental methods for studying ternary phase diagrams, ITC has been proven to be a simple, universal method at normal temperature and pressure. This method can be extended to investigate phase diagrams of multisalt aqueous solutions and related systems. Here, the phase equilibrium of one common ternary system NaCl + NaSO + HO) was determined using the ITC method with the aid of X-ray diffraction measurements. This isothermal and isobaric titration method can be used not only to determine the boundaries of different phase regions by analyzing changes in the slope of the observed heat vs solvent concentration plot but also to provide additional accurate data regarding the solid dissolution enthalpy and the dilution enthalpy. The dissolution enthalpy of 1 mol NaCl (NaSO) solid dissolving in water to just form a saturated solution is measured at 1.92 kJ/mol (-15.60 kJ/mol), and the dissolution enthalpy of the mixed solids can be evaluated with these data. The dilution enthalpy is strongly dependent on the solute concentration (0.08-0.23 kJ/mol at the range of concentration studied), and for the same range concentration of a solute, the dilution enthalpy of NaSO is positive and larger than that of NaCl. Therefore, the connection between the observed heat and the microscopic behavior and the separation between the dissolution and dilution data may result in the possibility to gain a clearer understanding about the solution behaviors.
等温滴定量热法（ITC）是热力学及相关领域中一种常用且强大的工具，溶液行为与原始滴定曲线之间的联系复杂、重要且值得研究，因此我们尝试用复杂的固 - 固 - 液相图来探讨这个问题。尽管有许多研究三元相图的实验方法，但ITC已被证明是一种在常温常压下简单通用的方法。该方法可扩展用于研究多盐水溶液及相关体系的相图。在此，借助X射线衍射测量，使用ITC方法测定了一个常见三元体系NaCl + Na₂SO₄ + H₂O的相平衡。这种等温等压滴定法不仅可通过分析观察到的热与溶剂浓度曲线斜率的变化来确定不同相区的边界，还能提供有关固体溶解焓和稀释焓的额外准确数据。测得1 mol NaCl（Na₂SO₄）固体溶解于水形成饱和溶液时的溶解焓为1.92 kJ/mol（-15.60 kJ/mol），利用这些数据可评估混合固体的溶解焓。稀释焓强烈依赖于溶质浓度（在所研究的浓度范围内为0.08 - 0.23 kJ/mol），对于相同溶质浓度范围，Na₂SO₄的稀释焓为正值且大于NaCl的稀释焓。因此，观察到的热与微观行为之间的联系以及溶解和稀释数据的区分可能有助于更清晰地理解溶液行为。
