Activity coefficients and phase equilibria of NaCl–NaI–H2O at 288.15 K

• The activity coefficient of sodium iodide under low concentration in the coexistence of chlorine and iodine was determined, clarifying the patterns of its thermodynamic property changes. • Model parameters were fitted based on the experimentally obtained activity coefficient data, relevant properties were calculated, and the non-ideality of these properties was described. • The stable equilibrium phase diagram of the NaCl-NaI-H 2 O system at 288.15 K was studied and determined using the isothermal dissolution equilibrium method, elucidating the phase equilibrium rules of the system. • A new set of model parameters was obtained based on a wide range of concentrations and various types of thermodynamic data (activity coefficient data at low concentrations and solubility data at high concentrations). These parameters were used to predict solubility, and the predicted results showed good agreement with the experimental values. The Nanyishan region of the Qaidam Basin is widely known for its extensive oilfield brine resources and substantial reserves. It contains not only sodium, potassium, calcium, and boron but also low concentrations of elements such as Li, Rb, Cs, Br, and I, which makes it exceptionally valuable for thorough development and application. The research on the thermodynamical properties of the coexisting NaCl and NaI in solution and their phase equilibrium relationships is essential. In this work, the mean activity coefficients of NaI in the NaCl–NaI–H 2 O mixed solution at 288.15 K were measured using the cell potential method. Additionally, the thermodynamic properties of this mixed solution were calculated using the Pitzer model. The phase equilibria of the ternary system at 288.15 K were determined using the isothermal dissolution equilibrium method. Finally, the solubility prediction of the NaCl–NaI–H 2 O system was carried out with obtained ion interaction parameters, and the calculated results were in better agreement with the experimental values. This demonstrates the reliability of activity coefficient and solubility data, and these key thermodynamic data can provide fundamental support for comprehensive utilization technology of liquid mineral resources.