Mean Activity Coefficients and Phase Diagram in NaBr-SrBr2-H2O Solutions at 308.2 K

The studies of mean activity coefficients of the NaBr-SrBr2H2O solutions are of great significance for the development of underground brines in the Sichuan Basin of China. In this article, the mean activity coefficients of NaBr in the NaBr-SrBr2-H2O solutions have been studied by the cell potential method. The range of the determined ionic strength fraction yb of SrBr2 is 0.0, 0.2, 0.4, 0.6, and 0.8, and the total ionic strength I ranges from 0.0100 to 1.0000 mol center dot kg-1. First, the cell potentials of NaBr in NaBrH2O solutions and NaBr-SrBr2-H2O solutions are measured by composite ion-selective electrodes. Then, the mean activity coefficients of NaBr in mixed solutions were calculated by using the measured cell potential data and the Nernst equation. Consequently, the mixed ion interaction parameters.Na+,Sr2+ and fNa+,Sr2+,Br- of the Pitzer model at 308.2 K were fitted by a multiple linear regression method. Finally, the above parameters and the Pitzer model were used to calculate the mean activity coefficients of SrBr2 and relevant thermodynamic properties (osmotic coefficient, water activity aw, and excess Gibbs free energy GE) in NaBr-SrBr2-H2O solutions. In addition, the solubilities of NaBr and SrBr2 in mixed solutions were predicted using the Pitzer model. The predicted theoretical values are in good agreement with the experimental values. The fitted mixed ion interaction parameters have good applicability for solubility prediction.