Mean Activity Coefficients and Phase Equilibrium Prediction in the Ternary System KBr-K2SO4-H2O at 278.15 K

Potassium and bromine are important resources in seawater and underground brines; consequently, the studies of thermodynamic properties and phase equilibria of salt-water systems containing potassium and bromide are of great significance for the development of potassium and bromine resources. The mean activity coefficients of the ternary system KBr-K2SO4-H2O at 278.15 K were studied by the cell potential method in this work. The cell potentials in KBr-H2O solutions were measured by ion-selective electrodes, and the electrode response slope kappa and the standard cell potential E-0 were fitted by linear regression. The results show that the ion-selective electrodes have a good Nernst response, which can be used for the determination of cell potentials in the mixed salt solutions. The mean activity coefficients of KBr in the mixed salt solutions KBr-K2SO4-H2O were measured by the cell potential method. The total ionic strengths of the mixed salt solutions in the system were I = 0.0100 - 1.0000 mol kg(-1), and the ionic strength fractions of K2SO4 were y(b) = 0.0, 0.2, 0.4, 0.6, and 0.8. According to the cell potentials determined by the experiments and the Nernst equation, the mean activity coefficients of KBr in the mixed salt solutions were calculated, and the mixed ion interaction parameters theta(Br,SO4) and Psi(K,Br,SO4) in the Pitzer equation were fitted by the regression program. Furthermore, the correlation equation between temperature and the mixed ion interaction parameter is established. In addition, the mean activity coefficients of K2SO4, osmotic coefficient Phi, water activity a(w), and excess Gibbs free energy G(E) were also calculated by using the Pitzer equation and mixed ion interaction parameters. Using the fitted Pitzer mixed ion interaction parameters, the solubilities of salts in the ternary system were calculated according to the Pitzer model, and the phase diagram of the corresponding ternary system was drawn and compared. The results show that the mixed ion interaction parameters of the Pitzer equation obtained in this work can be used to calculate the thermodynamic parameters and predict the phase equilibria. The data of thermodynamic properties obtained in this work are of interest to the separation and extraction of potassium and bromine resources from seawater in chemical processes.