Ternary System RbBr-Rb₂SO₄/NaBr-H₂O And CsBr-Cs₂SO< Sub>4/NaBr-H₂O Phase Equilibrium And Thermodynamic Properties

Brine is a very important treasure house of inorganic salts in our country.However,due to the various and complex components of natural brine,the development and utilization of brine resources are greatly restricted.Therefore,it is very important to study the phase equilibrium and thermodynamic properties of the components in brine.In this paper,four ternary systems of RbBr-Rb2SO4-H2O,CsBr-Cs2SO4-H2O,RbBrNaBr-H2O and CsBr-NaBr-H2O are selected as the research systems,the solid-liquid equilibrium and thermodynamic properties of the systems are studied.The solid-liquid equilibrium of the ternary systems RbBr-Rb2SO4-H2O,CsBrCs2SO4-H2O,RbBr-NaBr-H2O and CsBr-NaBr-H2O at 288.15 K,298.15 K and 308.15 K were studied by isothermal solution method.The density and refractive index of the saturated solution were also measured.The wet slag method and X-ray powder diffraction method were used to analyze the solid phase composition,which shows that there was no solid solution or double salt formation in the four systems at the experimental temperature.The effects of temperature and ion species on the crystallization region is compared separately,and the reasons for the changes in the crystallization region are explained from different perspectives.The change law of density and refractive index was discussed by drawing physical and chemical properties-composition diagrams.The empirical equation is used to fit the data of physical and chemical properties,and the results show that the calculated value is in agreement with the experimental value within the experimental temperature range.Therefore,the physical and chemical properties of other compositions at the experimental temperature can be calculated using this formula.The electromotive force method was used to study the thermodynamic properties of the ternary systems at 298.15 K.The ion-selective electrode pair constitutes a reversible cell:(a)Rb/Cs-ISE | A(mA0)| Br-ISE(b)Rb/Cs-ISE | A(mB0)| Br-ISE(c)Rb/Cs-ISE |A(mA),B(mB)| Br-ISECell(a)was used to calibrate the ion selective electrode Rb/Cs-ISE and Br-ISE to ensure that the ion selective electrode has a good response to the selected ion.Cell(b)was used to measure the selectivity coefficient of Rb/Cs-ISE to Na+and Br-ISE to SO42-.Cell(c)was applied to measure the electromotive force value of solution with different ionic strength under different ionic strength fractions(yB=0.25,0.50,0.75).The Pitzer equation and Nernst equation are used to deal with the experimental data to fit the interaction parameters between ions,and then the average activity coefficients of electrolyte A and electrolyte B,the osmotic coefficient and the excess Gibbs free energy values in the mixed electrolyte solution would be calculated.The law of each thermodynamic property under different ionic strength I and different ionic strength fraction yB were been discussed from the perspective of ion association and ion hydration.Through the study of phase chemistry and thermodynamic data of selected systems,the theoretical database of solution chemistry based on rubidium and cesium systems has been expanded,and which would provide basic experimental and data basis for the utilization and development of brine.