Study On The Thermodynamic Properties And Phase Equilibrium Of The RbX/CsX-NH₄XH₂O(X=Cl,Br) Ternary System

With the increasing reduction of solid mineral resources,the comprehensive and efficient exploitation and utilization of the sea brine resources have taken general attention.The composition of liquid mineral resources is complex and systematic,so it is particularly important to study the thermodynamic properties and phase equilibria of mixed electrolyte solutions.Ternary systems RbCl+NH4Cl+H2O,CsCl+NH4Cl+H2O,RbBr+NH4Br+H2O,CsBr+NH4Br+H2O and CsCl+(CH3)4NCI+H2O were research objects.The contents are as follows:Electromotive force method is applied to determine the thermodynamic properties of five mixed electrolyte systems of RbCl/CsCl+NH4CI+H2O,RbBr/CsBr+NH4Br+H2O,CsCl+(CH3)4NC1+H2O.The galvanic cells with no liquid connection potential were set up with Rb/Cs-ISE and CI/Br-ISE electrode:(a)Rb/Cs-ISE | A(mA0)|Cl/Br-ISE(b)Rb/Cs-ISE | B(mB0)| Cl/Br-ISE(c)Rb/Cs-ISE I A(mA),B(mB)| Cl/Br-ISEThe cells(a)were applied to determine if the electrode pairs composed of Rb/Cs-ISE and Cl/Br-ISE have a good Nernst response.The cells(b)were performed to study the selectivity coefficient.The cells(c)were applied to determine the potential value for electrolyte solutions of different ionic strength and different ionic strength fraction(yB=0.25,0.5,0.75)at 298.15 K.Then the Nernst and Pitzer equations are used to fit the data,so the thermodynamic properties of different systems such as the mean ionic activity coefficient,osmotic coefficient and excess Gibbs free energy were determined.The γ±A decrease with the increasing of the total ionic strength I at a constant ionic strength fraction yB,and it increase with the increasing of the ionic strength fraction yB at a constant I.We discuss the results from the effect of ion association and ion hydration,and the data were fitted with Harned rules.The phase equilibria and physicochemical properties of RbCl/CsCl+NH4Cl+H2O and RbBr/CsBr+NH4Br+H2O systems at 298.15 K and 308.15 K were determined.The results show that there are solid solutions in RbCl+NH4Cl+H2O and RbBr+NH4Br+H2O at 298.15 K and 308.15 K,while the ternary systems CsCl+NH4CI+H2O and CsBr+NH4Br+H20 are simple saturated system at both temperatures.Density and refractive index of systems have a break point in invariant point.The results of this study enrich the thermodynamic properties and phase equilibrium database of the rare alkali electrolyte solution,and provide basic data for the efficient development and utilization of natural liquid mineral resources,especially rubidium and cesium resources.