Thermodynamic Properties Of The Binary System （MgB₄O₇-H₂O） At Multiple Temperatures

In this dissertation, the stable phase equilibria of the binary system （MgB₄O₇-H₂O） at273.15K,283.15K,293.15K,298.15K,303.15K,308.15K,313.15K and323.15Kwere studied with the method of isothermal dissolution equilibrium method. Thesolubilities and physicochemical properties including pH and conductivities value inthe binary system at eight kinds of temperatures were also determined. The transformedbehavior of magnesium borate was studied at273.15K,298.15K,308.15K and323.15K. On the basis of experimental data, the diagrams of solubilities versustemperatures and the physicochemical properties versus compositions were plotted anda multinomial expression of the relationship between solubilities of MgB₄O₇anddifferent temperatures was given by the method of least square fitting.The thermodynamic properties have been studied for the system of （MgB₄O₇-H₂O）at298.15K and308.15K using isopiestic method. Reliability of isopiestic method hasbeen tested with two reference standards of NaCl and KCl whose solvent activities havebeen determined precisely. It is shown that accurate results can be obtained by theimproved isopiestic apparatus at very low molality. So the molality ranges of theexperiment for isopiestic method were extended. The osmotic coefficient and wateractivity of the MgB₄O₇-H₂O system from low to supersaturated concentration weredetermined using this isopiestic equipment at298.15K and308.15K, and relationshipsbetween osmotic coefficients and ionic strength, saturated vapor pressures and ionicstrength, water activities and molalities have been studied. The solubility of MgB₄O₇,the osmotic coefficient, and water activity for the binary system （MgB₄O₇-H₂O） at298.15K were also determined by isopiestic method. A comparison of the solubility data ofMgB₄O₇with the isothermal dissolved equilibrium method shows that the result agrees well. The Pitzer ionic interaction parameters and the solubility equilibrium constant lnKfor the MgB₄O₇-H₂O system at298.15K were obtained by fitting the experimental dataof osmotic coefficients from the literature and the present measurements of solubility ofMgB₄O₇and water activity by the isopiestic method using the modified Pitzer equationof osmotic coefficient through judging2value and the equilibrium constant equationsimultaneously. The standard deviation for the fit is0.00386. The solubility of MgB₄O₇was calculated as0.03510mol.Kg^-1with a iterative deviation of3.5×10^-5for lnK usingthe parameterized Pitzer equation and the solubility equilibrium constant lnK,. And arelative error of the calculated solubility is-0.00741when compared with theexperimental value. On the basis of experimental data of the MgB₄O₇-H₂O system at308.15K, The Pitzer ionic interaction parameters at308.15K were calculated by fittingthe modified Pitzer equation of osmotic coefficient with the standard deviation of0.00723for the fit. It is shown that the modified Pitzer model can represent thethermodynamic properties for MgB₄O₇-H₂O system and can be used to calculate thesolubility for the system containing MgB₄O₇at （298.15and308.15） K.