| Na2SO4 is the main constituent of seawater and natural brine, and the main component of solution system in industrial process. As the common metallic sulfate solution in manufacturing process, sodium sulfate is widely used in various fields such as chemistry and chemical engineering, geochemistry and industrial process. Thus the research of its physicochemical properties, especially the osmotic coefficient, activity of solvent and solute, thermodynamic model of reaction and relative thermodynamic properties are of great significance, which can help us understand the state of sodium sulfate in solution, separate and purify sodium sulfate and guide the related practical process.Great many research work have been put on the thermodynamic properties of sodium sulfate. Due to the chemical properties of Na2SO4 and disadvantages of traditional equipment, the isopiestic method is adopted to measure its activity coefficients. Based on the former work of other researchers, a set of equipment was developed to measure the properties of low concentration solution. The equipment was used to study the activity of sodium sulfate under low concentration with a range of 0.01?0.74 mol·kg-1.(1) Based on the weaknesses of the traditional equipment, we made improvement on three different perspectives. Firstly, taking the isopiestic apparatus into account, we developed a new one to accelerate the equilibrium speed and ensure high precision of the experimental result. We also developed the matched thermal conditioning device (including the rotation device) to lower the fluctuation of temperature by combination of the radial rotation of equal pressure device and up-down circulation of water under constant temperature. By assembly of new vacuum equipment and addition a set of buffer vacuum solution, the state in the equal pressure device was kept unchanged and the accuracy along with the precision of the experiment improved.(2) Using the improved equipment and method, we measured the isopiestic equilibrium concentration of Na2SO4-H2O with much lower concentration from 0.01 to 0.74 mol·kg-1 under 298.15 K and 303.15 K, and then calculated the corresponding osmotic coefficients and water activities. Two new sets of Pitzer parameters were obtained from resulting osmotic coefficients with osmotic coefficients of high concentration in references by multiple nonlinear regression, they are as following:T=298.15 K?, ?(0)= 0.01888,?(1)= 1.04250, C?= 0.00550, R2=0.99637T=303.15 K?,?(0)= 0.03063, ?(1)= 1.08205, C?= 0.00295, R2=0.99764The relative coefficients are both above 0.99, indicating feasibility of Pitzer for the experiment and reliability of the results.(3) After testify the reliability of Pitzer parameters, by using the fitted parameters to calculate the osmotic coefficients and water activities, we compared the calculated values with the experimental values. The result shows that relative deviations of osmotic coefficients between the two kinds of values are-1.51%?0.91%?-1.37%?1.27%. Meanwhile, the relative deviations of water activities at 298.15 K and 303.15 K are 0.017% and 0.025% respectively, suggesting Pitzer model can nicely present the thermodynamic properties of Na2SO4 solution. We also obtained the change law of osmotic coefficient and water activities with the mass concentration.(4) Comparing the solubility of Na2SO4 calculated by using different Pitzer parameters with the reported solubility, we testify the better reliability of our work. Therefore, the results of our work are more suitable for the calculation of activity with different concentration.(5) We calculated the mean activity coefficients, saturated vapor pressure and Gibbs energy of the Na2SO4-H2O system and the change laws of them by utilization of Pitzer parameters. Moreover, we used Gibbs-Duhem equation to testify the Na2SO4-H2O system. The calculated values and the measured values by isopiestic method are in reasonable agreement, further indicating reliability of the activity coefficients by using of Pitzer model. |
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