Solubility Determination and Thermodynamic Modeling of Solid–Liquid Equilibria in the Quaternary System NaCl–Na2SO4–NaHCO3–H2O at 323.15 K

The solubilities, refractive indices, and pH values in the NaCl–Na2SO4–NaHCO3–H2O system at 323.15 K were determined by the isothermal solubility method. The dry-salt diagram of the system consists of three isothermal solubility curves saturated with two salts, one invariant point, and three crystallization regions of NaCl, Na2SO4, and NaHCO3. Two models were applied to predict the solubilities in the system based on the Pitzer model. Model I assumes that there is only one carbon species HCO3– in the solution. Model II supposes that there are three carbon species corresponding to HCO3–, CO32–, CO2(aq), and OH– in the solution. Both the computed solubilities with the two models were basically consistent with experimental results. The concentrations of HCO3–, CO32–, CO2(aq), and OH– were calculated by model II. The dominant form of carbon species is HCO3–, and the concentrations of CO32– and CO2(aq) in the sodium bicarbonate solution can be neglected. Model II can be used to describe the experimental pH values. The mean activity coefficients of NaCl, Na2SO4, and NaHCO3, osmotic coefficients, and water activities were also calculated with the Pitzer model.