Isopiestic determination of the osmotic coefficients of Na2SO4(aq) at 25 and 50 degrees C, and representation with ion-interaction (Pitzer) and mole fraction thermodynamic models

Isopiestic vapor-pressure measurements were made for Na2SO4(aq) from 0.11 t o 3.74 mol-kg(-1) at 25.00 degrees C and from 0.12 to 3.57 mol-kg(-1) at 50 .00 degrees C, using NaCl(aq) as the reference standard. Published isopiest ic data, direct vapor pressures, emfs of reversible cells, freezing tempera ture depressions, boiling temperature elevations, heat capacities, and dilu tion enthalpies for this system have been critically assessed and recalcula ted in a consistent manner. Parameters for Fitter's model and a mole fracti on-based thermodynamic model were evaluated. The mole fraction-based thermo dynamic model for Na2SO4(aq) is valid from the freezing temperatures of the solutions to 150.5 degrees C. Fitter's model represents the osmotic coeffi cients and emfs essentially to experimental accuracy at 25 and 50 degrees C provided that the third virial coefficient was ionic-strength dependent.