Isopiestic Determination of the Osmotic and Activity Coefficients of K2HPO4(aq), Including Saturated and Supersaturated Solutions, at T=298.15 K

The osmotic coefficients of K2HPO4(aq) have been measured at T=298.15 K by the isopiestic vapor pressure method over the range of molalities from 1.3846 mola <...kg(-1) to 13.939 mola <...kg(-1) (oversaturation) with CaCl2(aq) as the reference solution. The molalities and osmotic coefficients of saturated solutions in equilibrium with K(2)HPO(4)a <...xH(2)O(cr) were measured simultaneously by the same method. Available literature osmotic coefficients of K2HPO4(aq) at T=298.15 K, and our new experimental data, were combined and modeled using an extended form of Pitzer's equation and the Clegg-Pitzer-Brimblecombe equation based on the mole-fraction-composition scale. These equations were used to calculate the activity coefficients of K2HPO4(aq) at T=298.15 K.