Bulk properties and hydration numbers of ammonium nitrate and ammonium chloride in solution: A structural and thermodynamic analysis

The apparent volumes of the salts in the systems H2O-NH4Cl (298 K) and H2O-NH4NO3 (273 K, 298 K, and 323 K) are reproduced with an accuracy of 0.03–0.01 cm3/mol by the equation ϕ = ϕ0 + Aw20.5 + Bw2, where w2 is the salt content (mass fractions). The study shows that there is a correspondence between the critical (for determining the hydration number) structural parameters-the intrinsic volume of the electrolyte and the volume of water in ion hydration shells-and the limiting (at w2 = 1) partial molar volumes of the components. The hydration numbers at infinite dilution are 6.9 for NH4Cl at 298 K and 9.1, 6.7, and 6.4 for NH4NO3 at 273 K, 298 K, and 323 K. The water volume in ion hydration shells decreases in the sequence: No3−, Cl−, and NH4+. The hydration numbers decrease with increasing salt concentration. The study shows that within a simpler model ϕ = ϕ0 + aw20.5, the hydration numbers are temperature independent.