Study of Ion-Pairing and Thermodynamic Properties of Sodium Fluoride in Aqueous Solutions at Temperatures from 298.15 to 353.15 K

Abstract

In this investigation, the water activity and osmotic coefficients of the binary solution NaF–H2O were determined using the hygrometric method, from dilution to saturation and from 298.15 to 353.15 K. The hygrometric measurement results obtained have been combined with NaF(cr) solubility data in water to construct a chemical model that calculates solute and solvent activities from 298.15 to 353.15 K and from dilute to saturated solution concentration [up to m(NaF) = 1.2 mol·kg−1 at 353.15 K], and solid–liquid equilibria in the NaF–H2O system. The solubility modelling approach, based on the fundamental Pitzer ion interaction equations, was employed. It was found, that the standard approach for 1–1 electrolytes, with three β (0) , β (1), and C ϕ single electrolyte ion interaction parameters, gives a very good agreement with osmotic coefficients data for unsaturated solutions, and with NaF(cr) equilibrium solubility data, used in parameterization. The raw experimental osmotic coefficients (ϕ) and model calculated mean activity coefficients γ(NaF) have been compared with reference experimental ϕ data, and with γ estimations made by using different calculation approaches. The comparison shows a good agreement between ϕ data at 298.15 K. There is a disagreement between γ(NaF) calculations made in this study, and calculations based on Scatchard equations, and an extended Debye–Huckel (DH) model. The Pitzer based model constructed here has been also used to calculate the thermodynamic solubility product $$ \left( {K_{\text{sp}}^{\text{o}} } \right) $$ of NaF(cr) from 298.15 to 353.15 K. The solubility isotherm m s for NaF–H2O and thermodynamic solubility product [given as ln $$ K_{\text{sp}}^{\text{o}} $$ (NaF(cr))] are presented. In the saturated solution of NaF(aq), from experimental data and Fuoss approach using the obtained parameters, the experimental and calculated association constants K ass of the ion pairs were estimated at temperatures ranging from 298.15 to 353.15 K. The results are in good agreement.