Vapour pressure osmometry and liquid density data for binary aqueous solutions of organic salts: Measurement and modelling

Abstract

In this work, considering the need for accurate thermophysical data to achieve a reliable thermodynamic description of electrolyte-containing systems, liquid density (298.15 or 313.15 K) and vapour pressure osmometry (313.15 K) studies were conducted for binary aqueous solutions of the organic salts: sodium acetate (Na[Acet]), sodium formate (Na[Form]), sodium gluconate (Na[Glu]), sodium succinate (Na₂[Suc]) and potassium formate (K[Form]). Liquid densities $\left(\rho \right)$ were found to be favoured by decreasing temperature and increasing salt molality, following the order Na[Acet] (1006.01–1099.51 kg·m^-3) < Na[Form] (1008.13–1120.56 kg·m^-3) < K[Form] (1007.73–1119.52 kg·m^-3) < Na[Glu] (1009.57–1117.90 kg·m^-3) < Na₂[Suc] (1005.43–1108.17 kg·m^-3) in the common composition range. This property was satisfactorily correlated with salt molality $\left(m\right)$ using second-degree polynomials, obtaining low standard deviations $(0.4<{\sigma }_{\mathrm{SD}}/\text{kg}·m^{-3}<1.2)$ and high determination coefficients $(0.9995<R^2<0.9999)$. Furthermore, vapour pressure osmometry (VPO) studies were carried out, at 313.15 K, for the same solutions, showing a significant reduction of vapour pressure at maximum concentration (Na[Acet]: 1.231 kPa; Na[Form]: 1.318 kPa; Na[Glu]: 0.450 kPa; Na₂[Suc]: 0.791 kPa; K[Form]: 1.187 kPa) and generally presenting osmotic coefficients $\left(\varphi \right)$ below unity, which implied negative deviations from an ideal solution. Finally, the osmotic coefficients were successfully described using the Extended Pitzer Model of Archer ($2.57<{\sigma }_{\mathrm{SD}}·{10}^3<9.56$ and $0.9368<R^2<0.9969$), which was later applied in the estimation of mean molal activity coefficients $\left({\gamma }_{\pm }\right)$ and excess Gibbs free energies $\left(G^E\right)$.