Heat capacity of NaNO3(aq) in stable and supercooled states. Ion association in the supercooled solution

Abstract

The heat capacity of NaNO₃(aq) from 0.1 to 10 mol kg⁻¹ was measured from 285 to 236 K, or lower temperatures, at 0.1 MPa, with a differential scanning calorimeter (DSC). A cooling scan method permitted supercooling of the aqueous solutions well below their normal freezing points. The uncertainties of temperature calibration and instrument heat calibration of the DSC were ± 0.08 K and ± 0.3% respectively. The heat capacity of dilute NaNO₃(aq) increases with decreasing temperature below 270 K, qualitatively similar to the effect observed for water and for dilute NaCl(aq). At larger concentrations of NaNO₃(aq), this behavior reverses and the heat capacity only decreases with decreasing temperature, qualitatively similar to the behavior observed for concentrated NaCl(aq). The present measurements and thermodynamic properties derived therefrom are compared to previous thermodynamic properties for NaCl(aq). This comparison shows that the anomalous effects for the two solutes are quite similar and suggests that the effects observed are general for strong univalent electrolytes. A multiple equilibrium model was fitted to selected measurements for NaNO₃(aq) to determine whether there are significant changes in ion association in the supercooled solution. A completely quantitative representation of the measurements was not obtained with this model. Nonetheless, the association constants for ion-pairing, obtained from the thermodynamic properties, increased significantly in the supercooled solution.