Ion association of dilute aqueous sodium hydroxide solutions to 600°C and 300 MPa by conductance measurements

Abstract

The limiting molar conductances Λ₀ and ion association constants of dilute aqueous NaOH solutions (<0.01 mol-kg⁻¹) were determined by electrical conductance measurements at temperatures from 100 to 600°C and pressures up to 300 MPa. The limiting molar conductances of NaOH_(aq) were found to increase with increasing temperature up to 300°C and with decreasing water density ρ_w. At temperatures ≥400°C, and densities between 0.6 to 0.8 g-cm⁻³, Λ₀ is nearly temperature-independent but increases linearly with decreasing density, and then decreases at densities <0.6 g-cm⁻³. This phenomenon is largely due to the breakdown of the hydrogen-bonded, structure of water. The molal association constants K _Am for NaOH( _aq ) increase with increasing temperature and decreasing density. The logarithm of the molal association constant can be represented as a function of temperature (Kelvin) and the logarithm of the density of water by

$$\begin{gathered} log K_{Am} = 2.477 - 951.53/T - (9.307 \hfill \\ - 3482.8/T)log \rho _{w } (25 - 600^\circ C) \hfill \\ \end{gathered} $$

which includes selected data taken from the literature, or by

$$\begin{gathered} log K_{Am} = 1.648 - 370.31/T - (13.215 \hfill \\ - 6300.5/T)log \rho _{w } (400 - 600^\circ C) \hfill \\ \end{gathered} $$

which is based solely on results from the present study over this temperature range (and to 300 MPa) where the measurements are most precise.