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

The limiting molar conductances Lambda(o) 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 degrees C and pressures up to 300 MPa. The limiting molar conductances of NaOH(aq) were found to increase with increasing temperature up to 300 degrees C and with decreasing water density rho(w). At temperatures greater than or equal to 400 degrees C, and densities between 0.6 to 0.8 g-cm(-3). Lambda(o) 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 moiai nssncialion consrant can be represenred os a function of temperature (Kelvin) and the logarithm of the density of water by log K-Am = 2.477 - 951.53/T - (9.307 - 3482.8/T)log rho(w) (25-600 degrees C) which includes selected data taken from the literature, or by log K-Am = 1.648 - 370.31/T - (13.215 - 6300.5/T)log rho(w) (400-600 degrees C) which is based solely,on results from the present study over this temperature range (and to 300 MPa) where the measurements are most precise.