Electrical conductivity measurements of aqueous sodium chloride solutions to 600°C and 300 MPa

Abstract

Electrical conductance measurements of dilute (<0.1 mol-kg⁻¹) aqueous NaCl solutions were made primarily to quantify the degree of ion association which increases with increasing temperature and decreasing solvent density. These measurements were carried out at temperatures from 100 to 600°C and pressures up to 300 MPa with a modified version of the apparatus used previously in the high temperature study in this laboratory. Particular emphasis was placed on conditions close to the critical temperaturelpressure region of water, i.e., at 5° intervals from 370 to 400°C. The results verify previous findings that the limiting equivalent conductance A_o of NaCl increases linearly with decreasing density from 0.75 to 0.3 g-cm⁻¹ and also with increasing temperature from 100 to 350°C. Above 350°C. A_o is virtually temperature independent. The logarithm of the molal association constant as calculated exclusively from the data≥400°C is represented as a function of temperature (Kelvin) and the logarithm of the density of water (g-cm⁻³) as follows:

$$log K_m = 0.997 - 650.07/T - (10.420 - 2600.5/T)log\rho _w $$

Note that this function also provides a good representation of the log K_m values obtained from 350 to 395°C at densities greater than ca. 0.6 g-cm⁻³. More precise conductance data now available in the literature suggest a systematic error of unknown origin may exist in the data obtained at lower densities in this region. The relevant thermodynamics quantities derived from differentiation of this equation with respect to temperature and pressure are listed in the text.