ELECTRICAL-CONDUCTIVITY MEASUREMENTS OF AQUEOUS SODIUM-CHLORIDE SOLUTIONS TO 600-DEGREES-C AND 300-MPA

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 our at temperatures from 100 to 600 degrees 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 temperature/pressure region of water, i.e., at 5 degrees intervals from 370 to 400 degrees C. The results verify previous findings that the limiting equivalent conductance Lambda(0) 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 degrees C. Above 350 degrees C, Lambda(0), is virtually temperature independent. The logarithm of the molal association constant as calculated exclusively from the data greater than or equal to 400 degrees 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 degrees C at densities greaser 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.