Multiple Ion Association versus Redissociation in Aqueous NaCl and KCl at High Temperatures

Abstract

The electrical conductivities of aqueous solutions of NaCl have been measured at 651 and 670 K at 28 MPa for molalities up to 1.0 mol kg -1 . These conductivities as well as the results of Hwang et al. (High Temp. High Pres. 1970, 2, 651-669), Ritzert and Franck (Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 798-808), andMangold and Franck (Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 21-27) for aqueous KCl have been fit to the conductance equation of Turq et al. (J. Phys.Chem. 1995, 99, 822-827) with a consensus mixing rule and either mean spherical approximation or Debye-Hückel activity coefficients. Except at one state point (NaCl at 670 K and 28 MPa), where the interactions are the strongest ( * ) 17.8), the simplest model that fits the experimental results reasonably well (( 2-3%) at molalities up to 4.5 mol kg -1 is one with only the limiting equivalent conductance and a pair association constant adjusted. Activity coefficients calculated with either the MSA and ionic diameters or with the Debye-Hückel equation of Oelkers and Helgeson (Geochim. Cosmochim. Acta 1990, 54, 727-738) (with no salting out) can be used with similar accuracy. At high concentrations this model predicts strong redissociation of the ion pairs that form at low concentrations. Oelkers and Helgeson (Geochim. Cosmochim. Acta 1993a, 57, 2673-2697) proposed a model with substantial multi-ion association (triplets and quartets, etc.). This model has two more adjustable parameters and does not fit the data without the physically unrealistic salting out coefficient used by Oelkers and Helgeson (Geochim. Cosmochim. Acta 1991, 55, 1235-1251), so this model is not recommended. For the point with the highest *, more complex models are needed at concentrations above 0.05 mol kg -1 . Good fits to the data were found for multi-ion association models (five adjustable parameters) and reasonable fits for more complex activity models (four adjustable parameters) with only pair association, so that the models are about equally accurate at equal complexity. The cluster model of Laria et al. (J. Chem. Soc., Faraday Trans. 1990, 86, 1051-1056 for the restricted primitive model is consistent with the qualitative predictions of our preferred model with only pair association.

Development of an Experimental Data Base and Theories for Prediction of Thermodynamic Properties of Aqueous Electrolytes and Nonelectrolytes of Geochemical Significance at Supercritical Temperatures and Pressures.

“…We have published the interpretation of our high concentration NaCl results as well as high concentration literature data on KCl (SHARYGIN et al, 2002). This work indicated that for most geochemical environments there is no need to invoke multi-ion clusters at concentrations up to 2 or 3 mole/kg because pair-wise association with mean spherical approximation activity coefficients fits the conductance data .…”

Extrapolation Methods for AC Impedance Measurements Made with a Concentric Cylinder Cell on Solutions of High Ionic Strength

“…a m NaCl = 0.001100 molÁkg -1 , b m NaCl = 0.009412 molÁkg -1 , c m NaCl = 0.05320 molÁkg -1 , d m NaCl = 0.1057 molÁkg -1 , and e m NaCl = 0.4938 molÁkg -1 and 10 kHz were used. This is the range that has been used in the past for cells with concentric cylindrical geometry albeit with larger cell constants [5,14,30,31].…”

“…In the course of measuring the impedance spectra of several solutes at high temperatures and pressures with the conductance instrument built at the University of Delaware in the lab of R. H. Wood, it was found that the extrapolation method was the largest contributor to the experimental uncertainty when measuring solutions with high concentrations [12,13]. This is especially important because high concentration measurements are required to accurately determine association constants involving the formation of triplet ions [14]. Hnedkovsky et al [7] first addressed this problem for this concentric cylindrical cell design, however, their conclusions were based on a smaller concentration range and comparisons between different extrapolation methods were applied all to the same set of measurements.…”

Electrical Conductivity in Hydrothermal Binary and Ternary Systems

“…A modifi ed cell is described (Sharygin et al, 2002) which avoids the cracking of the sapphire insulators by doing the pressure on them more uniform. This design used a CVD diamond washer was used to shield the surface of the sapphire in contact with the sample allowing the measurement of acids and bases.…”

“…A more detailed analysis of the multiple ion association has been given by Wood and co-workers (Sharygin et al, 2002) considering the high temperature electrical conductances of very concentrated NaCl and KCl solutions. Except for one experimental condition (NaCl at 670 K and 0.3 g·cm −3 ) the simple ion-pair association model fi ts the experimental results quite well up to 4.5 mol·kg −1 .…”