Limiting Conductivities of Univalent Cations and the Chloride Ion in H2O and D2O Under Hydrothermal Conditions

Frequency-dependent electrical conductivities of aqueous sodium chloride, potassium chloride, cesium chloride, potassium iodide and cesium iodide have been measured in both H2O and D2O between T = 298 and 598 K at p similar to 20 MPa at a ionic strength of similar to 10(-3) mol center dot kg(-1) using a high-precision flow-through AC electrical conductance instrument. Experimental values for the molar conductivity, I >, of each electrolyte were used to calculate their molar conductivities at infinite dilution, I > A degrees, with the Fuoss-Hsia-Fernandez-Prini conductivity model. Single-ion limiting conductivities for the chloride ion in H2O, lambda A degrees(Cl-), were derived from I > A degrees by extrapolating literature values for the transference number of Cl-, tA degrees(Cl-), in aqueous solutions of KCl and NaCl from similar to 400 and similar to 390 K up to the experimental conditions. Values for lambda A degrees(Cl-) in D2O were determined from literature values of tA degrees(Cl-) for KCl in D2O near ambient conditions, assuming the same temperature dependence as in H2O. The results were used to calculate values for the single ion limiting conductivities lambda A degrees(Na+), lambda A degrees(K+), lambda A degrees(Cs+), lambda A degrees(Cl-), and lambda A degrees(I-) in both light and heavy water. The values of lambda A degrees in D2O are the first to be reported at temperatures above 338 K. The temperature dependence of the isotopic Walden product ratio, , indicates that differences in the hydration of Cl-, K+ and Cs+ ions between light and heavy water at ambient conditions associated with hydrogen-bonding, the so-called "structure breaking" effects, largely disappear at temperatures above similar to 400 K. The value of for the "structure making" ion Na+ rises from 0.98 at 298.15 K to similar to 1.04 +/- A 0.02 at temperatures above similar to 375 K and remains approximately constant up to 600 K.