Limiting Conductivities of Univalent Cations and the Chloride Ion in H sub(2)O and D sub(2)O 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 H sub(2)O and D sub(2)O between T = 298 and 598 K at p ~ 20 MPa at a ionic strength of ~10 super(-3) mol.kg super(-1) using a high-precision flow-through AC electrical conductance instrument. Experimental values for the molar conductivity, Lambda , of each electrolyte were used to calculate their molar conductivities at infinite dilution, Lambda degree , with the Fuoss-Hsia-Fernandez-Prini conductivity model. Single-ion limiting conductivities for the chloride ion in H sub(2)O, lambda degree (Cl super(-)), were derived from Lambda degree by extrapolating literature values for the transference number of Cl super(-), t degree (Cl super(-)), in aqueous solutions of KCl and NaCl from ~400 and ~390 K up to the experimental conditions. Values for lambda degree (Cl super(-)) in D sub(2)O were determined from literature values of t degree (Cl super(-)) for KCl in D sub(2)O near ambient conditions, assuming the same temperature dependence as in H sub(2)O. The results were used to calculate values for the single ion limiting conductivities lambda degree (Na super(+)), lambda degree (K super(+)), lambda degree (Cs super(+)), lambda degree (Cl super(-)), and lambda degree (I super(-)) in both light and heavy water. The values of lambda degree in D sub(2)O are the first to be reported at temperatures above 338 K. The temperature dependence of the isotopic Walden product ratio, \( (\lambda logical or circ \eta )_{{{\text{D}}_{2} {\text{O}}} /(\lambda { logical or circ }\eta )_{{{\text{H}}_{2} {\text{O}}} \), indicates that differences in the hydration of Cl super(-), K super(+) and Cs super(+) ions between light and heavy water at ambient conditions associated with hydrogen-bonding, the so-called "structure breaking" effects, largely disappear at temperatures above ~400 K. The value of \( (\lambda logical or circ \eta )_{{{\text{D}}_{2} {\text{O}}} /(\lambda { logical or circ }\eta )_{{{\text{H}}_{2} {\text{O}}} \) for the "structure making" ion Na super(+) rises from 0.98 at 298.15 K to ~1.04 plus or minus 0.02 at temperatures above ~375 K and remains approximately constant up to 600 K.