PAULE. FIELDAND JAMES H. SHAFFER
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The Solubilities of Hydrogen Fluoride and Deuterium Fluoride in Molten Fluorides'.
by Paul E. Field'b Department of Chemistry, Virginia Polytechnic Institute, Blacksburg, Virginia 24061
and James H. Shaffer Reactor Chemistry Division, Oak Ridge National Laboratoyy, Oak Ridge, Tennessee $7830 (Received February 16, 1967)
The solubilities of HF and D F in molten LiF-BeF2 (66-34 mole %) were determined over the temperature range 500-700" and a t solute gas pressures between 1 and 2 atm. Using previously established experimental methods, the solubilities of both gases were found to mole of HF/mole of melt-atm) obey Henry's law. The Henry's law constants, K H for HF and DF, respectively, a t 500, 600, and 700" were: 3.37 f 0.13,2.96 f 0.07; 2.16 f 0.05, 1.83 f 0.03; 1.51 f 0.06, 1.25 f 0.03, where the values given were obtained by a linear least-squares fit of the experimental data as In K H vs. 1/T and the uncertainties are at the 95% confidence interval. The heats of solution, A H s , obtained from the leastsquares evaluation were -5.98 f 0.19 and -6.43 f 0.15 kcal/mole for HF and DF, reapectively. The comparison of AHs for H F in this melt composition with those obtained previously in melts ranging from 54 to 89 mole % LiF in BeFz reveals linear dependence of AHs on the mole fraction of LiF above and below 67y0 with a maximum of 67% LiF. Interpretation of the isotope effect is made by comparison of the difference in the entropies of solution between DF and HF with the difference in the calculated values of the entropies of the two gases a t 600".
Introduction Molten fluoride mixtures are currently in use as the fuel and secondary coolant in a nuclear power reactor experiment a t the Oak Ridge National Laboratory.2 Although these mixtures can be retained, almost indefinitely, at high temperat'ures in containers of INOR-8 (Hastelloy N), a nickel-base alloy, the presence of H F in the fluoride mixture results in metathetical corrosion of chromium and iron from the alloy. Hydrogen fluoride may be introduced as a dissolved impurity from the fluoride production process or generated by reaction of salt constituents with water vapor in the inert, blanket and sweep gases used with these systems. Thus, the behavior of H F in molten fluorides is of interest to the technology of high-temperature fusedsalt devices. Previous studies of the solubility of H F in various molten fluoride mixtures have shown interesting solThe Journal of Physical Chemistry
vent effects which appear as unresolved complex functions of the melt, c o m p ~ s i t i o n . ~Although ~~ the solubility of H F in these mixtures may be related to alkali fluoride-hydrogen fluoride compounds, the coordinating effects of other melt constituents with the alkali fluorides have a pronounced influence on the H F solubility. This investigation presents comparative measurements of the solubilities of H F and DF in a mixture of LiF (66 mole yo)in BeF2 as an attempt to elucidate further (1) (a) Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp. Presented in part a t the 152nd National Meeting of the American Chemical Society, New York, N. Y,, Sept 1966. (b) ORINS Summer Research Participant, 1965. (2) H. G. MacPherson, Power Eng., 71, 56 (1967). (3) J. H. Shaffer, W. R. Grimes, and G. 11.Watson, J . Phys. Chem., 63 8 1999 (1959). (4) J. H. Shaffer and G. M. Watson, Reactor Chemistry Division Annual Progress Report, April 29, 1960, ORNL-2584, Oak Ridge National Laboratory, p 31.
SOLUBILITIES OF H F AND DF
IN
3219
XOLTEN FLUORIDES
the solvent characteristics of molten fluoride mixtures. The observed isotope effect also provides a valuable preface to more experimentally difficult measurements of the solubility of tritium fluoride in similar systems. The solvent composition chosen for this study has been proposed as the fertile blanket for a thermonuclear breeder r e a ~ t o r . Tritium ~ would be produced by neutron irradiation of lithium-6 in the blanket salt mixture and recovered for subsequent injection as the fuel into the core of the fusion reactor. The formation of tritium in the current fission reactor, mentioned above, is minimized by the use of isotopically pure lithium-7 in its molten fluoride mixtures. Despite contrasting interests, the possible generation and dissolution of tritium fluoride in their molten fluoride systems may be common to both machines. Studies of isotope effect with hydrogen and deuterium may provide a basis for estimating some of the physicochemical properties of tritium, a highly radioactive synthetic element, and its compounds
Experimental Section Materials. Anhydrous H F was obtained from a cylinder containing 9 lb of liquid hydrogen fluoride supplied by the Matheson Go., Inc., East Rutherford, ;V. J. The liquid H P had a specified minimum purity of 99.9% and was used without further purification. Anhydrous DF was prepared specifically for this investigation by the Technical Division, Oak Ridge Gaseous Diffusion Plant, by reaction of elemental deuterium and fluorine.6 The fluoride mixture of 66 mole o/o LiF and BeFz was a 3.5-kg sample of material prepared by the Oak Ridge Xational Laboratory for use as the secondary coolant of the molten salt reactor experiment.’ The removal of oxides, sulfides, and structural metal fluoride impurities from this material was accomplished by treatment a t 600’ with a gas mixture of 10 mole yo H F in H, and at 700” with Hz alone. Results of chemical analyses showed that the fluoride mixture had the following impurities in ppm: Cr, 16; S i , 39; Fe, 123; S,
