NOTES
1144
Sodium Bifluoride : Decomposition Pressure by the Knudsen Effusion Methodl
for the 0.675, 1.03, and 1.88-mm diameter orifices respectively. The temperature dependence of the weight-loss rate was the same as the value found for effusion through the three orifices. Container reaction was negligible inasmuch as the crucible lost less than by Alan R. Miller 0.1 mg (5 X lo-' g-atom of Pt) while a total of 1.0 Aerojet-General Nucleonics, San Ramon, California 94685 g of H F (0.05 mole) was sublimed during the experi(Received October 17, 1966) mental program. The Pt cell was placed into a brass cup and attached to a vacuum system; the connection was sealed with In the course of work with tritiated sodium bia rubber O-ring lubricated with silicone grease. The fluoride we found it necessary to determine the desystem was equipped with a liquid nitrogen trap, a composition pressure of natural sodium bifluoride at silicone oil diffusion pump, and a mechanical pump. room temperature. The system has been studied at Residual pressures were read with a Veeco ionization higher temperatures by Fischerj2 who made direct gauge positioned on the opposite side of the manifold , ~ pressure measurements, and by Froning, et ~ l . who from the effusion cell. used a transpiration method. Their results can be exWhen residual pressure became less than torr, pressed, respectively, as an isothermal bath was raised around the sample logPH,(atm) = 6.594 - 3521/T°K (430-542') (1) chamber. Residual pressure during heating was usually less than torr and never larger than 5 X 10-6 logPH,(atm) = 5.821 - 3196/T°K (473-548') (2) torr. Temperatures were measured with a chromelfor the reaction alumel thermocouple fastened to the outside of the brass cup containing the Knudsen cell. NaHFz(s) = NaF(s) HF(g) (3) Heating times of 106-5568 min were chosen to give Extrapolation to 273°K gives, respectively, 5.0 X 5-17-mg weight losses. The effusion data were oblo-' and 1.3 X lov6 atm-values that are readily tained with three chargings of the cell; in each case, measurable by the Knudsen effusion t e c h n i q ~ e . ~ the initial composition was ca. 1mole % NaF in NaHF2. However, a room-temperature effusion experiment The composition shifted 3-6% during each effusion gave an immeasurable (less than 0.05 mg) weight loss, run. When the composition reached ca. 85-90% indicating that the apparent pressure was less than NaF in NaHF2, the sample was discarded and the cell l/loo the extrapolated value. Thirty-five additional recharged. Orifice sizes and temperatures were chosen effusion experiments were performed at temperatures randomly. from 310 t o 371°K in an effort to obtain a more meanThe diameters of the cylindrical effusion orifices ingful value. were determined by comparison with a calibrated Samples for NaHFz were produced by reaction of NaF slide under a microscope. Orifice lengths were measand H F in a Teflon vessel at room temperature. Anured with a vernier micrometer. Clausing factors hydrous H F was prepared at room temperature by calculated by Den/larcus7 were used to correct for the reaction of Hz and Fz which had been separately passed finite length of the orifice. through liquid nitrogen-cooled traps to remove water The experimental results are summarized in r4 '1 g ure and other impurities. The resulting white sample 1. The three solid lines in Figure 1 are linear leastgave a well-resolved X-ray diffraction pattern identical square fits (with 1/T the dependent variable) for the with that reported for NaHF2.6 three orifices; the root-mean squares of the 1/T Heating in a Pt crucible to constant weight produced deviations about the fitted curves are 0.01, 0.02, and a 32.3y0 weight loss and a material exhibiting only the (1) Supported by the Advanced Research Projects Agency through NaF X-ray diffraction pattern16 indicating that the a contract with the Office of Naval Research. sample had been originally NaHF2. (2) J. Fischer, J. Am. Chem. Soc., 79, 6363 (1957). Knudsen effusion experiments were performed in a (3) J. F. Froning, M. K. Richards, T.W.Stricklin, and S. G. TurnPt cell with one of three cylindrical orifice sizes: 0.675, bull, Ind. Eng. Chem., 39, 275 (1947). 1.03, or 1.88-mm diameter. The cylindrical cell was (4) M.Knudsen, Ann. Phyaik, 28, 999 (1909). (5) ASTM X-Ray Card 60479. 1/2 in. high by in. in diameter (inside dimensions). (6) ASTM X-Ray Card 40793. Loss of sample between the lid and crucible was ac(7) W. C. DeMarcus, Union Carbide Nuclear Co., Oak Ridge counted for by experiments in which a lid with no Gaseous Diffusion Plant, Report K-1302, Part I11 (1957), A D rifice was used. This loss amounted to 13,7, and 4.5% 124 579.
+
The Journal of Physical Chemistry
NOTES
1145
thalpy of 17.88 kcal to reaction 3; values in NBS Circular 50012lead to an enthalpy of 20.06 kcal. Although the decomposition pressures reported in ref 2 and 3 are not directly comparable with the present data because of a temperature gap of 60-100”, secondlaw heats were obtained by extrapolating eq 1 and 2 to 298°K and the NaHF2 free-energy functions to 500°K. These second- and third-law heats are 16.1 and 17.0 for ref 2 and 14.6 and 17.2 for ref 3.
a 0
--I 3.2
I
- I.88
mm orifice d i m .
- 1.03 mm orifice diam. - 0.675 mm orifice diam. Extrapolated equilibrium pressure
1
-
(8) K. Motzfeldt, J . Phys. Chem., 59, 139 (1955). (9) E. F. Westrum, Jr., and G. A. Burney, ibid., 6 5 , 344 (1961). (10) D. R. Stull, “JANAF Thermochemical Tables,” Dow Chemical Co., Midland, Mich. (11) T. L. Higgins and E. F. Westrum, Jr., J . Phys. Chem., 6 5 , 830 (1961). (12) F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine, and I. Jaffe, “Selected Values of Chemical Thermodynamic Properties,” National Bureau of Standards Circular 500, U. S. Government Printing Office, Washington, D. C., 1952.
I
I
I
2.8
3.0
2.6
IO~,,TOK
Figure 1. The HF partial pressure as a function of temperature over the NaHFz-NaF two-phase region.
Attractive Potentials between Fluorochemicals and Aliphatic Hydrocarbons
0.01 for the large, medium, and small orifice, respectively, (equivalent to cu. 1°K). The dashed line in Figure 1 was obtained by extrapolation to zero orifice size according to a method suggested by Motzfeldtf the equation of this line log PH,(atm) = 6.677
- 3940/T°K
(4)
leads to a second-law heat for reaction 1 of 18.0 f 0.5 kcal. The sublimation coefficient is calculated to be 0.006.8 Third-law heats (for reaction 3) were obtained by combining corrected apparent pressures with freeenergy functions for reaction 3. The individual apparent pressures were multiplied by the ratio of the equilibrium pressure to the least-squares-fit pressure (corresponding to the proper orifice) at 333.3”K. These factors are 3.85, 1.83, and 1.22 for the large, medium, and small orifices, respectively. The freeenergy functions for NaHF2 given by Westrum and Burneye were extrapolated from 300 to 400°K and combined with those for NaF and H F given in the “JANAF Tables”lo to yield
in cal/deg for reaction 3. A third-law heat of 18.5 f 0.3 was obtained. Higgens and Westrum11 obtained a heat of formation of -218.1 kcal/mole for NaHFz and assigned an en-
by J. H. Dymond and J. H. Hildebrand Department oj Chemistry, University oj California, Berkeley, California (Received November 1, 1966)
We reported in a recent paper‘ that the solubility of iodine in cyclopentane, cyclohexane, and two decalins agrees well with the equation
RT In ( a 2 / z 2 )= v20p12(62- 61)2
(1) with which a total 13 solutions of iodine conform, but that methyl- and ethylcyclohexanes are better solvents for iodine as calculated from eq 1 to the extent of -300 cal/mole, dimethylcyclohexane excels to the extent of 400 cal/mole, and furthermore that solvents such as triptane, 2,2,3-trimethylbutane, with one methylene hydrogen atom and 15 methyl hydrogen atoms per molecule, show an enhancement of solvent power for iodine amounting to -800 cal/mole. In eq 1, az is the activity of the solute referred to its pure liquid, z2 its mole fraction, vZ0its liquid volume, 91 is the volume fraction of the solvent; the 6’s are the two solubility parameters, evaluated from the square roots of their energies of vaporization per cubic centimeter. (1) J. H. Hildebrand and J. H. Dymond, PTOC.Natl. Acad. Sci. U . S.,54, 1001 (1965). See also J. H. Hildebrand, Science, 150, 441 (1965).
Volume 71, Number 4 March 1967
