SODIUMFLUORIDE POWDER
April, 1958
470
KINETICS OF THE REACTION BETWEEN SODIUM FLUORIDE AND URANIUM HEXAFLUORIDE. I. SODIUM FLUORIDE POWDER' BY F. E. MASSOTH AND W. E. HENSEL, JR.~ Goodyear Atomic Corporation, Portsmouth, Ohio Received December 11, 1967
Introduction (5) The reaction of uranium hexafluoride with SOdium fluoride has been found to give a solid addition where W t is the weight of sample at time t compound of the formula UF,j.3NaF.3v4 Since this W ois the initial weight of sodium fluoride reaction results in an increase in weight due to the M 0 is the molecular weight of sodium fluoride solid product formed, the kinetics may be studied M,is, the molecular weight of the complex, UF6.3NaF conveniently by a weight-change method at constant pressure. Although the reaction is reversible Hence, the functions f(A) and f(B) can be calat higher temperature^,^ dissociation can be ignored culated from the F values obtained experimentally by weight gain measurements. A plot of f(A) or safely below 100". Farrar and Smith5 have discussed the kinetics of f ( B ) versus t should give a straight line if the rate of solid-gas reactions in considerable detail. T.he reaction follows the linear or parabolic laws, retwo laws which are most often followed in these re- spectively. Variation of the rate constant with temperature actions are the linear and parabolic laws. Thus is usually treated by the Arrhenius equation. In Linear law: most cases when a diffusion mechanism prevails, the activation energy is that for the diffusion process. Pressure over a reasonably wide range is Parabolic law: generally not a variable in solid-gas reactions. In dx _ = -a this investigation, the uranium hexafluoride presdt x sure was maintained at about 90 mm. where x is the film thickness of product at time t Experimental and a is a proportionality constant. grade sodium fluoride powder ( J . Assuming spherical particles, the final integrated T.Materials.-Reagent Baker Chemical Company) of 99.9% purity was used. rate expressions as given by Farrar and Smith are This material was passed through a 100-mesh screen, heated Linear law: hVti ~
Ti
(1
-
F)1/3
- F)% -
(I
- F)%
t = 1
-
= f(A)
(3)
Parabolic law: Tiz
t = '/z
+
*/2(1
= f(B)
(4)
where kl is the linear rate constant kz is the parabolic rate constant ~i is the initial particle radius F is the fraction converted to product Tiois the molar volume of the solid reactant
Light microphotographs of the sodium fluoride powder show that the basic particles are cubic. Integration of the fundamental rate equations for cubic particles following the method of Farrar and Smith yields expressions similar to (3) and (4) with ~i replaced by li/2, where li is the edge of a cube. For convenience, the quantity (4kz/li2) = (k2/ri2) will be defined as k , the rate constant for the parabolic law. The fraction converted can be obtained by the relationship (1) This work was performed under contract AT-(33-2)-1, with the United States Atomic Energy Commission. (2) Presented before the Division of Physical and Inorganic Chemistry of the American Chemical Society at the 132nd National Meeting in New York. N. Y., September, 1957. (3) H. Martin, A. Albers and H. P. Dust, 2. anorg. alZgem. Chem., a m , 12s (1951). (4) G. I. Cathers and R. L. Jolley, "Formation and Decomposition Reactions of the Complex UFv3NaF," paper presented a t the American Chemical Society Meeting, Spring. 1957. ( 5 ) R. L. Farrar, Jr., and H. A. Smith, THISJOURNAL, 69, 763
(1955).
overnight a t 275" to remove surface moisture, and stored in a desiccator until used; it had a surface area of 0.33 sq. m./g. by nitrogen adsorpttion measurements. Uranium hexafluoride was purified by several liquefaction and flashing cycles until the theoretical vhpor pressure was attained: Apparatus.-Two types of reactors were used: UZ., a nickel reactor and a helix spring reactor. The nickel reactor was fabricated from one-inch tubing and was equipped with a small valve and connector. Sodium fluoride was introduced by unsoldering a coupling between the body and the valve. The total reactor weight was kept below 200 g. so that an analytical balance could be used for the weight measurements. The helix reactor, construc,ted of nickel and Pyres tubing joined by a Kovar seal, had over-all dimensions of 1.5 by 30 inches. A ground glass joint sealed with Halocarbon grease permitted disassembly. A quartz helix spring, having :t maximum load capacit,y of five grams, was employed for weight change measurement,s. Spring displacement was measured with a 10 cni. cathetometer. A small Pyres bucket held the sample. One-half and one gram analytical weights were used for calibration of the spring est,ension versus load, linear displacement being asaumed over the range measured. No change in spring conatant WBR observed upon recalibration after the run. A nicltcl manifold system provided for introduction and removal of gases. Pressure was measured by mems of a Booth-Cromer pressure transmitter6 used in conjunction with a iim-cury manometer. A thermostnted water-bath maintained reactor temperature to 3=0.1". Procedure .-The nickel reac,tor was loaded with approsimately one-half gram of sodium fluoride, heated under vacuum and exposed t o fluorine to minimize subsequent reaction of uranium hexafluoride with the metal surface. The sample was then allowed to react with uranium hexafluoride a t the bath temperature. At desired time intervals, the reactor was removed and weighed. In the procedure (6) S.Cromer, "The Electronic Pressure Transmitter and Self Balancing Relay," Columbia University, June 20, 1944, (hIDDC-803) Declassified March 20, 1947.
F. E. MASSOTH AND W. E. HENSEL, JR.
480
Vol. 62
for the helix reactor, about one-half gram of sodium fluoride was weighed into the sample bucket on an analytical balance and the apparatus was assembled. The system was evacuated overnight to remove adsorbed moisture. The sodium fluoride was then exposed to uranium hexafluoride and the spring extension was measured a t various time intervals with the cathetometer. Five readings were taken a t each interval and averaged; the deviation of the average was A0.03 mm. corresponding to a weight deviation of & 3 mg. A blank run with uranium hexafluoride resulted in no weight change, thus indicating negligible reaction of uranium hexafluoride with the Pyrex bucket.
Results IIMI, HIS.
Fig. l.-j(A)
andf(B) versus time.
The helix reactor was employed to establish the rate dependency of the reaction since measurements with this reactor were continuous, while the nickel reactor was more conveniently utilized in studying the reaction rate variation with temperature. The results of the run with the helix reactor are shown in Fig. 1. The curves of f ( A ) and f ( B ) versus time clearly show the parabolic dependence of,the reaction. The data obtained with the nickel reactor at several temperatures are presented in Fig. 2. Rate constants were calculated from the values of the least squares slopes of the curves in Fig. 2. These are presented in Table I together with the constant calculated from the helix reactor run. TABLE I REACTION RATE CONSTANT AT VARIOUS TEMPERATURES Temp., OC.
TIME, HIS.
Fig. 2.--f(B) versus time at several temperatures.
k X 108 hr.+
23.4 68.0 10.5 55.0 5.34 45.0 0 Helix reactor run.
Temp.,
OC.
34.0 25.5 24 0"
k X 108 hr.-1
2.38 1.54 1.335"
An Arrhenius plot of log k versus l / T is given in Fig. 3. From the least squares slope, the activation energy of the reaction is 13.1 f 0.2 kcal./mole of uranium hexafluoride and the frequency factor, 5.0 X lo6 hr.-l. Thus the rate constant is given by the expression k = 5.0 X 106e-1?"J0/nT hr.-1 (6)
Discussion I n the Arrhenius plot, Fig. 3, the value represented by the helix reactor run is in line with those obtained with the nickel reactors. This good agreement indicates that prefluorinatioll of the sodium fluoride in the nickel reactor runs apparently has no effect upon the reaction rate, since prefluorination was not employed when the helix reactor was used. Comparison of the curves of f(A) arid f ( B )versus time, Fig. 1, shows that the reaction follows the parabolic rate law very closely. However, the straight line in thef(B) plot does not pass through the origin. The same behavior is shown with the nickel reactor data (Fig. 2). The!@) fuiiction exhibits a large initial slope followed by a gradual reduction to a constant value. Examination of the basic differential form of the parabolic equation shows that as 2 approaches zero, the rate, dxldt, approaches infinity, which is impossible. Thus, deviations from the parabolic equation are to be expected at very low values of 2. Correction for this, by use of a modified expression, vix., dz/dt =
'
.
April, 1958
LIQUID-LIQUID
SOLUBILITY OF PENTAERYTHRITOL TETRAPERFLUOROBUTYRATE 481
+
'
a / ( l bx), as suggested by Mott,' still does not account for the large initial slope which is obtained experimentally. Hence, the results imply a faster initial rate of attack. It is probable that rapid chemical reaction or physical sorption occurs on the initial surface by a different mechanism, which prevails until a sufficient amount of surface is covered by the complex. Diffusion through the film layer then becomes rate-determining with parabolic dependency. The parabolic law does not take into account the initial surface reaction, and therefore cannot be expected to account for the early stages of reaction.* Additional evidence for a dual mechanism was obtained in a separate experiment. Sodium fluoride was allowed to react for three hours in the helix reactor a t room temperature, the excess uranium hexafluoride was removed, nitrogen admitted to a pressure of 5 p.s.i.g., and the system allowed to remain intact overnight. Upon re-exposure to (7) N. F. Mott, Trans. Faraday ~ o c . 36, , 1 (1940). J. Gregg, "Surface Chemistry of Solids," Reinhold Publ. Corp., Inc., New York, N. Y.,1951,Chapter XIV. (8) S.
uranium hexafluoride, a marked increase in initial rate was observed above that which would be expected by the parabolic rate law and similar to that observed in the beginning of a run. After continued reaction, the data again followed the parabolic law, with the slope of the f ( B ) versus time plot exactly equal to that obtained in the first threehour reaction period. This renewed activity of the sodium fluoride shows that the diffusion of uranium hexafluoride to the interior of the particle is the rate-determining step which kinetically follows the parabolic law. Since diffusion is the ratedetermining process, the uranium hexafluoride surface concentration is higher than that in the particle itself. Under this concentration gradient, uranium hexafluoride apparently migrated to the particle interior during the overnight standing. This process partially renewed the surface for subsequent rapid reaction with uranium hexafluoride. A similar occurrence has been observed for the oxidation of copper.9 (9) J. B. Brown, M. Dole and G. A. Lane, J . Chem. Phys., ST, 251 (1957).
LIQUID-LIQUID SOLUBILITY OF PENTAERYTHRITOL TETRAPERFLUOROBUTYRATE WITH CHLOROFORM, CARBON TETRACHLORIDE AND OCTAMETHYLCYCLOTETRASILOXANE BY K6z6 SHINODA' AND J. H. HILDEBRAND Contribution from the Department of Chemistry of the University of California, Berkeley, California Received December 8S, 1967
We have determined liquid-liquid solubility curves for mixtures of (a) CHCl,, (b) CCl4, and (c) c-(CHa)sSiaOa with (Cg7COOCH&C in order t o show the effects of great disparity in molal volumes. The molal volume of the pentaerythritol ester a t 25' is 542 cc., the others are (a) 81 cc., (b) 97 cc., (c) 312 cc. The critical temperatures and compositions, the latter expressed as mole % of (C3F,COOCHz)4C,are, respectively, (a) 43.5', 7.3; (b) 72.1", 9.1; (c) 123.5', 30.7. The critiFal compositions in the very unsymmetrical systems (a) and (b) agree well w t h an expression derived from a regular solution equation in which the entropy term is based upon mole fraction, not volume fraction.
The extraordinarily large molal volume of pentaerythritol tetraperfluorobutyrate, 542 cc. at 25", together with its compact structure, lends it unique value for studying the extent to which the thermodynamic properties of solutions are affected by disparity of the coniponeiits in molal volume alone in the absence of pronounced configurational factors. We have found already that the solutions of iodine in this liquid1&as well as in octamethylcyclotetrasiloxane,2 molal volume 312 cc., show no additional entropy of solution attributable to this disparity. The present investigation was uadertaken in order to learn whether this conclusion would be confirmed in the case of highly unsymmetrical liquid-liquid mixtures. Experimental The pentaerythritol tetraperfluorobutyrate, (C3F7COOCH&C, was obtained from Minnesota Mining and Manufacturing Company through the kindness of Dr. N. W. Taylor. It had been vacuum distilled a t ca. 0.5-2 mm. and (1) Department of Chemistry, Yokohama National University, Minamiku Yokohama, Japan. (la) K6z6 Shinoda and J. H. Hildebrand, THIBJOURNAL, 62, 295 (1958). (2) K6z6 Shinoda and J. H. Hildebrand, ibid., 61, 789 (1957).
160'. I t s refractive index was 1.3340 a t 25'. Its densitv has been determined to be: 20°, 1.703; 25', 1.699; 44.4', 1.6620; 59.2', 1.6357. The octamethylcyclotetrasiloxane, ( CH3)&3ia04,was pure material furnished by the General Electric Company through the kindness of Dr. R. C. Osthoff. It was dried over CaClz and vacuum distilled shortly before use in order to remove small amounts of polymer b.p. 66' a t 14 mm. The carbon tetrachloride, obtained from Eastman Organic Chemicals, "Spectro Grade," was dried over silica gel and distilled; b.p. 76.52" at 760 mm. The chloroform, "Baker Analyzed" Reagent, was shaken with mercury, washed successively with dilute sulfuric acid, aqueous potassium carbonate and water and then dried over CaClz with protection from light and distilled; b.p. 61.2' at 755 mm. Various amounts of the degassed ester were weighed into tubes 6 mm. in diameter and 12 cm. long. The second liquid was then added, the contents frozen, evacuated, sealed and reweighed. Consolute temperatures were observed by repeated heating and cooling while shaking the tubes. Successive observations agreed within 0.05' near the top of the liquid-liquid curves, and within 0.2" on the sides. The values a t 25' for the mixture with CCla were obtaiied by eva orating the volatile CCla a t 90" from weighed portions ofeach phase.
Results The results are shown in Table I and plotted in
