D. T. PETERSON AND D. G. WESTLAICE
1514
COMPARISON OF COVALUES OF N-ALKYLRESINSWITH N-Alkyl group
% qiiater-
CHs CaHb n-CsH7 n-C4H9
61 43 27 27
nized
&* 3.55 2.83 2.00
2.00
TABLE 111 DOWEX1-X8 OF
THOSE OF
Ethylene gl col
Methanol
cO*
&/cQ*
co*
CQ/CQ*
0.55 .42 .33 .33
1.0 1.6 1.3 1.2
0.56 .5L .40 .40
1.0 1.7 1.3 1.3
hydrophilic alcohols. With the less hydrophilic pentanone, the value of Co/Co* rises somewhat erratically with increasing size of the N-alkyl. This is probably due to the greater van der Waals forces with the resins of large N-alkyl groups. Acknowledgments.-The authors express their
THE
Vol. 63
SAMEPERCENTAGE QUATERNIZATION
Butanone CQ* CQ/CQ*
1.3
1.0 1.1 O.0G 1.2
1.0
2.4 2.4
Pentanone-2
*
CQ
2.5 3.3 5.7 5.7
cQ/cO* 1.0 1.1 0.95 1.5
gratitude to The Dow Chemical Company for generous financial support of this investigation and for the preparation of the special resins. The receipt from U. P. Strauss of the polysoap used in the determination of the interstitial volumes is also gratefully acknowledged.
THE RATE OF REACTION OF HYDROGEN WITH THORIUM1 BY D. T. PETERSON AND D. G . WESTLAKE Institute for Atomic Research and Department of Chemistry, Iowa State College, Ames, Iowa Received April dd, 1959
The reaction between thorium and hydrogen, which produced a surface layer of thorium dihydride, was shown to follow the parabolic rate law. At pressures slightly greater than the dissociation pressure of the dihydride, the absorption rate was very dependent on the pressure, but a t higher pressures the pressure dependency was less pronounced. I n c r w i n g the temperature accentuated the pressure dependency of the absorption rate a t the higher pressures. The temperature dependence of the absorption rate satisfied an Arrhenius type equation a t temperatures below 550" when the pressure was held constant at 120 mm. The activation energy for diffusion was found to be about 19.6 kcal. The absorption rate was the same for annealed thorium of two purity levels and for cold-swaged thorium.
Introduction Thorium reacts with hydrogen to form ThHz and Th4HIS. These compounds are intermediates in one process for preparing thorium metal powder and may have some use in nuclear reactions. The formation of a thorium hydride phase seems to be involved in the corrosion of thorium by aqueous media. However, no systematic study of the rate of reaction of hydrogen with thorium has been reported. The reaction between thorium and hydrogen would be governed by different rate-controlling processes depending on whether the hydrogen pressure was above or below the dissociation pressure of thorium dihydride. If the hydrogen pressure was above the dissociation pressure of the dihydride, the metal would acquire a dihydride layer through which the hydrogen must diffuse. If the dihydride layer were adherent and protective, the parabolic rate law would be expected. If the hydride layer were non-protective, a linear reaction rate would be observed. If the hydrogen pressure was below the dissociation pressure of the dihydride, nbsorption would occur by the solution of hydrogen in thorium and the rate would depend on diffusion into the metal if surface reactions were not rate controlling. I n this investigation, the reaction rates were determined over a temperature range from 350 to 700" a t pressures from slightly above the dissociation pressure of the dihydride up to 400 mm., but below the pressure range in which Th4Hlb is stable. (1) Contribution No. 743. Work was performed in the Amen Laboratory of the U. 8. Atomic Energy Commission.
Experimental Thorium prepared by different methods and of two purity levels was used. The crystal bar thorium was prepared at the Ames Laboratory from calcium-reduced Ames thorium. Ames thorium was repared by reducing ThF, with calcium. This metal was mexed in a B e 0 crucible under vacuum and cast into a graphite mold. The analyses are shown in Table I . Carbon was determined by combustion and nitrogen by the Kjeldahl method. The oxygen in Ames thorium was determined by weighing the hydrochloric acid in soluble residue as thorium oxide, whereas the oxygen in the crystal bar thorium was determined by vacuum fusion. All other elements were determined spectrographically.
TABLE I ANALYSIS OF THORIUM ~ ~ E Element
C
N 0 Fe Mn Cr Xi AI Ca Mg Si Be Hardness
Crystal bar Th, p.p.m.
145 70 60 20
.. 20
20 50 25
..
.. .. 35 Dph
T A L
Ames Th, p.p.m.
410 70 1300 130 20 20 20 40 50
20 55 170 69 Dph
The thorium was swaged into rods and vacuum annealed at 800' for one hour. Cylindrical specimens, approximately 6.5 mm. in diameter and 3.5 mm. long, were machined after the chuck and the cutting tool of the lathe had been cleaned with trichloro-ethylene. Immediately after the specimens had been washed in trichloroethylene and rinsed in acetone, they
1515
RATEOF REACTION OF HYDROGEN WITH THORIUM
Sept., 1959
0 04
-
'2
I
003
7'
E,
-
D
I
y
002
001
I
I
5
I IO
I
15
I
20 TIME
Fig. 1.-Amount
I
25
I
I
35
I
40
I 45
I
M
I
55
I I 60
(SeC!?.
of hydrogen absorbed us. square root of time.
were inserted into the loading tree of the reaction apparatus. The thorium samples were inserted into a furnace tube at constant temperature and under constant hydrogen sure. The apparatus was evacuated with a mercury sion pump and degassed until a static vacuum of less than I X 10-6 mm. was maintained after the stopcock to the diffusion pump was closed. The furnace tube was enclosed in a stainless steel tube which was 10 cm. long. This provided a zone 7.5 cm. long in which the temperature was constant to within 2' and in which the sample was centered. The temperature was measured by a thermocouple between the stainless steel cylinder and the furnace tube. The pressure in the system during degassing was measured with a cold cathode gauge. Purified hydrogen was generated by heating uranium hydride to about 400' and was stored in the pressure regulator. This device was a modification of that described by Belle, et aZ.,2 and maintained a constant pressure of hydrogen in the sample tube. Hydrogen was allowed to enter the system to the desired pressure, and a specimen was pushed into the sample tube. The reaction rate was observed by measuring the quantity of hydrogen drawn from the pressure regulator at known times. Liquid nitrogen cold traps were used to prevent mercury from the manometer, diffusion pump and pressure regulator from reaching the furnace tube.
Fig. 2.-Parabolio rate constant squared us. the pressure difference across the hydride layer.
Results and Discussion The absorption of hydrogen by thorium followed the parabolic rate law. Typical absorption curves are shown in Fig. 1. The failure of these curves to pass through the origin was due to the time required for the sample to reach the temperature of the furnace. No induction period was observed and the samples began to react with hydrogen as soon as they entered the furnace, but the constant parabolic rate was not obtained until the temperature of the sample reached the furnace temperature. The reaction rate was measured until between 10 and 30% of the thorium sample had reacted to form the dihydride. The reaction rate was not followed to completion since some deviation from the parabolic rate law would be expected when the thickness of the hydride layer became great enough to reduce significantly the thoriumthorium hydride interface area and change the gradient a t the corners of the sample. However, this deviation would not be due to a change in the rate-controlling process. The parabolic rate constant is related to the diffusion coefficient and the concentration gradient of hydrogen in the hydride layer. The parabolic
Fig. 3.-Log
-
&?-
(2) J. Belle, B. B. Cleland and M. W. Mallett, J . Electrochem. SOC., 101, 211 (1854).
I
I
1.2
13
I
1410s,1.5
I
I
I6
1.7
TK
k*
us. 1/T a t 120 mm. hydrogen pressure.
rate law can be expressed as m = kt'/a where m = weight of Hz absorbed per sq. cm. of surface, k = rate constant and t = time. The flux through the gas-solid interface, J , is dm/dt and if Fick's first law applies, J = - D (dcldx) = 1/2 kt--l/a. The concentration gradient in the direction of the flux is - (C, = Ci)/Ax = - AC/Ax if the gradient is constant through the surface layer. C, is the hydrogen concentration of the hydride at the gashydride surface, Ci is the hydrogen concentration a t the metal-hydride interface and Ax is the thickness of the hydride layer. As nearly all of the hydrogen remained in the hydride phase, Ax = Km where K is the volume of ThH2 which contains one mg. of hydrogen. Consequently Solving for the rate constant gives k 2 = 2 D A C / K . The hydrogen pressure dependence of the reaction rate arises from the change in AC as the hydrogen concentration at the gas interface increases with pressure. At 550" and below, the reaction rate was nearly independent of pressure above 100 mm. The values of k2 at several temperatures are plotted in Fig. 2 against the difference between the hydro-
NOTES
1516
gen pressure over the sample and the dissociation pressure a t the metal-hydride interface. The latter pressure was obtained from the pressurecomposition data of Mallet and C a m ~ b e l l . ~These curves must pass through the origin because the reaction must cease when the pressure of hydrogen over the sample is equal to the dissociation pressure of thorium dihydride. The pressure dependence of the reaction increased with increasing temperature. This behavior might be predicted from the hydrogen pressure-composition isotherms since the composition of the hydride phase changes more with pressure as the temperature increases. The diffusion constant often follows an Arrhenius type equation D = Do e-AH/RT. As the pressure dependence of the composition of thorium hydride was not known, the reaction rate could not be measured a t constant concentration difference across the hydride layer to determine the temperature dependence of the rate. However, log lc2 was plotted in Fig. 3 against the reciprocal temperature for runs a t a constant pressure of 120 (3) M. W. Mallett and I. E. Campbell, J . A m . Chem. Soc., 73,4850 (1951),
Vol. 63
mm. The apparent energy of activation obtained from this curve was 19.6 kcal. This value may be slightly larger than the activation energy for diffusion if the concentration difference increased over this temperature range. The rate of reaction of A m e s thorium was the same as that of crystal bar thorium. The difference in purity was not important in this reaction. Cold swaged samples of crystal bar thorium also gave the same reaction rate as annealed samples under comparable conditions. An appreciable induction time and a linear rate of reaction of hydrogen with thorium was reported by Straetz and D r a l e ~ . ~ The linear rate was undoubtedly due to formation of Th4Hla which is stable a t one atmosphere hydrogen pressure over most of the temperature range of their study. With careful handling of the specimens and thorough outgassing of the system, no induction time was detected in the present investigation. The induction period was probably caused by a thin film of contamination. (4) R. P. Straetz and J. E. Draley, "A Study of the Reaction Rate between Thorium and Purified Hydrogen," U. 9. Atomic Energy Commission Report CT-3045, 9145.
NOTES DECOMPOSITION OF ZINC OXIDE BY ZINC
VAPOR BY WALTERJ. MOORE A N D E. L. WILLIAMS
small that their apparent differences are not much outside the range of experimental uncertainty, but in the case of zinc vapor a greatly enhanced rate is clearly evident.
Chemical LabOTatOTy, Indiana University, Bloomington, Indiana Received February 6 , 1969
I n 1889 Morse and Whitel reported that the oxides of zinc and cadmium dissociated in the presences of the respective metals. They placed some zinc oxide and zinc in the end of a horizontal tube divided by a small dam. The tube was evacuated and heated. Just after the fusion of the zinc, they were able to collect some oxygen. The zinc evaporated and condensed beyond the dam, but as soon as some zinc collected there, the zinc oxide evaporated again and condensed further along the tube. After all the zinc had distilled, no more of the zinc oxide was found to evaporate. We have made some quantitative measurements of the rate of evaporation of zinc oxide in flowing streams of oxygen, nitrogen, and zinc vapor in nitrogen. Three single crystals of zinc oxide (from 10 to 30 mg. each) were mounted in the ends of 1-cm. lengths of quartz capillary tubing, which were placed in a lavite holder inside a resistance furnace regulated to f2'. The crystals were close to the mid-point of the furnace cross-section and mid-way from the ends of the furnace. One end was open and the other served as the entry for the flowing gases. The crystals were removed a t intervals and weighed to determine their changes in weight. The results shown in Table I are the means of two or three quite concordant runs.
These data show that the rate of evaporation of ZnO in a stream of Zn vapor is about 100 times that in a stream of Nzunder the conditions cited a t 1030". The weight losses in O2 and N:! were so (1) € N. I. Morse and J. White, Jr., A m . Chsm. J , , 11, 258 (1889).
TABLE I EVAPORATION RATESOF ZINCOXIDE Gas flowing 0 2 0 2 0 2
Nz Zn
Time, Flow rate, Temp., hr. mmples/ OC. min.
30 46 67 185 52
980 980 1100 1030 1030
3.1 6.5 3.1 4.0 0.6
Rate of evaporaCalcd: tion of ZnO, equilibrium m./hr. rate
2 . 2 A 1.1 2 . 2 k 1.0 3.5+2.3 1.1 A O . 0 120& 10
0.03 0.065 1.5 110 lo-?
The standard free energy change for the reaction ZnO(c) = Z n k )
+
+ 1/2 O d d
is AGO = - 114640 51.657' (ca1.).2 Hence we can estimate the equilibrium constant Kp = PZnP02'". If we assume that the flowing gas is completely saturated with the equilibrium concentrations of zinc and oxygen, we estimate the evaporation rates given in the last column of Table I. The rate in Nz is only about 1% of the equilibrium rate, the rate in O2a t the highest temperature is still somewhat above the equilibrium value, but the rate in zinc is about lo9times the equilibrium value. The first hypothesis that would serve to explain the enhanced decomposition of ZnO in Zn vapor would be the occurrence of a volatile suboxide of (2) J. A. Kitchener and S. Ignatowicp, Trana. Faradav Soc., 47, 1278 (1951).
. a
