Oct., 1953
EXCHANGE REACTION OF HYDROGEN AND TRITIUM
723
THE EXCHANGE REACTION OF HYDROGEN AND TRITIUM1v2 BY LEONM. DORFMAN AND H. C. MATTRAW Contribution f r o m the Knolls Atomic Power Laboratory, The General Electric Company, Schenectady, N . Y . Received February 19, 1866
The rate of exchange of hydrogen and tritium, initiated by the tritium @-radiation,has been investigated a t room temperature. Over the range of conditions studied, half-times of from 260 to 818 minutes have been obtained. G values for the initial rate of formation of tritium hydride range as high as 450 molecules/100 e.v., indicating the reaction to be a chain urocess. The initial rate was found t o be directly to the total pressure and to the square-root of the absorbed _ proportional _ &intensity. for the analyses. Sensitivity calibrationse showed that hyIntroduction drogen and tritium have equal mass spectrometric sensitiviThe exchange reaction of hydrogen and deu- ties. A test of a H2-T2 sample at approximately 50 microns terium has been the subject of a number of previous pressure in the expansion bulb of the mass spectrometer investigations. Extensive studies of the rate of the showed that the composition of the analytical sample remained constant for over half-an-hour, a considerably thermal exchange reaction have been made by longer time than that required for a n analysis. Some six to Farkas and Farkas3 and more recently by Van ten analyses were done on each run over a period of 10 to 40 M e e r ~ c h e . ~These studies have established that hours depending on the composition of a particular run. the mechanism of the thermal exchange is a chain Results and Discussion process involving atom-molecule interactions. The progress of the reaction, as in the thermal Mund, et CZZ.,~have reported results of the radiationinduced exchange of hydrogen and deuterium H2-D2 exchange13may be represented by the expoinitiated by a-particles from radon. I n all these nential equation cases analyses have been carried out by the thermal (HT), - (HT)t = (HT)(1) conductivity method. where (HT)t and (HT), denote the boncentrations We have investigated the rate of the hydrogen- of tritium hydride at time t and at equilibrium, tritium exchange respectively. The concentration of HT is not zero a t the start of a run since the initial tritium conHz + T2 = 2HT tained some 10% of tritium hydride. The conThis reaction, initiated by the tritium ,&radiation, stant k is fixed for a given run by the composition has been studied a t room temperature using a mass and pressure. (HT) ,has been determined experispectrometer as the analytical instrument. mentally6 for a number of compositions and pressures. The experiment,al values are slightly higher Experimental A series of runs has been carried out extending over a than the theoretical values. The differences are of range of total pressures from 59.0 to 399.9 nim. with tritium significance in the determination of the equilibrium pressures between 33.2 and 147.7 mm. Hydrogen was puri- constant; their effect is negligible in the calculation fied by absorption on a uranium bed followed by pumping of exchange rates based on equation I. The caland subsequent desorption. The hydrogen was stored over culated value of the equilibrium constant is 2.57 a Fugassi valve to avoid any contamination by grease. Tritium, containing approximately 95% tritium and 5% a t 25". We have obtained6 an experimental value hydrogen, was separated from impurities prior to each run of 2.87 f 0.06. by diffusion through a palladium thimble. The validity of equation I in representing the Runs were carried out in spherical Pyrex bulbs with a tot$ progress of the exchange may be seen from Fig. 1 volume of 109.5 f 0.5 cc., at a temperature of 28.0 f 1.0 . Each bulb was fitted with two vacuum stopcocks in series which shows a plot for some of the runs of 1 enclosing a capillary lock to permit removal from time to (HT)t/(HT), vs. time in minutes, on a semitime of a small analytical sample. The volume of the cap- logarithmic scale. The rate of exchange a t time illary lock was approximately 0.18 cc. so that removal of a t is then given by sample for analysis resulted in a negligible pressure decrease in the reaction bulb. Fluorothene grease was used as the lubricant to minimize any exchange or interaction with the tritium. A new reaction bulb was used for each run. Prior to the run the bulb was pumped out and thoroughly degassed for several hours after which tritium was introduced by means of a Toepler pump. The run was started by adding hydrogen. The bulb was then connected to the sample manifold of the mass spectrometer by means of a ground joint lubricated with Fluorothene grease. The gas present in the capillary lock at the start of the run was discarded and analytical samples were taken from the reaction bulb. A General Electric analytical mass spectrometer was used (1) The Knolls Atomic Power Laboratory is operated by the General Electric C o . fos the Atomic Energy Commission. The work reported liere was cariied out under Contract No. W-31-109 Eiig.42. (2) Presented at the 122nd meeting of the Ameriran Chemical Society in Atlantic City, N. J., September, 1952. (3) A. Farkas and L. Faikas, Proc. Roy. Soc. ( L o n d o n ) , 152A, 124 (1935). (4) M. Van Meersole, Bull. soc. china. Belg., 60, 99 (1951). (5) W. Mund, T . de Menten de Hoines and M. Van Meersche, ibid., 56, 386 (1947).
and the true initial rate of HT formation is given by = t-0
The half-time,
7,of
k(HT),
(111)
the reaction is given by 7
=
O.G93/k
(IV)
where 1; is obtained from the slope of the straight lines in Fig. 1. The values of k and the half-times in minutes are shown in Table I in which the data for the series of six runs are tabulated. A correlation of the exchange rate as a function of pressure and absorbed pintensity is dependent on a knowledge of the fraction of the P-energy absorbed by the gas in the spherical reaction bulb. The (6) H. C. Mattraw, C. F. Paohuoki and L. M. Dorfman, J. Chem. Phus., 20, 926 (1952).
724
LEONM.
DORFM-4N AND
H. c. MATTRAW
VOl. 57
extrapolation of the half-thickness curve, which is very nearly linear a t the low-energy end, give a value of p / p = 4.7 cm.2/mg. for tritium @-particles in aluminum. Examination of the range energy curve12 (which requires no extrapolation) on the basis that the half-thickness is one-tenth the range, indicates that this estimated value for p / p is a t least of the right order of magnitude. The mass absorption coefficient for different absorbers is very nearly proportional to Z / A . This gives a value, for nuclear @-particles in hydrogen, of ( p / p ) ~= 2.1(P/p)Al, and in tritium of ( P / ~ ) T = 0 . 6 9 ( ~ / ~ ) ~Calculation 1. of this ratio for hydrogen on the basis of the Bethe-Block e q ~ a t i o ishows i ~ ~ ~agreement ~~ within 19% with the value based on proportionality in Z / A . Comparison16 of calculated with experimental values for absorbers of low atomic number indicates that the absolute values agree within 209.’. The fraction of the &energy absorbed has therefore been calculated from
h+ =1ZO
percentage absorption has therefore been estimated in the following manner. Tritium has a half-life of 12.46 years’ with a @having an end-point energy of 18 kev. and an average energy of 5.69 kev. The mass absorption coefficient for nuclear 6-particles in aluminum is given by the equationlo
for 0.1 < E,
1 2 3 4 5 6
50.0 119.3 123.7 213.2 295.4 399.9
p/p =
(V)
22/Eo1ea3
< 3.0, EObeing the end-point energy in
46.1 64.6 81.6 33.2 147.7 133.6
0.320 .454 ,418 ,248 ,458 ,411
8.47 12.6 13.3 13.7 19.5 26.7
818 550 522 504 355 260
e-(N/p)x
(VI)
where x is the absorber thickness in mg./cm.2. The average energy, 5.69 kev., has been used in calculating the amount of absorbed energy. The source of radiation in this case is uniformly distributed throughout a spherical bulb. Lind16 has calculated that the average distance from all points within a sphere t o the walls is 0.814 times the radius. The diameter of the reaction bulbs is 5.8 em., so that the average path length through the gas is 2.36 cm. The initial rates of formation of tritium hydride are shown in col. 7 of Table I. Column 8 lists the estimated fraction of the incident @-energy absorbed for each run. G-Values for the initial rate of formation of HT have been calculated and are listed in the last column. It is evident from these G-values which range as high as 450 molecules/100 ev. (corresponding to an ion-yield of 150 molecules/ ion pair) that the exchange reaction involves a chain process.
8.55 36.4 36.7 38.7 140.6 233.9
0.13 .25 .26
.-lo
.51 .62
1.52 1.95 1.65 1.28 1.41 1.65
220 350 270 450 200 440
hlean 1.58
mev.; or it may be obtained from the half-thickness curvell for nuclear @particles. Equation V and (7) 0. 11. Jeiiks, F. N. Sweeton and J. A. Gliorrnley, Phys. Rev., 80, 990 (1950). ( 8 ) G. C. Hanna and IJ. Pontecorvo, ibid., 75, 083 (1949). (9) E. R. Graves and 1).I . hleyer, i b i d . , 76, 183 (1949). (IO) R. D. Evans, “The Science and Engineering of Nuclear Power,” Vol. I, Addison-Wesley Press, Inc., Cairihridge, Mass., 1947, p. 53. (11) C. D. Coryell and N. Sugariiian, “Radiocheiiiical Studies: The Fission Products,” McGraw-Hill Inc., Xew York, N . Y . , 1951, Book 1 , p. 18.
It may be shown from these data that the initial rate of exchange is proportional to the total presmre and to the square root of the absorbed @intensity. This relationship ( 1 2 ) L. E. Glendenin, Nucleonics, 2 (No. 1). 12 (1948). (13) H. Bethe, 2. Pkusik, 76, 293 (1932). (14) I i . Block, ibid., 81, 363 (1033). (15) G. L. Brownell, “Conference on Absolute Beta-Counting,’’
Preliiiiinary Report No. 8, Nuclear Science Series, Paper 6 (1950). (16) S. C. Lind, J . Am. Chem. Soc., 41, 531 (1919).
*
F
Oct., 1953
KINETICS OF SILVER BROMIDE WITH
may be tested in the following manner. equations 111and VI1 k(HT), = K(I,bB)*/2P
From (VIII)
and since the incident &intensity is direct>lyproportional to the tritium concentration this may be written Where A ~ H Tis~the mole fraction of HT a t eqnilibrium. The values of K' obtained in this correlation are shown in col. 9 of Table I. Reasonably good constancy obtains over a wide range of pressures and absorbed intensities. The mean value is K' = 1.58 X (min.-l rnole-'/z liter-'l2). The individual values have an average deviation of 0.17 X or a standard deviation of 0.23 X 10-2.
The kinetic expression for this radiation-induced exchange differs from that obtained in the thermal El2-D2 exchange,3 in which the order of the reaction was found to be 3/2,in that P< in'the latter case is replaced by This difference stems from the different primary processes. In the thermal cxc'hange reaction the primary dissociation at a given temperature is pressure dependent. In the H2-T2 exchange a t room temperature the primary
6.
AN
ADSORBED LAYEROF ALLYLTHIOUREA
725
dissociation is dependent on the radiation intensity, or in other words on the tritium pressure. The square-root dependence in both cases indicdtes that the exchange proceeds by way of atommolecule interactions. The mechanism may be written Hz -w+ 2H Tz -w+ 2T H T -w+ H H Tz IIT T €12 I _ IIT 2H +Hz 2T ---t Tz H+T+HT
+ +
(1)
+T +T
+H
(2) (3) (4) (5) (6) (7) (8)
The exchange occurs chiefly by way of reactions (4) and (5) rather than react,ion (8) as indicated by the high ion yields. The ion yield is inversely proportional to the square-root of the radiation intensity. Mund, et a1.,5in one of their two runs have obtained an ion yield for H D formation greater than lo3. This may be explained by the fact that their ionization intensity was on the order of 1/200th of the intensity in these H2-T2runs. Acknowledgment.-We are indebted to Mr. C. F. Pachucki for his assistance in carrying out the mass spectrometric analyses, and to Dr. D. L. Douglas with whom we have had many helpful discussions.
KINETICS OF THE REACTION BETWEEN SILVER BROMIDE AND AN ADSORBED LAYER OF ALLYLTHIOUREA BY T. H. JAMES AND W. VANSELOW Communication No. 1544 f r o m the Rodak Research Laboratories Received February $6, lS6d
Allylthiourea is strongly adsorbed by silver bromide to form a monolayer. The adsorbed allylthiourea prevcnts adsorption
of certain dye ions, e.g., phenosafranin and 3,3'-diethylosacarbocyanine, but the dyes are rapidly adsorbed after reaction tJetween the allylthiourea and silver bromide occurs. Use is made of this fact to measure the rate of reaction in the adsorbed
layer. The rate of reaction varies as the logarithm of the reciprocal of the bromide ion concentration in the surrounding solution. The rate varies as the 1.5 to 2 power of the hydroxyl ion concentration, depending on the bromide ion connentrat)ion. The reaction curve is not autocatalytic in shape, but reaction occurs faster on a silver bromide surface which contains sulfide, either from previous reaction with allylthiourea or from reaction with sodium sulfide. Chromatographic experiments indicate that the surface after reaction with one layer of allylthiourea is intermediate in adsorpt,ive properties between a pure silver bromide surface and a silver sulfide surface, but more closely resembles the former. The rate of reaction of subsequently adsorbed layers of allylthiourea continues to increase with increasing amount of sulfide formed in previous reaction, far beyond the amount of sulfide which corresponds to a uniform layer one molecule thick. The rate of the sulfide-catalyzed reaction varies as the first power of the hydroxyl ion concentration. The over-all energy of activation of the uncatalyzed reaction is 32 kcal./mole in the absence of excess bromide, and 40 kcnl./mole in the presence of 0.0001 d l bromide ion. The activation energy of the sulfide-catalyzed reaction is 22 kcal./mole. The mechanism of the reaction is discussed on the basis of the kinetic results. The rate of reaction of derivatives of thiourea a t pH 7.2 increases in the order: l,&diethyl, 1,1dimethyl, methyl, (thiourea), allyl, phenyl, acetyl. 1,1,3,3-Tctramethylthioureais inactive,
Introduction Thiourea and many of its derivatives are strongly adsorbed by silver bromide. The adsorbed layer prevents adsorption of certain dye ions, such as the phenosafranin and 3,3'-diethyloxacarbocyanine cations. Under suitable conditions of temperature and pH, the adsorbed thioureas react with the silver bromide to form silver sulfide, and the surface l'ormed by this reaction very rapidly adsorbs the
dye cations. Use has been made of these facts to study the kinetics of the reaction of adsorbed thioureas, particularly allylthiourea, with silver bromide. Probably the technique can be extended to other reactions where the adsorptive properties of the reaction product differ markedly from those of the adsorbed reactant layer. The present investigation was undertaken because of the importance to the theory of photo-
