1542
G. LEMAIRE, C. FERRADINI, AND J. PUCHEAULT
crystals1' and BrOH- and IOH- from frozen alkali halide solutions. l8
atories, Christie Hospital and Holt Radium Institute, for providing facilities.
Acknowzedgment' The authors thank the staff Of the Department of Physical Chemistry, Hebrew University of Jerusalem, Israel, and of the Paterson Labor-
(17) R. C. Catton and M. C. R. Symons, J . Chem. Soc. A , 446 (1969). . . (18) I. Marov and M. C. R. Symons, ibid., 201 (1971).
Radiolysis of Water by Tritium P Rays: Scavenging of Hydrogen Peroxide Precursors by G. Lemaire, C. Ferradini, and J. Pucheault* Centre nationale de la Recherche Scientifique, 94 Ivry, France
(Received October 18, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
The influence of OH radical scavengers on the yield of HzOz formation in 0.4 M HzS04 solutions irradiated by tritium 6 rays was measured. Comparison with the results obtained with y rays shows differences which can be explained by the increase in the number of "short tracks" (in the terminology of Mozumder and Magee).
Introduction I n an earlier study,I we measured the initial yield of hydrogen peroxide formation in aerated 0.4 M HzS04 solutions irradiated by tritium P rays. The value obtained, Go(H202)= 1.5 f 0.05 molecules/100 eV, was in agreement with the molecular and radical yields determined by Collinson, Dainton, and Kroh,2that is: GH = 2.9, G H O= ~ 0, GOH = 2.0, G H ~ = o ~1.0, and G H = ~ 0.55. We felt it would be interesting to study the inhibition of molecular products by appropriate scavengers in order to see if this inhibition could be compared to that of y rays. From this point of view, the use of bromide and chloride ions which inhibit the formation of H202 in the presence of air is particularly well suited. The mechanism of their action has been clearly established notably by Sworski3 as due to the occurrence of halide r e a c t i o n ~i.e. ,~ OH+S--+S+OH-
(1)
S + HzOz ---t S- + H+ + HO2
(2)
in competition with those in the bulk of the solution OH
+
H202 ---t
H HO2
+
0 2
+ HO2
HOz
+ HzO
+HO2
+H2Oz
+
(5)
which do not change the formation yield of hydrogen (2) is equivalent to (3) peroxide since (1)
+
The Journal of Physkal Chemistry, Vol. 76, No. 11, 197.2
=
+
G H ~ o ~ '/z(GH
- GOH)
Using the equation of material balance ( ~ G H ~4-o ~ GOH= ~ G H GH), ~ this yield can be expressed in t e r m of the molecular yields
+
G(Hz0z)
=
~ G H~ G O H~ ~
(1)
On the other hand, when S- ions are present in high ~ ~ ~ concentrations, it is recognized that the G H molecular through the competition, yield is diminished by AGI.I~o~ in zones of high radical densities, of reaction 1 with the local formation of hydrogen peroxide OH
+ OH +HzO2
(6)
The fate of S, after diffusion, being identical with that described previously (reaction 2), it is as though supplementary OH radicals were recuperated by reaction 3, and the variation of the final yield can be reached by differentiating expression I, that is AG(H2oz)
=
~AGH~oZ
(11)
provided that the G H 2 yield is not modified by this heterogeneous process which can thus be expressed by
(3)
(4) 0 2
G(Hz0z)
(1) G. Lemaire and C. Ferradini, Radwchem. Radioanal. Lett., 5 , 175 (1970). (2) E. Collinson, I?. S. Dainton, and J. Kroh, Proc. Roy. Soc., 265, 422 (1962). (3) T.J. Sworski, J . Amer. Chem. Soc., 76, 4687 (1954). (4) E.J. Hart, Radiat. Res., 1, 53 (1954).
1543
RADIOLYSIS OF WATER BY TRITIUM p RAYS the corresponding decrease in the global yield of hydrogen peroxide. We thus used C1- and Br- scavengers in order to be able to compare these G H 2 0 1 inhibitions in the case of tritium p rays with those corresponding to other qualities of radiation. It is known that the hydrogen yield can be modified in high concentrations of halides and that in certain cases even the elemental halogen may Thus we have limited our study to a region of concentration ([Cl-] < 1M , [Br-] < low2M ), where these two phenomena do not interfere appreciably. * -lo
Experimental Section 1 . Irradiations by Tritium p Rays. Tritium is a pure p emitter with a half-life of 12.4 years, The maximum energy of the p rays is 18.6 keV while the median energy is 5.6 keV. We used tritiated water as “vector,” The maximum path length of tritium p rays in water is 6 p and wall effects were thus avoided. While there are no external irradiation problems, those of internal contamination are considerable since all the p rays emitted by a tritium source internal to the organism are totally absorbed by it. This consideration led us to performall the manipulations in aventilated glove box. All the chemicals used were of Merck manufacture. The Pyrex containers were washed with boiling aqua regia. They were then rinsed in ordinary water, then in monodistilled water, and finally in triple-distilled water. When they had been dried they mere put into an oven (600’) for 6 hr. The tritiated water came from CEN (Saclay). Its activity was about 1 to 5 Ci 0111-3. Using a volume of tritiated water of about 1-2 cm3, the following operations were performed: after a first dilution to 20 em3 (with triple-distilled water), four distillations were carried out: the first in the presence of potassium permanganate, the second in the presence of barium hydroxide; the third and fourth were simple distillations performed in a Pyrex bidistillator. After this treatment, the average conductivity of the water was around mho cm and its pH about 5.7, just after the end of the distillation. A second dilution with triple-distilled water whose conductivity was the same occurred when the active solutions were put in the Pyrex storage recipients. The analyses were performed on samples of 1 ems taken a t regular intervals, taking into account the activity of the different solutions. The quantitative determination of HzOzwas done by cerimetry. The 0.4 ill sulfuric solutions of ceric sulfate (-7 X M) are slightly unstable and in order to take into account the eventual variations of CeIV during the day, analyses were made each morning in two ways: (1) by spectrophotometry at 320 nm with E 5580 M-l cm-l, and
(2) by volumetry using a standard solution of ferrous ammonium sulfate. Doses were determined by Fricke’s method by adopting G(FelI1) = 12.7 as the oxidation yield during radiolysis by tritium p rays, a value established by Hart.4 I n order to do this, a tritiated 2 X loe3 M ferrous ammonium sulfate solution in 0.4 M H2S04 was used. The quantities of ferric ions that had appeared were measured by cerimetry on 1-cm3 samples of the solution taken at regular intervals. A constant rate of growth that leads to a dose rate of 5.68 X 10l6eV hr-’ was found in this particular case. 2 . Irradiations by Cobalt-60 y Rays. The same experiments were also carried out using ‘Wo as a source of radiation (source of 300 Ci). The dosimetry was done using Fricke’s method, taking G(FelI1) = 15.7 and determining FeIII concentrations by spectrophotometry.
Results The experiments show that the formation of H~OZ in aerated 0.4 M HzS04solutions is linear with dose for the concentrations of chloride and bromide ions used and depends on these concentrations. Table I summarizes all the results. Table I [KCll, M
0
5
x
5
x
10-4 10-3 10-2 10-2 10-1 10-1 1
[KBrl, M
0
5
x
10-4 10-8 10-3 10-2
Tritium 9, rays G(Ha0d
W o y rays G(Ha0z)
1.5 1.39 1.16 1.03 0.82 0.7 0.13 (0.1
1.20 1.13 1.08 0.94 0.87 0.70 0.53 0.18
G(Hz0d
(;I(HzOa)
1.5 1.28 1.04 0.83 0.58
1.20 1.12 1.01 0.81
One can see that, for irradiation by tritium rays, G(H2O2)is lowered from 1.5 to 0.13 molecules/100 eV when the concentration of chloride ions varies from 0 to 5 X lo-’ M . For a concentration of C1- equal to or greater than 1 M the inhibition is almost total. (5) A. R. Anderson and B. Knight, Proc. and Int. Congr. Radiat. Res., Harrogate, 57 (1962). (6) 8.A. Brusentseva and P. I. Dolin, Proc. All-Union Conf. Radiat. Chem., l s t , Moscow, 69 (2957). (7) A. M. Kabakchi, Russ. J . Phys. Chem., 30, 1906 (1956). (8) A. M.Koulkes-Pujo, A n n . Chim., 5, 707 (1960). (9) J. Pucheault and C. Ferradini, J . Chim. Phys., 58, 606 (1961). (10) A. Rafi and H. C. Sutton, Trans. Faraday Soc., 61, 509 (1965). The Journal of Physical Chembtry, Vol. 76, N o . 11, 1972
1544
G. LEMAIRE,C. FERRADINI, AND J. PUCHEAULT
4
log kc
Figure 1. Influence of halide concentration on G(H202): 0, with k c l - = 4 X 1Og M - 1 sec-1 for chloride; A, k ~ ? -= 1.6 X 1010 M-1 sec-1 for bromide.
G(H202) is lowered from 1.5 to 0.58 molecules/100 eV when the concentration of bromide ions increases from 0 to 10-2 M . I n the case of irradiation by e@Coy rays, G(H202) is lowered from 1.2 to 0.18 when the concentration of chloride ions rises from 0 to 1 M and from 1.2 to 0.81 for bromide concentrations increasing from 0 to
M. I n conclusion, both in the case of y rays and that of 0 rays, the identity of the mechanism for the two scavengers used is shown by the curve in Figure 1, which repreplotted against log sents the ratio G(H202)/G~(H202) ICC, where Go(H202)is the yield in absence of scavenger, C is the scavenger concentration at which the yield is G(H202),and IC is the rate constant of its reaction (1) with OH, that is, 4 X lo9 M-’ sec-l for C1- and 1.6 X lo1@ M-’ sec-l for Br-.ll It can be seen that, for each type of radiation, the points corresponding to the two halides, as we might expect, are on the same curve. Moreover the inhibition of the HzOz yield for tritium 0 rays i s greater than for y rays (see experimental points in Figure 2). We will now discuss these results in relation to theoretical and other experimental work.
Discussion Many mathematical analyses have been made to explain theoretically reactions which occur during the diffusion of radiolytic entities in the presence of solutes which scavenge these entities. These calculations have been treated notably by n/Iagee12and by Kupperman,la who suppose that initially the H and OH species are formed in sites that are microscopic in size and that during diffusion there occurs a combination between two identical radicals (reaction 6, for instance). This sort of model, called the “one radical model,” coupled with well chosen constants, gave results which are in reasonable agreement with experiments using y rays. I n this case, the sites where the radicals were produced-called “spurs”-were considered to consist on the average of isolated spherical zones which enclosed the tracks of secondary electrons of about 100 eV, thus having initial dimensions of about 10 A and The Journal of Physical Chemistry, Vol. 76, No. 11, 1978
8
6
10
1
log k c
Figure 2. Inhibition of G H ~ by o ~halides. Theoretical curves:
-, isolated spurs; cylindrical tracks: , 8.5 x 107 radicals/cm; - - -, 34 X l o 7 radicals/cm. Experimental points: tritium p rays: 0, C1-; 0, Br-; GoCo y rays: X, C1-; f, Br-.
containing about ten radicals. Schwarz, for instance,14 showed the validity of this model since, for many scavengers, the results could be plotted on curves similar to those in Figure 1, but calculated according to the preceding principles. For many years, the subject of the influence of the nature and energy of the radiation was treated in terms of stopping power or LET (linear energy transfer, dE/dz). So defined, LET was considered to be the parameter which reflected this influence in a univocal way. The calculations were carried out following the model of the “string of beads’’ and of the “cylindrical tracks.’’ For LET higher than that of electrons produced by 6@Co y rays, the mean zones of heterogeneity are no longer considered to be isolated from one another along the track. As the LET increases, they draw closer together until there is an “overlapping;” this occurs when their average distance from one another is less than their diameter for an expansion where the radical-radical reactions became negligible (if, by diffusion, the radius grows ten times, the rate of the radical-radical reactions decreases by a factor of lo6, simply as a result of this expansion). However, according to this model, tritium 0 rays differ little from 6OCo y rays: for a track of an electron of 5.6 keV (the average energy of the 0 spectrum), the average LET is about 10 keV/p, which corresponds to 107 radicals/cm and to a separation of spurs of 100 A that is about ten times their initial diameter. The radical and molecular yields should thus be little different from those for isolated ‘‘spurs” and, if one only takes into account these mean parameters, the model “string of beads” cannot explain differences that we have noted between the efficiencieswith which halide ions decrease the molecular hydrogen peroxide yield for the two sorts of radiation. (11) M. Burton and K. C. Kurien, J. Phys. Chem., 63, 899 (1959). (12) J. L. Magee, J . Chim. Phys., 52, 52 (1955). (13) A. Kupperman in “Actions Chimiques et Biologiques des Radiations,” 5th Series, M. Haissinsky, Ed., Paris, 1961. (14) H. A. Schwarz, J. Amer. Chem. Soc., 77, 4960 (1955).
RADIOLYSIS OF WATERBY TRITIUM p RAYS I n fact, it has been known for a long time that the zones of heterogeneity are of different sizes and that merely taking an average value for the corresponding parameters does not permit the interpretation of all radiolytic phenomena. There exists, in fact, a spectrum of secondary electron energies which correspond to cross sections of energy transfer which can be determined, for the lowest energies, according to the excitation function of the irradiated solvents and by using Born’s approximation (Moller’s formula, for example) for the highest. Thus Mozumder and Magee insisted particularly on the importance of this energy distribution of secondary electrons (classed in order of increasing energy as ((spurs,” “blobs,” “short tracks,” and “branch tracks”) and elaborated a corresponding method of c a l ~ u l a t i o n . ’ ~Burch,17 ~~~ in a similar calculation, has already shown that by taking into consideration short tracks for tritium p rays, one calculates for the oxidation yield of Fez+in aerated sulfuric solution a value a,s low as 12.7. I n the case of the inhibition of molecular products by scavengers, an application of this method is all the more necessary in that the results are determined by the characteristics of the zones in which the formation and inhibition of these products do actually take place; thus they cannot be determined with mean parameters which take into account zones in which these phenomena are not of primary importance. For a certain number of these zones of heterogeneity (and above all short tracks which correspond to energy transfers of between 500 and 5000 eV), the mean density of the radicals formed along the secondary tracks thus defined is such that it is necessary to use the cylindrical model of diffusion in order to treat the problem of the scavengeability by various solutes. This treatment was carried out notably by Kupperman for high LET radiations and, comparing wi>h the diffusion model for isolated spherical spurs, he came to the following diff erent conclusions. (a) Until about 100 keV/p, t h e density of radicals in cylindrical tracks is initially smaller than that in spurs as they were defined above as mean zones of heterogeneity for y rays. The radical expansion by diffusion of the radiolytic species is certainly slower as is the corresponding lowering of concentrations, but this growth of importance of the time factor hardly favors a t all the radical-radical reactions compared with the radical solute reactions. Theoretically it follows that, while the molecular yields are greater than for isolated spurs, the action of the scavengers should be more efficient in order to diminish these yields. This is the reverse of the prediction of Mozumder18for the short tracks of tritium p rays. (b) For higher LET, for example the mean LET along the track of the zlOPoa rays, the density of the radicals isJ 011the contrary, higher from the very beginning of the diffusion than in the case of isolated spurs.
1545 The production of the molecular products must thus not only increase considerably but must also be inhibited by the scavengers with greater difficulty. This was remarked on by Burton and Kurien,’l who irradiated halide solutions with external a ray sources using the same experimental principles as those described here. I n Figure 2 are drawn the inhibition curves for GH2O2, curves which are calculated according to Kupperman theory (page 128 of ref 13) and concern the three types of diffusion examined qualitatively above: isolate spurs, cylindrical track with 8.5 X lo7 radicals/cm, and high density tracks (34 X lo7 radicals/cm). We have also plotted on this figure our experimental points for C1- and Br- scavengers; the calculation of GHZO2 from G(H202)was done according to expression I taking for G H P the values of 0.45 for y rays and 0.55 for tritium p rays.2 On the one hand, we can see that the results for the y rays correspond closely to the theoretical curve for isolated spurs; the values of the parameters of these mean spurs had furthermore been chosen to account for analogous phenomena. On the other hand, insofar as tritium p rays are concerned, the experimental points are even slightly to the left of the calculated curve for cylindrical tracks of low density which express an even greater scavengeability. It would seem that the major part of the hydrogen peroxide was thus formed (and eventually inhibited) in heterogeneous zones analogous to those of the ‘‘short tracks” of electrons from 500 to 5000 eV (50 to 10 keV/ p ; 5.10’ to lo7 radicals/cm). The difference that was noted between the two sorts of radiation thus springs essentially from the difference in the distribution of the diverse heterogeneous zones; the proportion of energy spent in “short tracks” is much greater in the case of tritium p rays (primary electrons of low energy in the p spectrum, end of trajectories secondary electrons) while for 6oCoy rays, the majority of the heterogeneous zones in which HzOz is formed are smaller (“blobs” and (‘spurs’’) and have characteristics which are closer to those used for calculations of diffusion with spherical symmetry. From this point of view, the 50-keV Xrays used also by Burton and Kurien” are close to the “Co y rays, and effectively, in Figure 2, their experimental points for the inhibition of G(H202) by C1ions can be placed between the two curves for the two sorts of radiation examined here. Recently, Appleby and Gagnonlg published a study on the inhibition by Cu2+ of the molecular yield of hydrogen in solutions of tritiated water. These authors (15) A. Mozumder and J. L. Magee, Radiat. Res., 28, 203 (1966). (16) A. Mozumder and J. L. Magee, ibid., 28, 215 (1966). (17) P. R. J. Buroh, ibid., 11, 481 (1959). (18) A. Mozumder in “Advances in Itadiation Chemistry,” M . Burton and J. L. LMagee,Ed., Interscience, New York, N . Y., 1969, p 71. (19) A. Appleby and W. F. Gagnon, J . Phys. Chem., 75, 601 (1971).
The Journal of Phgsical Chemiatrg, Val. 76, No. 11, 1972
1546
C. C. CHAOAND J. H. LUNSFORD
noted that, in contradiction to our results for HzOz,it is ~ in the case more difficult to inhibit the yield G H than of y rays. Their interpretation is essentially based on the hypothesis that the differences in the spectrum of electrons for the two qualities of radiation brought about a variation in the relative importance of the reactions responsible for the formation of Hz, that is
+ eaa- +Hz + 20Hea,- + H +Hz + OH-
ea,-
H+H+Hz and no doubt a fourth process whose nature is still little understood. I n each of the entities described by Mozumder and Magee (spurs, blobs, short tracks, and branch tracks) these processes would evolve differently and their inhibition by a scavenger would be different. This accounts for the differences of scavengeability observed for HZproduced by tritium p rays and by 6oCoy rays, for which the distributions of the above entities are different. Even though the origin of molecular HZ is still a matter of controversy, it seems that this hypothesis is
very reasonable. The results of Appleby and Gagnon would thus not be in contradiction with our own interpretation on the formation of HzOz. The latter comes from a process which seems to be less complex and is thus more appropriate for the study of heterogeneities during radiolysis. We intend, in another paper, to present a method of analytical calculation which would permit the determination of the percentage of energy spent in “short tracks” for various qualities of radiation, calculations Which can easily be applied in cases of complex spectrum like that of tritium p rays.20 The present discussion, based on critical experimental work, underlines the inadequacy of the linear parameter LET = -dE/dz and the need to take into consideration a LET that is doubly differentiated. A first approximation treats the secondary electrons which arise from energy transfer greater than 100 eV, for example, as independent; this leads to the concept of “LET spectra,” advocated for a long time by many radiobiologists (see, for example, ref 21). (20) J. Pucheault and G. Lemaire, to be published. (21) P. Howard-Flanders, Advan. Biol. Med. Phys., 6 , 553 (1958).
An Electron Paramagnetic Resonance Study of the Cu+-NO Complex in a Y-Type Zeolite by C. C. Chao and J. H. Lunsford* Department of Chemistry, Texas A I M University, College Station, Texas 778@ (Received December 28, 1971) Publication costs assisted by The Robert A . Welch Foundation
The Cu+-NO complex in a Y-type eol lite was formed by the reaction of Cu+ with adsorbed nitric oxide. The epr hyperfine structure reveals that the unpaired electron spends 20% of its time on the Cu+ ion and is distributed about evenly between the 3d,~and 4s orbitals on the copper. The molecular axis of NO is essentially parallel to the axis of the 3d,2 orbital.
It is known that Cu2+ in a Y-type zeolite can be reduced to Cu+ with CO, and at low temperatures NO molecules can be adsorbed on the Cu+ ions reversib1y.l I n this article we report on t h e results of epr studies on the Cu+-NO complex in Cu+-Y and the structural information deduced from this work. The Cu2+-Y samples were prepared by exchanging NaY in Cu(NO& solutions three times. The extensively exchanged samples were degassed to 500” and reduced in an atmosphere of 500 Torr of CO a t the The Journal of Physical Chemistry, Vol. 76,No. 11, 1972
same temperature. The extent of reduction was monitored by observing the intensity change of the epr spectrum of the Cu2+ ion. Purified nitric oxide was admitted onto the samples a t room temperature and usually at a pressure of 20 Torr. The epr measurements were carried out with an X-band spectrometer at 77°K and a &-band spectrometer at about 120°K. (1) C . M. Naccache and Y. Ben Taarit, J. Catal,, 2 2 , 171 (1971), and private communication.
