60 1
MM[UIVI A T I O N S T O THE E D I T O R
Scavenging of t e Molecular Hydrogen Yield f r o m Water Irradiated w i t h Tritium p Particles’ Publication costs as&ted Rutgers Universita
iiy Department of Environmental Sciences,
Sir: It is well established that the yield of molecular products, H2and R202,from y-irradiated aqueous solutions is lowered in the presence of certain reactive scavengers.2 This effect is usually attributed to reactions of the scavenger with free radical precursors of the molecuh product within the “spur.” Recently, it has been suggested on theoretical grounds that a major fraction of *,he energy deposited in water by lowenergy electyons gives rise to “short tracks” rather than spurs. I n an attempt to distinguish experimentally the properties OF spurn and short tracks, we have measured the scavengeability by copper ions of the molecular Hz yield produced by tritium fl radiolysis (average energy of tritium fl = 5.69 IreV). We find that this molecular Hz yield is significantly less scavengeable than that produced by 6oCo? radiolysis. Solutions containing approximately 40 mCi ml-l of tritiated water were used. The water was purified by standard techniques involving multiple distillation, y irradiation, and uv The solutions all cond l RBr as OR scavenger and were detained oxygenated by bubbling with argon. I n dilute bromide solutions the measured yield of hydrogen G(H2) is equal to the molecular yield O H 2 (G = molecules produced per 100 eV). Hydrogen evolved was analyzed by sweeping out with argon into a gas chr~matograph.~I n order to obtain efficient sweeping action, a wetting agent (LO-* M tertiary butyl alcohol) was used which was found to be without effect on the hydrogen yields from y-irradiated Gus04 solution. Cu2+ reacts sufficiently rapidly with hydrogen atoms5 so that the lowest conM ) is enough to prevent the centration used formation of additicinal IIr through hydrogen abstraction from the alcohol. After each analysis, accumulated hydrogen pero Kide was removed by uv photolysis and the oxygen which was evolved was swept out with argon. The solutio11 was then left to accumulate hydrogen for a further period. I n this way the solution was exposed for times ranging from 5 to 18 hr (2550 to 9200 rads) and a plot of hydrogen evolved as a function of dose was obtained at each CuSOa concentration. The plots, each consisting of at least five points, were all linear with dose and passed through the origin. Dosimetry was by calculation from the specific activity of
the tritiated water used, as measured by serial dilutions and liquid scintillation counting. The results are shown in Figure 1 n-bere the ordinate is the ratio of the observed G H z to G H ? (the extrapolated value a t zero Cu2+ concentration, 0.5’75). Figure 1 also shows the same ratio for 6oCoy-irradiated solutions containing C U ~ + .The ~ cube root of the Cu2+ con-
0
7-p-
5
6QCo-a
1
0
01
OE
03
04
OS
06
07
-
08
09
A 10
-.
[cUSO, 5H,~]y3 Figure 1. Decrease in GH,/GH,Owith concentration of copper sulfate: 0, tritium p radiolysis; 0, y radiolysis, ref 6.
centration is used for the abscissa to spread the data. A distinct quantitative difference is apparent in the efficiency with which copper ions scavenge the molecular hydrogen yield produced by tritium fl radiolysis as compared x-ith cobalt y radiolysis. Three spur processes are important in hydrogen formation. They are
+ eaq- + 2 H20-+ Hz + 20Heaq- + H + HzO --+HZ+ OH-
eaQ-
H+H+H2
(1) (2)
(3)
together with an initial molecular yield of (1) Paper of the Journal Series, New Jersey Agricultural Exyeriment Station. Rutgers-The State University of New Jersey, Department of Environmental Sciences, New Brunswick, N. J. (2) See, e.g., A. 0. Allen, “The Radiation Chemistry of Water and Aqueous Solutions,” Van Nostrand, New York, N. Y., 1961. (3) A. Moenmder and J. L. Magee, Radiat. Res., 28, 203 (1966). (4) J. W. Swinnerton, V. J. Linnenbom, and 6 . H. Cheek, Anal. Chem., 34,483 (1962). ( 5 ) &I. Anbar and P. Neta, Int. J . A p p l . Rad&. Iaotop., 18, 493 (1967). (6) H. A. Schwarz, J . rlmer. Chem. SOC., ’77,4961)(1955). (7) H. A. Schware, J . Phys. Chem., 73, 1928 (1969).
The Journal of Physical Chemistry, Vol. 76, -Yo. 4, 1971
w 2
7, the relative contributions to the total molecular hydrogen yield of these four processes have been calculated to be, respectively, 31.6%, 29.7%, 3.9%) and 34.8%0.7It is possible that the large difference in scavengeability of the molecular Hz yield when it originates fronn radiations giving predominantly short tracks rather than spurs is due to a change in the relative importance of these four processes, and hence a change in the naiure of ttle major precursor. It is noteworthy that our dat a can be brought into coincidence with the y-ray data if a constant normalizing factor of 0.02 is applied to the Cu2+ concentration even though this varies over four orders of magnitude. The value of 0.02 is very dose to the relative reactivities of Cu2+ with hydrogen atoms and with electron^.^
The Journal cf Physical Chemistry, Vol. 7 6 , No. 4, 1971
COMMUNICATIONS TO THE
EDITOR
An increased contribution of spur reactions involving
H atoms has been suggested to explain the observed low yield of H atoms at high LEI';* our results, if explicable on the same basis, would lend support to a picture of tritium f? radiolysis in terms of short tracks of high local LET. Further work is in progress t o establish the generality of the effect reported here. (8) A. Appleby and H. A. Sohwarz, J . Phgs, Chmn., 73, 1937 (1969).
DEPARTMENT OF ENVIRONMENTAL SCIENCRS A. APPLEBY* RUTGERS-THESTATE UKIVERSITY W. F. GAGNON NEWBRCNSWICIC, NEWJERSEY 08903 RECEIVED OCTOBER 19, 1990
