June, 1959
EFFECT OF SOLUTE CONCENTRATION IN THE RADIOLYSIS OF WATER
899
EFFECTS OF SOLUTE CONCENTRATION I N RADIOLYSIS OF WATER’ BY MILTONBURTON AND K. C. KURIEN Department of Chemistry, University of Notre Dame, Notre Dame, Indiana Received February 14, 1969
Effects of halide scavenger on G(H202)in aerated 0.8 N HzS04solutions irradiated in separate experiments with average24 MeV. X-rays, Corn y-rays, 50 kev. X-rays and 3.4 MeV. alphas are in substantial agreement with the expanding-spur treatment of the theory of the radiolysis of water, both in the Ganguly-Magee form and in the earlier Magee form. On the other hand, Go, the 100 e.v. yield of initial decomposition, seems to vary with velocity of the impingent particle in a manner not predicted by any current theory. Furthermore, there appears to be an inconsistency in the application of the present, necessarily simplified, theory to the results. The GO values found are significantly larger than those corresponding to the Ganguly-Magee model. At the same time, they suggest participation of both ionized and excited molecules in the radiolysis but to a degree considerably less than might be expected on the basis of the vapor phase result of Firestone. The latter is shown to correspond to the assumption that all primarily ionized and excited molecules contribute to the chemistry of the vapor. Apparently, part of the excitation energy is dissipated in the liquid state prior to decomposition t o scavengeable free radicals. The results of experiments with “24 Mev.” X-rays are qualitatively similar to, but quantitatively different from, those with Coeo 7-rays. The expanding-spur theory would not in any presently existent OH + X OH- have been calculated treatment predict such a result. The specific rates of the scavenger reaction Xon the basis of the Ganguly-Magee model to be 1.6 X 1010 1. mole-’ sec.-l for Br- and 4 x 109 1. mole-’ sec.-l for C1-.
+
1. Introduction
A study of fission-fragment-induced decomposition of aqueous thorium nitrate solutions by Boyle and Mahlman2 shows, on detailed analysis, that G(N2)increases linearly with the first power of the nitrate concentration and that G(H2) decreases linearly with the square-root of the same concentration. Such results suggest that the nitrogen is the product of the direct interaction of the radiation with the nitrate and that the hydrogen results from a simple diff usion-controlled reaction involving free radicals, presumably hydrogen atoms. An indication that the true situation is somewhat more complicated was given earlier by Sworski3who pointed out that in air-saturated acid solution of KBr, G(Hz02)decreases linearly with the cube-root of concentration when the solution is irradiated with Co60 y-rays. Shortly thereafter, Magee4 presented a quantitative theory of diffusion-controlled reactions related essentially to the number of spurs per ionization track. The conclusions of Magee pertinent to the results may be summarized as follows. The cuberoot relationship is fortuitous and only approximate. No single exponential relationship holds over the en tire range of solute concentration. However, if the approximate value of the exponent is considered, it will be found to vary with the energy of the impingent radiation from a low value for slow particles like alphas to values approaching 0.5 when the number of spurs per ionization track approaches infinity (as in fission-recoil tracks). The values of the “exponents” are characteristic of the radiation and not of the solute nor indeed of the solvent system under iiwestigation. Following the initial results of Sworski, a significant number of aqueous systems irradiated with Cosoy-radiation was found t o obey the cube-root relationship. They are summarized in Table I.3,5-12 (1) Contribution from t h e Radiation Project operated by the University of Notre Dame and supported in part under U. S. Atomic Energy Commission Contract AT(ll-I)-38. Abstract from a portion of a thesis submitted by K. C. Kurien in partial fulfillment of the requirements f o r the degree of Doctor of Philosophy. (2) J. W. Boyle and H. A. Mahlman, Nuc. Sei. E n g . , 2, 492 (1957). (3) T. J. Sworski, J . Am. Chem. Soc., 76, dG87 (1954). (4) J. L. Magee, J. chim. p h y s . , 64,528 (19.55). (5) T. J. Sworski, Radiation Research, 2, 26 (1955).
+
I n addition, Sowden13 found a similar relationship for Hzproduction with NO1- ion scavenger in an unbuffered Ca(NOJ2 solution irradiated in a pile with mixed fast neutrons and gammas. A more elaborate version of the theory by Ganguly and MageeI4 considered the distribution of spurs along the ionization track and made more detailed predictions regarding effect of solute concentration on the so-called “radical yield” in water. These predictions are qualitatively supported in a limited range by the results of Schuler and and also by those of Donaldson and Miller.16 Schwarz17has studied the effect of Gos0 y-rays, 18 MeV. deuterons and 33 MeV. helium ions on Hz yield as affected by NO2- concentration and on HzOzyield as affected by Br- concentration. This paper is a report of a study of H202 yield in radiolysis of bromide and chloride solutions as a function of halide concentration over a broader range of incident-particle velocities than has heretofore been described. 2. Experimental 2.1. Chemicals, Solutions, Analyses.-All chemicals used in this work were Baker’s A. R. grade. Water used was twice distilled from alkaline permanganate and finally from acid dichromate solution. Special care was exercised to avoid contamination by organic vapors. Thc general method employed in this work followed Swor~ki.~ssAirsaturated solutions of 0.8 N containing various concentrations of KBr and KC1 were employed. Hydrogen peroxide yield was determined by ceric reduction. Spec(6) A. 0. Allen and R. A. Holroyd, J. Am. Chem. Soc., 7 7 , 5852 (1955). (7) H. A. Schwara, ibid., 77, 4960 (1955). (8) H. A. Schwarz and A. 0. Allen, ibid., 77, 1324 (1955). (9) C. J. Hochanadel and J. A. Ghormley, Radiation Research, 3, 227 (1955). , (10) H. A. Mahlman and J. TV. Boyle, J. Chem. Phys., 47, 1434 (1957). (11) H. A. Schwarz and J. M. Hritz, J. Ann. Chem. Soc., 80, 5036 (1958).
(12) K. C. Kurien, P. V. Phung and M. Burton, Radiation Research, in press. (13) R. G. Sowden, J . Am. Chem. Soc., 79, 1263 (1957). (14) A. K. Ganguly and J. L. Magee, J. Chem. Phye., 26, 129 (1856). (15) R. H. Schuler and A. 0. Allen, J. Am. Chem. Soc., 79, 1565 (1957). ( 1 6 ) D. hl. Donaldson and N. Miller, Radiation Research, 9, 487 (1958). (17) H. A. Sohwarz, J. hl. Caffrey and G. Sclioles, J. Am. Ckcm. Soc., 81,in press (1959).
MILTON BURTON AND I
