THERADIOLYSIS OF SOME HEAVYWATERSOLUTIONS AT pD 1.3-13 chromatographic analyses, and the Analytical Group for magnesium analyses. The authors also wish to express their indebtedness to Dr. R. K. Steunenberg
511
for several helpful discussions. This study was done under the auspices of the U. S. Atomic Energy Commission.
The Radiolysis of Some Heavy Water Solutions at pD 1.3-13 by Z. D. Draganie, 0. I. MiEit, and M. T. Nenadovie Boris KidriE Institute of Nuclear Sciences, VinEa, Yugoslavia (Received June 12, 1967)
The yields of primary products in heavy water y radiolysis are determined over the pD range from 1.3 to 13. Various combinations of the following scavengers are used: Cu2+, Br-, oxalic acid, ethanol, and oxygen. Results are compared with the corresponding values from the preceding work with the same systems in light water solutions, and the following ratios of primary yields were obtained: G--H~o/G-D,o = 1, G I E ~ ' ~ / G < R ~1,' ~ and GMHao/GMDZO > 1. Relative rate constants are determined for some reactions of D, OD, e,,-, 0- ion radicals, and the isotopic effect is discussed.
Introduction
There are some indications that the yields of primary products in water radiolysis differ for HzO and DzO, but the values reported in the literature are contradictory.'-10 On the other hand, as it was recently pointed out,ll~Lz published values for primary yields of water radiolysis differ mainly according to the system from which they have been derived. This is why our main interest was to determine the ratio of primary yields derived from measurements on the same solute system in light and heavy water. I n this paper, we present the results of measurements in heavy water systems similar to those made in our preceding works with light water,13-17 using the same scavengers under the same conditions. The determination of the isotopic effect seemed useful for a better understanding of the radiolysis of heavy water solutions and for the possibility of comparing corresponding reactions in the two media. Therefore, rela1,ive rate constants were also determined for some reactions of D, e,,-, OD, and 0- ion radicals; the corresponding data for HzO were given in our previous publications. 14,16,17 Various combinations of the following scavengers (pD 1.3-6) were used: Cu2+, CzH60D,Br-, and 02. The results of studies on the oxygenated heavy water oxalate solutions (pD 1.6-13) are presented. Also, ferric yield in heavy water Fricke dosimeter is measured.
Experimental Section In this work, only some more important details are given since the techniques used were fully described elsewhere.14J6*17 Xolutims. Triply distilled water with the deuterium (1) J. Baxendale and G. Hughes, 2. Physik. Chem. (Frankfurt), 14, 323 (1958).
(2) H. Mahlman and J. Boyle, J. Am. Chem. SOC.,80, 773 (1958). (3) T. Hardwick, J. Chem. Phys., 31, 226 (1959). (4) D . Armstrong, E. Collinson, and F. Dainton, Trans. Faraday SOC., 55, 1375 (1959). (5) K. Coatsworth, E. Collinson, and F. Dainton, ibid., 56, 1008 (1960). (6) P . Phung and M. Burton, Radiation Res., 7, 199 (1957). (7) E. Hayon, J . Phys. Chem., 69, 2628 (1965). (8) H. Mahlman, J . Chem. Phys., 3 2 , 601 (1960). (9) P. Dyne, J . Fletcher, and L. Roy, Can. J . Chem., 39, 933 (1961). (10) F. Dainton and D . Peterson, Proc. Roy. SOC.(London), A267, 443 (1962). (11) A. Allen, Proceedings of the Fifth Informal Conference on the Radiation Chemistry of Water, University of Notre Dame, Radiation Laboratory, AEC Report COO-38-519, Notre Dame, Ind ., Oct 1966, p 6. (12) M. Halssinsky, "Actions chimiques et biologiques des radiations," Vol. 11, Masson et Cie, Paris, 1967. (13) I . Draganib, J. Chim. Phys., 56, 16 (1959). (14) Z. Draganib, I. Draganib, and M. Kosanib, J . Phys. Chem., 68, 2085 (1964); 70, 1418 (1966). (15) Z. Draganib and M. Nenadovib, Intern. J. A p p l . Radiation Isotopes, 17, 319 (1966). (16) 2;. DraganiO, M. Kosanib, and M. Nenadovib, J . Phys. Chem., 71, 2390 (1967). (17) 0. Mibid and I. Draganib, ibid., 70, 2212 (1966).
Volume 72, Number 2 February 1968
512
oxide content 99.77 f 0.02% (checked by IR analysis) was used. The chemicals, HzC204, CuSO4, KBr, H2S04, ethanol, and metallic sodium were Merck or BDH AR grade products. The pD of the solution was adjusted by adding heavy water solution of concentrated H804 or NaOD. The latter was made by directly dissolving metallic sodium in heavy water in an argon atm0~phere.l~The pD values given are the values obtained by the addition of 0.4 unit to the actual reading on a pH meter.18 The oxygen was introduced into degassed solutions and its concentration was 0.60 f 0.02 mM. Only when the influence of oxygen concentration on radiolytic yields was studied was the oxygen concentration varied from 0.24 to 0.99 mM. Irradiation. Irradiations were performed in the 2000 Ci (nominal) radioactive cobalt source. The dose rate, as determined by Fricke dosimeter (G(Fes+) = 15.5), was 1.8 X l O l e eV ml-l hr-l. In the absorbed dose c a l c ~ l a t i o n ,it~ was ~ taken into account that for D2O the mass density is 11% higher and the electron density 10% lower than for HzO; the corresponding correction was made also for Fricke dosimeter. Analysis. The molecular deuterium, carbon dioxide, and initially present oxygen were determined by gas chromatography. 2o Calibration curves were determined for pure Hz and Dz samples as well as for a mixture of Hz, HD, and Dz(1.02:2.03:1.00). The amount of D20zwas determined spectrophotometrically using, with some modifications,’* the method developed by Ghormley. The acid and alkaline solutions were neutralized by adding NaOH or HzS04, and the molar extinction coefficient for D202 was 25.500 M-I cm-I at 24”. Ferric ions were measured spectrophotometrically by the standard procedure but taking into account that the ratio of the molar extinction coefficient of ferric ions in heavy and light water is 1.070.21 The molar extinction coefficient for Fe3+ in light water was measured as 2.198 M-l cm-l at 24’. All G values were calculated from the concentrationdose curves obtained from at least five determinations in the dose range of 0.3 X 10l8 to 4 X 10l8 eV m1-I.
Results Various Combinations of Cu2+, C a b O D , Br-, and 0 2 , p D 1.3-6. GD,. The molecular deuterium yield was measured in degassed solutions of CuSO4 (lo-* M ) at pD 5. The value G(DJ = 0.38 i 0.02 was obtained. According to the radiolysis mechanism proposed for CuSOd solutions under these conditions, one obtains GD, = G(D2). GD,~,. The deuterium peroxide yield was measured for different bromide ion concentrations in oxygenated KBr solutions at pD 1.3 and -6. When the measured yields are presented as a function of (KBr)”’, two parallel straight lines with the slope of 1.44 were obThe Journal of Physical Chemistry
%.
I>. DRAGANIC, 0. I. RllrC16, AND M. T. N A N A D O V I ~
tained. Extrapolation to zero bromide concentration gives GO(D202) = 1.27 0.03 and 1.00 f 0.05 at pD 1.3 and -6, respectively. According to the reaction scheme, under these conditions GD,o, = 0.5 [GO(DZOZ) G(D2)l. Hence, it can be calculated that at pD 1.3 and -6, G D , ~ ,= 0.82 0.03 and 0.68 f 0.04, respectively. GD Geap-. G(HD) values were measured in degassed heavy water ethanol solutions at pD 1.3. Independently of the ethanol concentration (1 X 5 X M ) and taking into account that for these G,,,GD, = G(hydrogen)t,tal, conditions GD the yield GD Ge,,- = 3.69 f 0.18 was obtained. G(HD)values were also measured in degassed ethanol solutions at pD 1.3, in the presence of different cupric sulfate concentrations. With the increase of cupric sulfate concentration the decrease in the measured yields of hydrogen is in agreement with the reaction scheme
*
+
*
+
+
+
+
+ D = HD + C2H4OD Cu2++ D = Cu+ + D + Dap++ eaq- = D + DzO Cu2+ + cap- = Cu+ + D 2 0
C2H60D
(1) (2) (3) (4)
From the usual kinetic treatment one obtains G(HD) =
GD k2(CU2+) kl(C2H60D)
+
The competition curve given in Figure 1 was obtained in working conditions where (D+) >> (Cu2+). From the intercept on the ordinate we obtain GD Geaq- = 3.6 0.11, and from the slope and the intercept, the value for the ratio Ic2/kl = 2.6 0.3. The yields of reducing radicals are also derived from G(D202) values in the system CZHBOD 0 2 . Reto 1 X gardless of ethanol concentration (5 X M ) , the measured G(D202) was 3.96 0.11 for pD 1.3 and 3.36 f 0.10 for pD -6. Using these figures and the relation GD Ge,,= G(D202) GD,, derived from the reaction scheme, we obtained for total reducing yield the values 3.58 and 2.98 at pD 1.3 and -6, respectively. GD. If the experimental conditions are adjusted SO that (C2H60D) >> (Cu2+) and (Cu2+) >> (D+), then
*
+
*
+
*
+
-
(18) P.Glasoe and F. Long, J. Phys. Chem., 64, 188 (1960). (19) H. Fricke and E. J. Hart in “Radiation Dosimetry,” Vol. 11, Academic Press, New York, N. Y., 1966, Chapter 8. (20) Lj. PetkoviE, M. KosaniE, and I. DraganiE, Bull. Inst. Nucl. bci., “Boris KidriE” (Belgrade), 13, 77 (1962). (21) J. Boyle and H. Mahlman, Radiation Res., 16, 414 (1962).
THERADIOLYSIS OF SOMEHEAVYWATERSOLUTIONS AT pD 1.3-13
513
The usual kinetic treatment gives
0
0.05
0 2 ) are the yields measured at different scavenger ratios, and GO(D202) are the values derived from the cube-root plots at zero KBr concentration. The competition diagram in Figure 3 is obtained by using eq 9 and the experimental data obtained at pD 1.3 and -6, and corrected according to Hummel and Allen.22 The intercepts on the ordinates give for G O D the values 2.95 f 0.17 and 2.5 f 0.12 for pD 1.3 and -6, respectively. Oxalic Acid Solutions Containing Oxygen, pD I .6-13. The radiolytic products C02, D2, and D202 were measured in irradiated oxalic acid solutions (50 mM) containing oxygen (0.6 mM). Figure 4 shows the values of the measured yields as a function of pD. Dotted lines correspond to the data previously obtained for light water.
0.20
0.10 0.15 [Cuz+l/ [CZHIOD].
Figure 1. Competition between cupric sulfate and ethanol for D atoms: dotted lines represent the data for corresponding Hz0 solutions.1'
I
0
0.2
0.4
0.6 [CuZfl/[D+1.
0.8
1.o
Figure 2. Competition between CuZ+ and D + ions for solvated electrons: dotted lines represent the data for corresponding HzO solutions.17
eq 5 becomes GD, = G(hydrogen)t,t,l - GD,. In the solutions of C2H60D(2 X lod2M ) and CuSO4(1 X M ) , at pD 5.5, one can calculate GD = 0.49 f 0.05. Geaq-. In degassed solutions of ethanol and cupric sulfate, the experimental conditions can be chosen so that (C2H60D)>> (Cuz+). When the D+ ion concenM , a competition tration varies from to occurs between Cuz+and D + ions for solvated electrons. The total hydrogen measured in these conditions and the reduced form of eq 5 served for constructjon of the competition diagram in Figure 2. From the intercept on the ordinate, we obtain G,,,- = 2.5 f 0.2, and from the intercept and the slope the ratio k4/k3 = 4.4
0
0.1
0.2
0.3 0 [Br-I/[CzHsOD].
1
2
3
Figure 3. Competition between Br- ions and ethanol for OD radicals: left side, p D 1.3; right side, pD -6; dotted lines represent the data for corresponding Hz0 solutions.'@
f 0.6.
GOD. There is a competition between ethanol and Br- ions for OD radicals in oxygenated solutions of ethanol and potassium bromide. With the increase of KBr concentration, the measured yield of hydrogen peroxide decreases due to the reactions where G(D2-
+ OD = HOD + CzHiOD Br- + OD = OD- + Br Br + = D + + Br- + DOz
C2H60D
D202
(6)
---------------_- - - - - _ _ _ _-.___.___ __ _ _ _ _ _ _ _ _ _ _____ ____ 1
2
3
4
5
6 7 8 9 1 0 1 1 1 2 1 3 1 4 PH (PD).
Figure 4. The influence of pD on measured yields of C02, DZ, and DZO2in 50 mM oxalate solutions in the presence of 0.60 mM Oz: dotted lines represent the data for corresponding HzO s0lutions;1~0, COZ; X, DzOZ; and A, Dz.
(7) (8)
(22) A. Hummel and A. Allen, Radiation Rea., 17, 302 (1962).
Volume 72, Number 2 February 1968
2. D. DRAGANI~, 0. I. M1616, AND M. T. NENADOVI~
5 14
0.5
I i
0
0.2
0.1
0.4
0.3 (HzCzO~] '/a.
0.5
0.6
Figure 5. Cube-root plots: measured molecular deuterium yields as a function of oxalic acid concentration in presence of 0.60 mM 02:X, acid solution (natural p D 1.4-1.9); 0, neutral solutions (pD 5.8-7.6); and 0, p D 13; dotted lines represent the data for corresponding HzO ~ o l u t i o n s . ~ ~
Figure 5 shows the oxalic acid concentration influence on deuterium yields measured at pD 1.41.9, 5.8-7.6, and 13. It is interesting to point out that the cube-root plots obtained for solutions in heavy water are parallel to those derived for solutions in light water. The trend of experimental data given in Figures 4 and 5 is in good agreement with our previous data obtained in H20 solutions under the same conditions. We therefore consider that from the scheme proposed earlier13+14
+ +
DzC204 (or DC204-, C2042-) OD = DzO (or OD-) COz COOD (or COO-)
+
O2 O2
+ D (or e,,-)
=
DO2 (or 0,-)
+ COOD (or COO-) = DO2 (or 0,-) + COZ OD- + OD = 0- + D2O + 0- + D2O = 20D- + C02 + COO+ 0- = = 202- +
c20d2-
0 2
03-
0 2
203-
202- (or DOZ) = D202
+ OZ
(10) (11) (12) (13)
The competition plot in Figure 6 was made by using eq 20 and the COzyields measured at pD 13 for different Oz/oxalate ratios. The best line, obtained by the least-square method, gives k15/k14 = 107. The corresponding value in light water was 140. It can be seen that the rateconstant ratio for 0- ion radicals is 1.3 times higher in light than in heavy water. A similar effect was obtained for the OH radical: relative rate constants in the system ethanol-KBr-oxygen differ by a factor of 1.4, when measured in H 2 0 and DzO solutions. Equation 19 is valid in acid and neutral media. In order to see at which higher pD values this equation can be applied, we have to know the ratios k15/k14 and kl3/klO and also their pD dependence. However, klS/k14 is known only for H 2 0 solution at pH 13, and published ~ O H + O H - dataz3-z5 differ by a factor of 10. The data on the corresponding reactions in D2O are not available in the literature. Nevertheless, one estimation is still possible. Taking klo = 6 X lo6 M-I sec-l,I6 k13 = 3.6 X 108 M-1 sec-l,23 and correcting by the factor 1.4 for the isotopic effect in hydroxyl radical reactions, one obtains k13/k10 = 43. This means that G(C02) could be a direct measure for OD radical yields at all pD / 1G M ~ ~ ~ water. The difference includes not only the isotopic efIt is interesting to compare our ratio (GeSq-)H20/ fect in diffusion velocities of reactive species, but also the (Ge,,-)D,o = 0.92 and 0.93 obtained from Dorfman's isotopic effect in the reaction mechanism itself. Thereworkz7 and 0.9 from Hart's.28 This is an excellent fore, for instance in the case of the competing reactions agreement, in spite of the fact that the absolute yields of Cu2+and ethanol for H or D atoms, the value of a = differ considerably. These authors are using high con1.4 should be in the first place attributed to reaction 1, centrations of scavengers, which may influence the where the abstraction of H atoms occurs.29 Reabsolute values but not the yield ratios. action 2 is an electron transfer and the probability Our G H / G D value is in good agreement with results of its influence on a is very low. of Hayon7 and Hart.28 However, we feel that the A pronounced effect ( a = 2.7) can be seen in the use of this observation for any general conclusion concompetition reaction of H + (or D+) and Cu2+ for solcerning the origin of hydrogen atoms would be unvated electrons. One would expect that the mobility justified as the experimental error is of the order of of solvated electrons and cupric ions in heavy water magnitude of the effect observed. is ~maller,~O but the corresponding decrease in the Results obtained in this work are especially in conrate of reaction 4 cannot be more than about 20%. trast with those of Phung and Burton6 and H a y ~ n , ~ who have reported lower yields for all primary products (26) W. Seddon and A. Allen, J. Phys. Chem., 71, 1914 (1967). (27) L. Dorfman and I. Taub, J. Am. Chem. Soc., 8 5 , 2370 (1963). in DzO radiolysis. On the other hand, our data de(28) E.Hart, Proceedings of the Fifth Informal Conference on the rived from measurements on a large scale of acidities Radiation Chemistry of Water, University of Notre Dame, Radiaare in reasonable agreement with some data reported tion Laboratory, AEC Report COO-38-519,Notre Dame, Ind., Oct 1966,p 24. for acid media.4~6~8 This is especially the case for acid (29) C. Roginski, "Theoreticheskie Osnovii Izotopnih Metodov medium with Mahlman and Boyle data.2 Izuchenia Hemicheskih Reakcii," Akademiya Nauk SSR, Moscow, The ferric ion yield we have measured (16.5 f. 0.3) 1959, p 21. is higher than the recent Hayon's value (lLO),' but in a (30) C. Swain and D. F. Evans, J. Am. Chem. Soc., 88, 383 (1966).
+
The Journal of Physical Chemistry
THE RADIOLYSIS OF SOME HEAVYWATERSOLUTIONS AT pD 1.3-13 Also, the change in the degree of dissociatjon of the cupric ion, when replacing H 2 0 with D20, should have no influence on the rate-constant determination of reaction 4: measurements were made with fairly low M ) so that cupric sulfate was concentrations practically fully dissociated. On the other hand, the relative decrease of the rate of reaction 3 in DzO solutions can be larger due to the hydronium ion, but by not more than 40%, i.e., considerably less than observed. Also, since reaction 4 is an electron transfer, a measurable isotopic effect could hardly be expected. It seems, therefore, that the observed effect should be mainly attributed to reaction 3. Data concerning a long-lived activated complexa1and observations made by Brown, el al.,a2about a low disappearence rate of esq- in heavy water support this idea. It may be important to note that the observed isotopic effect is close to the kinetic isotopic effect in protonization processes. In accord with observations on acid catalytic hydration of olefins,aa the decrease in the rate of reaction 3 may be explained by the fact that the transfer of deuteron from D30f ion to solvated electron is slower than the corresponding proton transfer in H2O.
517
It can be seen that for competition reaction of hydroxyl radicals, the isotopic effects were 1.3 and 1.5 for pD 1.3 and -6, respectively. Neither the mass difference of OH and OD radicals nor the smaller mobility of the Br- ion in heavy water (which is not more than 20%)a0 can account for this. The reason should be looked for in the complexity of the observed reactions. However, it is interesting to note that the parameter a depends only slightly on pD, though the ratio k ~ l k eis strongly dependent on pD (about ten times higher in acid than in neutral medium). For 0- ion radicals the isotopic effect measured was 1.3, close to that observed on hydroxyl radicals. Acknowledgment. The authors are indebted to Dr. J. Sutton for reading the manuscript and to Dr. I. Draganib for many useful discussions during the work. Thanks are also due to Mrs. M. Marovid for technical assistance in the experiments. (31) C. Lifahitr and G. Stein, J . Chem. Phya., 42, 3330 (1965). (32) D. Brown, F. Dainton, and J. Keene, Proc. Chem. SOC., 266 (1964)
.
(33) V. Gold and M. Kessick, J. Chem. SOC.,6718 (1965)
Volume 7.9, Number 2 Februaru 1868
