E. HAYON
2628
In these expressions each a, is &,(a$), and each 6, is P,(a,b). They are given by crl(U,b)
=
uu
+ + (u)
ab(a,b)
=
uaua(u)3
P1(a,b) = 5
Pz(a,b) = ual -
u2
-
+-
P3(a,b) = ua2 - u2a1 ua
Radiolysis of Heavy Water in the pD Range 0-14
by E. Hayon Service de Chimie physique, C. E. N . Saclay (S. & O.),France
(Received February 4, 1966)
Radical and molecular yields from various systems irradiated with 6oCoy-rays in heavy water in the pD range 0-14 were determined and are compared with data obtained in light water. Radical yields were found to increase in acid and alkaline solutions as compared to neutral solutions. This is interpreted as due to scavenging in the “spurs” of the radicals (DzO)- and OD by Df and OD- ions, respectively. This is similar to the conclusions reached regarding HZO. At all pH values, the values of Gb,and GD,o,are lower in D20 than those of GH, and GR,o, in HzO, while the yields of and GODare only slightly lower in DzO than the corresponding yields in HzO.
Few detailed studies have been made of the yields of radicals and “molecular” products in the radiolysis of heavy water. Results extrapolated to zero scavenger concentration are available mainly in 0.8 N sulfuric acid solutions,l-7 a few in neutral solution^,^^^^^^^-^^ and none in alkaline solutions. In view of the importance possible differences in the primary yield from irradiated HzO and DzO may bear on theories of the nature and diffusion of the primary radiation-chemical species, an attempt was made to determine the yields of radicals and “molecular” products in DzOas a function of pD in the range 0 to 14. The results obtained are presented below. Experimental A 200-curie 6oCoy-source was used, and the dose rate was 7.7 X l0ls e.v./ml. min. based on the ferrous sulfate The Journal of Physical Chemistry
dosimeter in light water, taking G(Fe3+) = -15.5. In order to minimize the consumption of heavy water, ~
~
~
~
~~
~~~~~~
~
(1) H. A. Mahlman and J. W. Boyle, J . Am. Chem. SOC., 80, 773 (1958). (2) J. H.Baxendde and G. Hughes, Z. physik. Chem., 14,323 (1958). (3) T. J. Hardwick, J. Chem. Phys., 31, 226 (1959). (4) C. N. Trumbore, J . Phys. Chem., 64, 1087 (1960). (5) K.Coatsworth, E.Collinson, and F. S. Dainton, Trans. Faraday SOC.,56, 1008 (1960). (6) F. S. Dainton and D. B. Peterson, PTOC.Roy. SOC. (London), A267, 443 (1962). (7) L. M. Dorfman and I. A. Taub, J . Am. Chem. SOC., 85, 2370 (1963). (8) E.J. Hart, W. R. McDonnell, and S. Gordon, PTOC.Intern. Conf. Peaceful Uses At. Energy Geneva, 7, 593 (1955). (9) H. A. Mahlman, J . Chem. Phys., 32, 601 (1960). (10) P. J. Dyne, J. W. Fletcher, W. M. Jenkinson, and L. P. Roy, Can. J. Chem., 39, 933 (1961). (11) P. Riess and B. E. Burr, Radiation Res., 16, 661 (1962).
RADIOLYSIS OF HEAVYWATER
2629
5-ml. samples of solution were used throughout; other1: Hydrogen Yielda Obtained on Irradiation of wise, the procedures described e l s e ~ h e r ewere ~ ~ ~ ~ ~Table Various Air-Free Deuterium Oxide Solutions strictly followed. Each G value given is the result of six measurements on the yield-dose curve. Soluk PD G(hydrogen) The deuterium oxide was refluxed in alkaline perM KBr Neutral 0.36 manganate for 24 hr. and then doubly distilled. It was 10-8 M KBr 1.0 0.335 further purified by radiolysis and then photolyzed at 10-8 M KBr 0.5 0.32 5 x 10-4 M moaa Neutral 0.32 2537 A. to destroy the radiation-produced hydrogen 2 x 10-3 M KN0a5 Neutral 0.30 peroxide. On infrared analysis, this final product was 1 X 10-2 M KNOs“ Neutral 0.24 found to contain 99.72 f 0.02 mole % deuterium 3 x 10-’,M KNOa” Neutral 0.193 oxide. The acidity and alkalinity of the solution were 10-1Methanol+ 5 X 1 0 d 4 MKNOa Neutral 0.79 normally adjusted using HzS04 and NaOH. At pD 10-1 M ethanol 0 . 8 N H z S 0 4 3.88 values below 1.0 and above 13.0 the results were a In presence of 10-3 M KBr. checked using reagent grade DzS04and NaOD (supplied by Merck Sharp and Dohme) to adjust the pD. In all cases the G values obtained were within *3% of with the value of 0.37 obtained by Mahlmang using those determined using HzS04 and NaOH. The actual NaN03 in DzO. pD was obtained using the correction term found by In alkaline solutions, the formation of “molecular” Glasoe and Long14 to hold over the whole pH range: hydrogen remains essentially unchanged up to pD 13.5 pD = pH meter reading 0.40. (see Table 11, column 4), when it decreases slightly at The gaseous products were measured by gas chropD 14.0 to GD, = 0.34. This behavior is similar to that matograph~.’~Calibration was carried out using Hz, observed in light water.13 Dz,and an equilibrated mixture of Hz,Dz,and HD. In this way it was possible to show that the response of the instrument to Hz,HD, and Dz was in the ratio of 1.0: Table I1 : Irradiation of Air-Free 10-8 M KNOa and 0.92 :0.70, respectively. ,411 other methods of esti5 X 1 0 - 3 M Formate Ions mation and extinction coefficients were identical with those used p r e v i o ~ s l y . ~ All ~ ~ ’of~ the chemicals used PD G(hydrogen)T U(NOz-) G D ~ GRek were analytical research grade. 6.0 0.77 2.32 0.31 2.78
+
Results The systems used in this work to determine the yields of radicals (DzO)-, D, and OD, and of “molecular” products Dz and DzOz, have all been employed in radiolytic studies of light water. It is because the reaction mechanism in each of the systems is thought to be understood, and is expected to be the same in DzO as in HzO, that they have been used here. The various systems studied will be referred to below. Molecular Hydrogen Yields. The production of hydrogen in a dilute degassed KBr solution is taken to be equal to the yield of “molecular” hydrogen. 6, was found to be equal to 0.36 i 0.01 in neutral solutions of fM KBr, and 0.34 i 0.01 and 0.32 h 001 at pD 1.0 and 0.5, respectively (Table I). This small but significant decrease of the Dz yields in acid solutions is of the same order as that observed in light water.15 As a further check, the values of GD,were measured in neutral solutions of potassium nitrate; see Table I. Plotting these yields against the cube root of the KN08 concentration gives on extrapolation to “infinite dilution” a value of Go, = 0.36. This value compares well
12.1 12.9 13.3 13.5 14.0
0.63 0.47 0.41 0.38 0.29
2.52 2.89 3.04 3.13 3.30
I n absence of 5 x 10-3 M formate. G(hydrogen)T GDZ.
-
...
0.31
...
0.31 0.29
’ GW
2.84 3.05 3.14 3.20 3.30 = G(NOz-)
+
Hydrogen Peroxide Yields. Aerated solutions of potassium bromide have been used16 to determine the yield of “molecular” hydrogen peroxide. From the mechanism postulated,16 the net peroxide production is given by G(peroxide)T = G(D2Oz) f l/z(GRed
- GOD)
(A)
where G(peroxide)T is the total measured yield of per(12) E.Hayon, Trans. Faraday SOC.,61, 723 (1965). (13) E. Hayon, ibid., 61, 734 (1965). (14) P.R.Glasoe and F. A. Long, J. Phys. Chem.,64, 188 (1960). (15) E. Hayon, ibid., 65, 1502 (1961); C. H. Cheek, V. J. Linnenborn, and J. W. Swinnerton, Radiation Res., 19, 636 (1963). (16) T.J. Sworski, J. Am. Chem. SOC.,76, 4687 (1954); A. 0.Allen and R. A. Holroyd, ibid., 77, 5852 (1955).
Volunw 69, Number 8 August 1966
E. HAYON
2630
Table Ill : Yields of Radicals and Molecular Products on ?-Irradiation of Water Da0”
- , ,
PD/PH
GRed
GOD
QDl
GDaOa
G(-DzO)
0.50 0.85 1.25 2.05 2.85 3.30 6.00 12.10 12.90 13.30 13.50 14.00
3.60 3.55 3.43 3.26 3.07 2.94 2.83 2.84 3.05 3.14 3.20 3.30
2.86 2.81 2.69 2.52 2.33 2.20 2.12 2.21 2.52 2.73
0.32
0.60
...
...
...
4.15 4.10 3.99 3.81 3.62 3.49 3.40 3.42 3.64 3.73
0.36 0.34
0.41 0.37
This work.
2.99
...
0.34
... ...
...
...
0.36
0.56
0.36
0.49
... ... ...
... ...
...
...
3.86
GRed
GOH
3.66 3.61 3.51 3.31 3.07 2.90 2.85 2.93 3.12 3.23 3.27 3.41
2.97 2.94 2.86 2.69 2.50 2.40 2.25 2.43 2.72 2.90 3.04 3.31
--
H20b GH2
0.40
...
GH202
G(- H20)
0.78
4.50 4.46 4.36 4.16 3.92 3.80 3.70 3.76 3.95 4.05 4.12 4.18
...
... ... ... ...
0.41 0.42
... ... 0.45 0.43 0.43
0.71 0.65 0.60 0.56 0.53 0.45
...
0.43 0.37
‘ See ref. 13.
oxide, G(Dz02) is the yield of molecular peroxide a t any KBr concentration, and GRed = G(D,o,- -t GD. Using the stoichiometric relationship G(-DzO)
=
+
~GD,
GRed =
~ G D , o-k , GOD (B)
eq. A can be converted to G(per0xide)T
=
2G(D202) - GD,
The Journal of Physical Chemistry
+
+
GD G(D,o)(C) The deuterium atom yield was obtained on irradiation of neutral solutions of a two-solute system’’: 10-1 M ethanol to react with the D atoms to form HD by abstraction from the alcohol, and 5 X M KKOS to react with (D20)-. In this system G(hydrogen)T = GD G(D2) = 0.79, where G(D2) = 0.32 is the yield M KN03. of molecular Dz in the presence of 5 X One obtains, therefore, GD = 0.47 f 0.02. h similar run made in light water gave a value of GH = 0.55 f 0.02. It would appear, therefore, that GE is about 15% greater than GD. GRed a t pD 0.5 (0.8 N HzS04)was obtained on irradiation of an air-free solution of 10-1 M ethanol. I n this GD,. The total hysystem, G(hydrogen)T = GRed drogen yield was found equal to 3.88, from which one can calculate GRed = 3.56. Dorfman and Taub7 found G(hydrogen)T = 3.87 on irradiation of 0.5 M CZHsOD in 0.8 N DzSO4, and 3.81 in 0.8 N H2S04,both in DzO solutions. Supposing GD to be independent of pD as is GH in light water,18the variation of GRed with pD-and therefore of G(D20)--waS determined by measuring the hydrogen peroxide formed on irradiation of aerated 5 x M ethanol solutions, as was done with light GRed
Taking GD, = 0.36, one can calculate G(D2O2)a t different bromide ion concentrations. I n all KBr solutions irradiated in the presence of air, G(peroxide)T was produced linearly with dose (0-5 X lo8 e.v./ml.) but with a positive intercept on the peroxide axis of 4-6 p M , thought to be due to the presence of organic impurities in the water.16 From the plot of G(D2O2) against [KBr]’/*one can draw a straight line to give a value of GD~O, = 0.56 on extrapolation to infinite dilution. The peroxide yield in alkaline solutions was derived by measuring the yield of oxygen formed on irradiation of deaerated solutions of ferricyanide in the presence of 40 pM ferrocyanide, as described e1~ewhere.l~I n this system, Fe(CN)63- reacts with peroxide to yield equivalent quantities of 0 2 . The results obtained are given in against [OD-]”* one Table 111. On plotting GD,O~ obtains a straight line to give a value of G D ~ =~ ,0.56 on extrapolation to infinite dilution. The decrease of GDzo, with increase in [OD-] is interpreted, as was done for light water,13 to the reaction OD OD- --t 0DzO occurring in the spurs, with the rate constant for combination of OD radicals being significantly greater than that for combination of 0- radicals to give D202. The ‘Lmolecularl’peroxide formed in 0.8 N sulfuric acid solution was determined by measuring the yield of O2 produced on irradiation of an 800 pM Ce(SO& = G(O2) = 0.60 f 0.02 was found. The solution. GD*o*
+
slight increase of GD202in acid compared to neutral D 2 0 solutions is similar to that found in Hz0.l6 Radical Yields. On irradiation of light water hydrogen atoms as well as electrons (HzO)- are formed. Similarly, one would expect the yield of total reducing species in DzOto be
+
+
(17) J. T. Allan and G. Scholes, Nuture, 187, 218 (1960); J. Rabani, J . Am. Chem. Soc., 84, 868 (1962). (18) E. Hayon, J . Phys. Chem., 68, 1242 (1964).
RADIOLYSIS OF HEAVY WATER
2631
+ HCOO- +HCOO + ODHCOO + +C02 + HO, DO2 + OD(Dz0)- + (D2O)- + Fe(CN)63- +Fe(CN)64- + D 2 0 DO2 + Fe(CN)63Fe(CN)64- + D + + O2
water.12 From the mechanism postulated by Jayson, et aZ.,19we have G(peroxide)T
=
+
GD~O,
+ GOD)
'/2(GRed
OD
0 2
(D)
0 2 ---+
The nature of the reducing species formed on irradiation and of the H02 or DO2 species is not important in this system; neither is the deuteration of C2H50H in DzO. From eq. D one can calculate (GRed GOD), taking the values for G D ~ given o ~ above. These results are given in Table IV.
--f
+
~
PD
G(D@d
3.83 3.78 3.64 3.46 3.26 3.13 3.02
a
GDZOZ GRed
0.60
+ GOD
6.46 6.38 6.12 5.78 5.40 5.14 4.92
... ...
... ...
...
0.56
Calculated yields (see Discussion).
D202
+ 2Fe(CN)2-
GReda
GOD"
3. 60b 3.55 3.43 3.26 3.07 2.94 2.83
2.86 2.81 2.69 2.52 2.33 2.20 2.09
' Taking
GRed =
G(-FeY
+ NOa- +NO2 + 20DOD + HCOO- +HCOO + ODD + HCOO- +COO- + HD HCOO + D202 +OD + COz + HOD D + OD(Dt0)NO2 + HCOO -+- NOz- + H+ + C02 NO2 + COO- +NOz- + C02
~~~
For the purpose of simplicity reactions involving deuterated formate ions were not put down. On the basis of the above reactions.
+ G(hydrogen)T - 6,
the yields of G(N02-), G(hydrogen)T, and GD, in this system were determined and are given in Table 11. In order to determine GOD yields in alkaline D2O solutions, the ferricyanide formate 0 2 system was used, as was done in light water.la In this system the following reactions are considered to occur in alkaline medium
+
0 2
+
2Fe(Cn')~~- 2D+
+ O2
+
=
GRed
+ GOD+ ~ G D , o ,
(E)
~
~
~
~~
Table V: Irradiation of Aerated Solutions Containing 800 N M Ferricyanide and 5 x 10-8 M Formate Ions
(Dz0)-
G(N02-1
--f
+
3.60.
The yields of total reducing species in alkaline solutions were determined from the irradiation of dilute aqueous solutions of KN03 and formate ions in the absence of oxygen. The mechanism is similar to that postulated in light waterla
=
+D202
Values of G(-FelI1) in 800 p M ferricyanide and 5 X M formate ions in the pD range 12-14 are given in Table V. Taking the GD,o, values given in Table VI and GEM values given in Table I1 it was possible to calculate GODfrom eq. E above. ~~
GRed
+ DO2
The above reactions are not dependent on the precise nature of the reducing species, of H02 or DO2, or of OH or 0- formed in alkaline solutions and give a net reduction yield.
~~~~
Table IV : Irradiation of Aerated Solutions of M Ethanol in D10 5X
0.50 0.85 1.25 2.05 2.85 3.30 6.00
DO2
a
PD
G( -FeX1I)
12.1 12.65 12.95 13.3 14.0
6.11 6.33 6.53 6.71 7.03
GRedQ
GOD
2.84 2.95 3.05 3.14 3.30
2.21 2.38 2.52 2.73 2.99
Values taken from Table 11.
Table VI: Yields of [KBrl, M
5x 2 x 5 x 1x
10-4 10-8 10-8 10-2
D202
in Aerated Bromide Solutions G(peroxide)
G(Dz0d
0.73 0.66 0.60 0.51
0.505 0.47 0.44 0.395
Discussion The increase in radical yields in acid and alkaline HzO solutions was a t t r i b ~ t e d ~ to ~ v al ~scavenging by H + or OH- ions of the radicals (H20)- and OH produced in the "spurs." Similarly, in D2O solutions one (19) G. Jayson, G. Scholes, and J. J. Weiss, J . Chem. SOC.,1358 (1957).
Volume 69, Number 8 August 1966
E. HAYON
2632
would expect to observe the same scavenging reactions occurring in the spurs (Dz0)OD
+ D+
+ OD-
+ DzO 0- + 20D-
4D
---f
resulting in a decrease of the back reaction to form water (Dz0)-
+ OD +DzO + OH-
In calculating GRed and GODvalues in the aerated ethanol system, it was assumed that scavenging by D+ ions in the spurs leads to equivalent increases in the yields of oxidizing and reducing species, as this is a requirement for material balance. In this way the change in the yields of GRed and GODwith variation in [D+]was calculated on the basis that GRed = GOD = '/Z(GRed GOD). These values of @Red and GOD are given in Table IV, columns 4 and 5. On the basis of the radical diffusion theory and the properties of the medium (the relaxation time and viscosity of DzO are both larger than in HzO),it was suggested5that a more extended initial distribution of radicals in DzO relative to HzO would give lower molecular product yields and higher radical yields in DzO compared to HzO. The radical and molecular yields obtained above have been summarized in Table I11 and are compared with the respective yields obtained13 in H20. It can readily be seen that at all pH values the material balance obtained in DzO is poor compared to that obtained in H2O. The reason for this inaccuracy is not apparent. Comparison of the yields in DzO and H20 solutions, Table 111, shows that GD,and G D ~are o ~ about 20% lower than the corresponding values in H2O. The yields of GRed and GODare, however, about 5-10010 lower in DzO than in HzO. Results in the literature on the yields in DzO solutions vary considerably. Thus, in neutral solutions GD, values of 0.28,30.30,80.36,60.37,9and 0.4911 have
+
The Journal of Physical Chemistry
been obtained. I n 0.8 N HzS04,GD, values of 0.29,a 0.38,l and 0.446 were obtained. The hydrogen peroxide yield was found to be 0.66in neutral solutions and 0.6E1,~ 0.77,6and 0.79l in 0.8 N HzS04. Values of G R of~ 3.48 ~ and 3.243 were obtained in neutral OZ 4- HCOOH; GRed = 3.7 in the HZ-k DzO system10; and 3.94l and 4.13 in 0.8 N HzSO4 solutions of Fe2+ 0 2 . In 0.8 N HzS04values for GODof 3.045 and 3.12l have been reported. As a check to the results obtained in this work, M ferrous ions in 0.8 N HzS04 aerated solutions of in the presence of loF3M NaCl were irradiated. After making the usual correction for the change in density of DzOsolutions compared to HzO
+
+
+ 2&,o2 = 15.0 f 0.2
G(Fe3+) = 3 G ~ ~ d GOD
was obtained, taking the extinction coefficient for ferric ions at its maximum absorption as 2375 at 25°.20 The yield of G(Fe3+) = 15.0 f 0.2 found experimentally is in fairly good agreement with the yields of radicals and molecular products obtained in this work for 0.8 N HzS04 solutions and summarized in Table 111. This G(Fe3+) value is to be compared with values of 16.22,516.72,' 16.95,3and 17.14previously reported. It does not seem possible to explain these different results. More work is clearly needed to confirm the yields of products formed in the radiolysis of heavy water before one can attempt to interpret them on the basis of the diffusion theory. Acknowledgments. The author thanks Dr. H. Hering for bringing to his attention the factors inherent in the determination of deuterium gases by gas chromatography, and Miss C. Cercy, Service Isotopes Stables, Saclay, for kmdly preparing an equilibrated mixture of Hz, HD, and Dz. (20) J. W. Boyle and H. A. Mahlman, Radiation Res., 16, 416 (1962).
