Z. D. DRACANI~ AND I. G. DRAGANI~
3950
Studies on the Formation of Primary Yields of Hydrogen Peroxide and Molecular Hydrogen (GH,o~and GH,) in the
Radiolysis of
Neutral Aqueous Solutions by Z. D. DraganiC and I. G. DraganiE* Boris KidriE Institute of Nuclear Sciences, Belgrade, Yugoslavia (Received J u n e 22, 1071) Publication costs borne completely by T h e Journal of Physical C h e m i s t r ~
The formation of GH201and G H was ~ studied in yirradiated aqueous solutions containing selected mixtures of scavengers for both oxidizing and reducing primary free radicals. Experimental verification was made of various assumptions based on the free-radical model of water radiolysis. It was confirmed that efficient scavenging of OH radicals not only reduces G E ~ but o ~ also increases GHz by depressing the extent of water reforming reactions and making more eas- available for increased Hz formation. Similarly, it was shown that the removal of eaq-leads not only to a decrease in GH2but also to an increase in G H ~ o ~ These . effects are additive, and empirical relations were proposed to correlate the expected primary molecular yields with the reactivities toward OH and Unifying curves were obtained for the dependence of observed fractional yield changes, GM/GsIo, on reactivities. They point out that the secondary spur reactions of radicals with products of radical-solute scavenging reactions are rather exceptions even at solute concentrations of about 1M and reactivities of 1010 sec-1.
Introduction The free-radical model of water radiolysis,l which has furnished satisfactory explanations for most of the experimental observations, 2 - 4 assumes that in irradiated aqueous solutions the primary free radicals (OH, eaq-, H) disappear mainly by the following reactions.
+ OH HzOz OH + eaq- -+ OHOH + H HgO OH + S1 Pi 2Hz0 eaq- + eaqHz + 20Heaq- + H30++H + H20 eaq- + H +Hz + OHOH
---t
--f
.--t
__f
(1) (2)
(3)
(4) (5)
(6)
HzO
eaq-
+ Sz
+Pz
H+H+Ht
+
(7) (8) (9)
H Sz +P3 (10) Accordingly, the formation of primary hydrogen peroxide (GH~OJis in reaction 1, and the reactions 5, 7, and 9 account for the major part of primary yield of molecular hydrogen (GHJ. I n neutral water or dilute aqueous solutions G H ~ = o ~0.67 and G H ~= 0.45, as derived from various measurements.1 Recent studies have clearly confirmed that an increase in concentration of an efficient O H scavenger, SI, leads to a decrease in GH~o,which is proportional The Journal of Physical Chemistry, Vol. 76, N o . 26, 1071
only to VOH = k O H t S 1 [ 8 1 ] , in reciprocal secondse6 Also the observed decrease of GH2was found to be proportional to the reactivity toward the hydrated electrons, veaq- = keaq-+s2[S2],in reciprocal seconds, regardless of the chemical nature of S2.6 The above findings represent an important argument for the assumption that the origin of primary molecular yields should be sought in recombination reactions (eq 1, 5, 7, and 9). However, new sources of primary molecular products and some limitations of the diffusion model (eq 1-10) have been suggested in recent considerations of very early effects of water radiolysis.’ It was, therefore, interesting to get further information concerning the formation of primary HzOz and Hz. If the recombination reactions are indeed of importance for the origin of GH2O2and GH2,then the yield decreases should also be observed in solutions containing simultaneously larger amounts of scavengers for both oxidizing and reducing primary species (SI, Sz, and 53); in the above-mentioned studies, the irradiated solutions con(1) A. 0. Allen, “The Radiation Chemistry of Water and Aqueous Solutions,” Van Nostrand, Princeton, N. J., 1961; I. G. DraganiO and F. D . Dragani6, “ T h e Radiation Chemistry of Water,” Academic Press, New York, N. Y., 1971. (2) A. Mozumder and J. L. Magee, Radiat. Res., 28, 215 (1966). (3) A. Kuppermann in “Radiation Research 1966,” G. Silini, Ed., North-Holland Publishing Co., Amsterdam, 1967, p 212. (4) H. A. Schwars, J . Phys. Chem., 73,1928 (1969). (5) 2.D . DraganiO and I. G. DraganiE, ibid., 73,2571 (1969). (6) E. Peled and G . Czapski, ibid., 74,2903 (1970). (7) W. H. Hamill, ibid., 73, 1341 (1969); P . L. T. Bevan and W. H . Hamill, Trans. Faraday Soc., 66, 2533 (1970); T. Sawai and W. H. Hamill, J.Phys. Chem., 74,3914 (1970).
FORMATION OF PRIMARY YIELDSOF H202AND H2 tained only the scavenger reacting with the precursor of the molecular product measured (i,e., OH scavenger in HzOz or eaa- in Hz studies). The model represented by eq 1-10 could also be tested by experimental verifications of the following assumptions. (a) Efficient removal of ea,- should not only re~ also increase G H ~ o ~The . reason is that duce G H but more OH radicals should remain available for reaction 1 because of the depression of reactions 2 and 3. (b) Efficient removal of OH radicals should not only reduce C H ~but O ~also increase G H by ~ depressing the extent of water reforming reactions (eq 2 and 3) and making more e,, - available for increased formation of Hz in reactions 5 and 7. The purpose of this study was the experimental verification of the above assumptions. Simultaneous presence of larger amounts of scavengers for both oxidizing and reducing primary species (reactivities up to 1O1O sec-l) made the conditions in most of the cases studied deliberately more complex than in other studies on the origin of primary molecular products. It was expected that in some cases PI, Pz,and P3 would react with primary oxidizing and/or reducing radicals. This should cause each system to behave quantitatively different, making the reaction scheme more complex than given above by eq 1-10. Also, it mas interesting to compare these experimental results with theoretical predictions of the diffusion model, derived for simpler conditions (initial cCincentrations of SI, Sz, and S3 set equal to 1 X l o w 3Ill),where the secondary spur reactions of radicals with products of radical-solute scavenging reactions are not expected to occur. Experimental Section All the chemicals used in this study were (Merck or BDH products) of the highest purity available and were not subjected to any additional purifications. The purification of water and the sample preparation were carried out by the standard procedures previously de~ c r i b e d . ~Because of acetone loss on degassing its concentration was always determined; under well standardized working conditions a good reproducibility ( = t 2 % ) was reached (e.g., 0.60 M starting concentration gave 0.41 M acetone in deaerated solution). Irradiations were carried out using a 3000-Ci (nominal) radioactive cobalt source giving 2.4 X 1019eV g-l hr-l. Absorbed doses varied from 2 X lo1' to 12 X 10'' eV g-' and were corrected for the electron density of the solution studied. Hydrogen peroxide was determined by the 1 1 ' method8 with an accuracy of ~ 1 ~ 2 7exceptionally 5; it was =t4% in the case of very large amounts of scavengers present and low yields measured. The optical density measurements were made in 4-cm c e h and against water. Reference samples contained the solutes at the concentrations under study and the reagent; they were prepared and measured simultaneously with ir-
3951 radiated samples. Before the analysis some Hz02 (about 1 X M ) was always added to eliminate the errors due to the presence of trace amounts of reducing impurities. Optical densities were stable with time except for solutions containing both formate and acetone, where the measurement conditions had to be standardized. Molar extinction coefficients were between 23,800 and 21,720 M-l cm-l at 350 nm and 24". The value of 23,800 M-l cm-' was found to be correct for the dilute solutions of all of the scavengers studied in this work as well as for 1 M solutions of NO3-, ethanol, isopropyl alcohol, and their mixtures. For 0.41 M acetone and its mixtures with ethanol and isopropyl alcohol €350 = 23,250 M-l cm-l. In the case of 1 M solutions of HCOO- and Nos- the molar extinction coefficient was 22,060 Ill-l cm-'; €360 21,720 M-' cm-l was found for 1 M formate solutions. Molecular hydrogen was measured by gas chromat o g r a p h ~ ;hydrogen ~ was separated on a 6-m column of silica gel at 50" and argon as the carrier gas. The accuracy was =k2%. The accuracy in all radiation chemical yield measurements was better than =t4Oj,. Results Table I summarizes the yields of hydrogen peroxide measured in aqueous solutions containing various combinations of substances known as efficient scavengers for OH and ea,-. The yields of molecular hydrogen measured in irradiated solutions of selected mixtures of free-radical scavengers are presented in Table 11. Table I11 shows how the yields of HzOzand HSdepend on the concentration of one solute which is at the same time efficient scavenger for both oxidizing and reducing primary radicals. The yields given in Tables 1-111 were calculated from concentration-dose plots which were linear over the studied absorbed dose range. I n some of the HzOzmeasurements the dosage plots were not straight lines, and the corrected values had to be used as the initial yields. These were obtained as the G(Rz02) readings at zero dose on diagrams where pointby-point peroxide yields were plotted against dose. Discussion The systems used in the present study were chosen in such a way that the measured values of G(H202) and G(Hz) represent the corresponding primary yields, G H ~ oand ~ GBz. I n constructing the yield-reactivity curves (Figures 1-4) the experimental values from Tables 1-111 were directly used. The reactivities were calculated as the products of scavenger concentration and the rate constant. Table I V summarizes the values of rate constants used in these calculations. It (8) A. 0. Allen, C. J. Hochanadel, J. A. Ghormley, and T. W. Davis, J. P h w . Chem., 56, 675 (1952); H.A. Schwarz and A. J. Salzman, Radiat. Res., 9,502 (1958). (9) Lj. PetkoviA, M. Kosani6, and I. Drag&&, Bull. I n s t . Nucl. sei., Boris Kid& (Belgrade), 15,9 (1964). T h e Journal of Physical Chemistry, Vol. 76, N o . 26, 1971
3952
2. D. DRAGANI~ AND I. G. DRAGANII~
Table I : Yields of Hydrogen Peroxide Measured in Deaerated Aqueous Solutions Containing Various Concentrations of Efficient Scavengers for OH and eaq-
NaNOa Scavenger for OH, IM
CxH60H 1 x 10-8 2.5 X 5 x 10-3 1 x 10-2 2.5 X 0.1 0.25 0.3 1.0 2.5 HCOO - a 2.5 X 1 x 10-2 2 . 5 X IOp2 0.1 0.25 1.o
_______________
1 X 10-2
2.5 X 10-8
5 X 10-2
0.1
0.25
0.5
0.67 0.67 0.63
0.66
0.67
0.71 0.69
0.71
0.75
0.77 0.76
0.78
0.72 0.66 0.59
...
...
...
0.65
...
... ... I
.
.
...
...
...
...
0.51
0.47 0.34 0.28
...
...
...
...
. I .
*
...
... ... -
3
.
*..
...
...
9 . .
I
I
...
. . I
,
.
...
. . I
.
I
.
I . .
. . I
I . .
...
*..
...
0.30
0.32
... ...
2.5 X 10-2
5 X 10-1
0.10
...
0.72
0.74
I
,
,
(
I
.
...
...
0.35
0.36
...
,
.
,
I
...
...
...
... 0.58
... ...
...
...
...
I
.
.
...
*..
I . .
1.0
*.. .
I
.
...
I , .
* I .
0.38
0.41
... ...
0.48
*.. ...
0.76 0.72 0.67 0.61 0.54 0.37
...
...
...
0.40
0.50
0.55
1.5
...
...
...
0.34
...
*.. I . .
... ...
...
*.. ...
...
meaeured in the presence of eaq- scavenger,
...
.
0.37
.,.
... I
...
...
0.68
...
...
...
...
0.67
...
0.36
... ...
...
1 X 10-2
...
I , ,
*.. ...
0.66 0.60 0.54 0.48
...
7--------------G(HzOz)
. . I
...
...
.
CzHaOH
a
...
0.58 0.50
5 X 10-8
(CH3)zCHOH 1 x lo-* 2.5 x 10-2 2.5 x 10-1
- scavenger, M----------------
5 X 10-8
Scavenger
0.1 1.0
e.,
2.5 X 10-8
for OH, M
2 x 10-8 1 x 10-8
U(Hz0z)measured in the presence of
...
...
...
...
. . I
I . .
... . . I
...
...
...
...
... ... ...
0.41
I
.
,
0.72 0.59 0.39 0.77 0.66 0.53
0.80
...
...
...
...
0.43
... ... ... ...
I , ,
. . I
...
...
...
... .
I
,
0.47
* . I
...
...
These data have been obtained by Mrs. N. MrkiC as apart of her B.Sc. thesis at the University of Belgrad, Yugoslavia, 1971.
should be noted that for eaq- reactivities the dependence of the rate constant on the ionic strength was taken into accountO6 Also, in calculating the OH reactivities account was taken of recent findings concerning the pH dependence of rate constants of reactions between halide ions and OH radicals.1°
Dependence of Measured G H ~ o n~ Reactivities toward OH and eaQ-. Figure 1 summarizes the results on the ~ effect of OH scavenger concentration on G H ~ omeasured in the presence of a constant amount of eaqscavenger. It can be seen that increased reactivity toward OH radicals leads to a decrease in the observed HzOz yields also in the presence of larger amounts of eaq- scavenger: 0.25 M nitrate (curve l), 0.025 M nitrate (curve 2), and 0.0025 M (curve 3). I n the presence of larger amounts of eaq- scavenger the absolute values of HzOzare larger, pointing to an increased formation which takes place even at high OH reactivities. Curves 1-3 are practically parallel, indicating that the The Journal of Physical Chemistry, Vol. 7 6 , N o . 26, 1871
mechanism of formation of primary hydrogen peroxide is practically the same. In most of the cases presented in Figure 1 ethanol in various concentrations was used as OH scavenger. To verify the general character of the effect observed, ethanol was replaced in some experiments by sodium formate or isopropyl alcohol (2.5 X to 1 M ) , both efficient OH scavengers. Also, acetone (0.41 M ) was used instead of nitrate in some irradiations. These cases are also presented in Figure 1. It can be seen that the data fit well the yield-reactivity curves, confirming that the effect observed does not depend on the chemical nature and the combination of the scavengers used; it is dependent on the reactivity only. Figure 2 shows how the increased reactivity toward eaq- leads to an increase in measured G H ~ in o ~the pres(10) ,M.Kosanid and I. Draganid in “Proceedings of the Third Tihany Symposium on Radiation Chemistry,” J. Dobo and P. Hedvig, Ed., Academiai Kiado, Budapest, 1971.
FORMATION OF PRIMARY YIELDSOF H20zAND H2
3953
Table I1 : Yields of Molecular Hydrogen Measured in Deaerated Aqueous Solutions Containing Various Concentrations of Efficient Scavengers for e&q-and OH 0.7Scavenger for eaq-, M
G(H2) measured in the presence of OH scavenger, M 5 X 10-4
0.1
0.45 0.43 0.34 0.19 0.09
0.44
1.0
0.50
0.54
I-
NOS2.5 X 10-4 2.5 x 2.5 x 10-2 0.25 1.0
Nos-
...
2 . 5 10-4 ~ 0.25
...
NOS2.5
0.5
x
...
10-4 1
x
lo-*
HaOe"
*..
. a . I . .
I
0.21
I .
0.24
... Br..
,
..
0.49 0.23
I , .
CNS-
.
0.44
0.5
0.1
$0.5-
0
...
0.44 0.29 0.18
0.45 0.43 0.38 0.27 0.17
Cu2+
1 x 10-4 1 x 10-3 1 X 10-8 0.1 1.0
0.47 0.40 0.34 0.29 0.22
0.46
*
.
I
1.5
0.51 a
*..
...
...
0.28
0.30
0.34
. I ,
I
,
,
0.54 0.53 0.48 0.37
I
. , a
I
I
IO'
IO6
0.40
I
I
0.1'
1.0
0.48
...
-
0'31
. ,.. ,,
Br-
2 X 2 x 10-3 2 X 10-2 0.2 2.0
-
1
I
I@ koH,s,CSll, sec".
1
I
IO'O
109
Figure 1. Dependence of observed primary hydrogen peroxide yield on hydroxyl radical reactivity in deaerated solutions containing eaq- scavenger. Curve 1: 0,0.25 M NO3CzHaOH; A, 0.41 M (CH3)zCO CzHsOH; V, 0.41 M (CHs)zCO (CH3)zCHOH; 0 , 0.25 M NOaHCOO-. Curve 2: 0, 0.025 NosC&,OH. Curve 3: X, 0.0025 M NosCzHsOH.
+
+
+
+
+
+
I . .
Br-
,..
0.47
...
*.. ...
...
...
0.32
...
*..
0.49
0.53 0.51 0.45 0.38
..,
... 0.33
...
...
a These data have been obtained by Mrs. N. Mrkif as a part of her B.Sc. thesis at the University of Belgrade, Yugoslavia,
1971.
Table I11 : Yields of Hydrogen Peroxide and Molecular Hydrogen Measured in Deaerated Aqueous Solutions of Acrylamide
t
Solute, M
G(Hz0i)
G(Ha)
2 x 10-4 2 . 5 x 10-4 1 x 10-3 2.5 x 10-3 1 x 10-2 2.5 x 10-2 5 x 10-2 0.1 0.25 0.5
0.68
...
...
0.65 0.63 0.60 0.54 0.53 0.47 0.41 0.36
0.46
*..
0.42 0.32 I
,
,
0.28
0.k10'
I
I
I
1
to9
IO6
I
1
10'O
kaiq+s,tSzl, sec-I.
Figure 2. Dependence of observed primary hydrogen peroxide yield on hydrated electron reactivity in deaerated solutions containing OH scavenger. Curve 1: X, 1 X M CZH~OH Nos-; V, 1 X loe3 M CzH60H (CH3)nCO. Curve 2: 0 , 0.1 M CzH60H NO$-. Curve 3: 0, 1.0 M CzHbOH NO3-; A, 1.0 M CzH50H (CH3)&0. Curve 4: 0, 1.0 M HCOONos-.
+
+
+
+
+
+
0.19
ence of larger amounts of OH scavenger: 1 M ethanol (curve 3), 0.1 M ethanol (curve 2), and 1 X 10-8 M (curve 1). Curve 4 represents the data with 1 M formate ion instead of ethanol. The data with acetone (1 X to 1.5M ) , used instead of nitrate as hydrated electron scavenger, agree well with the available yieldreactivity curves. As in the above case, we see that the formation of primary hydrogen peroxide yields depends
on the reactivity only and not on the chemical nature or the combinations of the scavengers used. Dependence of Measured GHI o n Reactivities toward eaq- and O H , Figure 3 shows that increasing reactivity toward eaq- causes a decrease in G Halso ~ in the presence of larger amounts of OH scavengers. The absolute values of the yields increase with increasing OH scavenger concentration: 1 M iodide or 1.5 M bromide V iodide, or 1 X M bromide (curve 1), 5 X lod4 1 (curve 2). The trends of yield-reactivity curves are The Journal of Physical Chemistry, Vol. 76,N o . 26, 1971
Z.D. DRAGANI~ AND I. G. DRAGANI~
3954 Table IV: Rate Constants Used in the Reactivity Calculation"
NOa-
