J. Phys. Chem. 1994,98, 190-192
190
Observation of C103 Radical in Aqueous Chlorate Solution by Pulse Radiolysis Masafumi Domae, Yosuke Katsumara; Pei-Yun Jiang, Ryuji Nagaishi, Chisato Hasegawa, and Kenkichi Ishigure Department of Quantum Engineering and Systems Science, Faculty of Engineering, University of Tokyo, 7-3- 1 Hongo, Bunkyo- ku, Tokyo 1 1 3, Japan
Yoichi Yoshida Nuclear Engineering Research Laboratory, Faculty of Engineering, University of Tokyo, 2-22 Shirakata Shirane, Tokai-mura, Ibakaki 31 9- 1 1 , Japan Received: August 9, 1993; In Final Form: October 25, 19938
A pulse radiolysis study on concentrated aqueous chlorate solution was carried out. In the radiolysis of the chlorate system, C102 and ClO, radicals are produced as a result of direct action of radiation on chlorate ion with G values of 1.O and 1.5, respectively, whereas the slow formation process of ClO, radicals is not observed. The absorption peak of C103 radicals is around 330 f 10 nm and the absorption coefficient is evaluated as 4700 M-1 cm-1. The rate constants of some reactions of ClO, radical with additives were determined.
1. Introduction The radiolysis of diluted aqueous solutions has already been studied by many researchers. However, studies on the radiolysis of concentrated aqueous solutions are sparse probably because of the following problems: (i) the direct effect of radiation on solute, and (ii) thepossibleparticipationofsolutein spur reactions. It is of interest to elucidate the mechanism of the radiolysis in concentrated systems. Consequently, we have investigated in the pulse radiolysis of concentrated nitric,' sulfuric,2 and phosphoric acid solutions.3 In the radiolysis of each system, radical formation of NO3, SO4-, and H2PO4 (HPOd-, P042-) has been demonstrated to be through two processes, a fast one from the direct action of radiation on the solutes and a slow one from the reactions of OH radicals with solutes. In this work, as one of the extended studies of the radiolysis of concentrated aqueous solutions, the radiolysis of concentrated aqueous chlorate solutions was investigated. On the analogy of previous work, the formation of C103radicals by the direct action of radiation on chlorate ion is expected. There seems to be no report on C103radical in aqueous solutionsalthough the formation of C103 radicals in the gas phase has been observed previ~usly.~ Thus, this work is the first attempt to produce C103 radical in the liquid phase.
Figure 1 was observed when sodium chlorate solutions were irradiated by fast electron pulses. The transient absorption was also observed immediately atfter the electron pulses when sodium chlorate solutions were saturated with Ar or N20. No slow formation process was observed in contrast to nitric, sulfuric, or phosphoric acids. This results is in agreement with the findings reported by Buxton et al.' that chlorate ion reacts with hydrated electrons, OH radicals, and H radicals with rate constants less than 106 M-I s-1. The absorption spectrum shown as filled circles in Figure 1 is similar to Cl2-or C102 radicals which are well-known. However, the possibility of Cl2- radicals is ruled out on the basis of the following experimentalresults: (i) the formation of the absorption is very fast and completed in the duration of electron pulses, and (ii) the intensity of the absorption increases in proportion to the electron fraction of sodium chlorate. From this evidence, it is disproved that the absorption is due to the reaction products such as C12-, which can hardly be formed directly from the decomposition of C103-. Thus, the absorption must be attributed to the products resulting from the direct action of radiation on thesolute. The following are assumed as the direct decomposition processes of chlorate ion on the basis of radiolysisl-3 and photolysis8 of other oxyanions in aqueous solutions:
2. Experimental Section
ClO,-
Electron pulses of 10 ns and 28 MeV from a linear accelerator at the Nuclear Engineering Research Laboratory, University of Tokyo, were used. Details of pulse radiolysis system have already been described elsewhere.5 For dosimetry, lOmM KSCN solution saturated with N20 was used with G = 6.1 and t(472 nm) = 7580 M-I cm-I for (SCN)z-.6 G indicates the number of molecules or radicals formed per 100 eV absorbed. The absorbed energy in concentrated solutions was corrected for the electron density of the solution. Chemicals were of the highest purity available and were used as received without further purification. All experiments were carried out at room temperature (ca. 20 "C) in aerated solutions if not otherwise stated.
ClO,-
3. Results and Discussion
3.1. Absorption Spectra and Identification of Decomposition Products. A broad absorption band shown as filled circles in *Abstract published in Aduunce ACS Absrfucrs. December IS, 1993.
0022-3654/94/2098-0190$04.50/0
ClO,-
----
+ eC102 + 0C102- + 0 ClO,
(1)
(2)
(3)
According to these decomposition processes, the absorption spectrum shown as filled circles in Figure 1 must be attributed to ClO3, Cl02, Cl02-, or their superposition. The absorption spectra of C1029radicals and C102-Io have already been reported. The absorption peaks of Cl02 radicals and C102- are 360 and 260 nm, respectively. Clearly, Cl02- is excluded as a candidate. Now it is possible that the absorption spectrum shown in Figure 1 is assigned to ClOz and C103radicals or their superposition. Since the absorption spectrum of Cl02 radical is sharp as indicated in Figure 1, the absorption spectrum is not explained without the C103 radical. Thus, it seems to be reasonable that the spectrum shown as filled circles in Figure 1 is a superposition of Clop and C102 radicals. Furthermore, the absorption spectrum of CIO, radical was separated as shown in Figure 1 (see section 3.2). The absorption spectrum of C103 radical in aqueous solution 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 191
CIO, Radical in Aqueous Chlorate Solution
/M
Concentratlon of CIO;
0
I
0
Br
I
0.2 0.4 0.6 0.8 Electron fraction of solute (f,)
1
Figure 3. Dependence of G((SCN)2-)on electron fraction of solute: ( 0 ) NaClO3 solution containing 1 mM KSCN air saturated; (M) NaClO3 solution containing 1 mM KSCN and 0.4 M ethanol saturated with N20.
CI
c
2.0
3.0
3.3
3.6
Electron afflnlty / eV
Figure 2. Correlation between electron affinities of halogen atoms and wavenumbers of absorption peaks of halogen compounds: (0)XO (X; ha10gen);~JlJ~ (A)xoj;11,14 ( 0 )X2-;I3 and (M) X0-.l6
shown in Figure 1 is much broader than in the gas phase and its absorption peak is at 330 f 10 nm, which is shifted to the red compared with that in the gas phase.4 Amichai and Treinin have pointed out the linear relation between the transition energies of oxyhalogen compounds in the aqueous solution and the electron affinities of halogen atoms." In Figure 2, the correlation in the cases of XO, X03, X2-, and X0- (X, halogen atom) is represented.7J1-16Similar correlations are observed in the cases of X02-and X03-, although not included in Figure 2. According to Figure 2, the absorption peak of C103 radicals is expected to be at 328 nm, which is in good agreement with the experimental result. 3.2. Yield. Two experiments were conducted to determine the yield of the decomposition products; one is to determine the sum of the yield of oxidizing radicals, and the other is to determine the yield of C103 radicals. The total yield of the oxidizingradicals was evaluated by adding 1 mM KSCN in sodiumchloratesolution under aeratedcondition. The absorption of (SCN)2- formed by the following scheme was observed:I7J8
SCN-
+ OH(0-)
-
+ ClO,
--
SCN-
S C N + ClO,-
SCNOH1.1 x 10" M-' C'(1.8
+
+ 0, H + 0,
X
(6)
(7) The absorption of hydrated electron decays rapidly because of
0;
HO,
1.9 X 10" M-' s-'
(8)
2.1 X 10" M-' s-l
(9)
The yield of (SCN)2- is equal to the sum of the yields of Clop and 0- radicals produced directly from chlorate ion and OH radicals from water, Cl02 cannot oxidize SCN- because of lower redox potential of C102 than SCN.I9 Thus, the following equation is obtained, G((SCN),-) =fwG(OH) +f,IG(ClO,) + G(O-11 (10) wheref wandf, are the electron fractions of water and the solute, respectively. Equation 10 isderived from the hypothesis that the decomposition of water and the solute by radiation in a concentrated system occurs in proportion to their electron fractions. Experimentalresults areshown as filled circlesin Figure 3, where G((SCN)2-) varies linearly as the electron fraction of the solute increases. From this result, eq 10was proved reasonable. By extrapolatingthe experimental result tof, = 1, eq 11 is obtained. G(ClO,)
+ G ( 0 - ) = 2.5
(11)
Since 0-is formed as the result of the decomposition process (2) and the yield of 0- radicals is equal to that of C102 radicals, the eq 12 is obtained. G(ClO,)
+ G(Cl0,)
= 2.5
(12) The second experiment was carriedout with the sodiumchlorate solutions containing 1 mM KSCN and 0.4 M ethanol saturated with N20. In this system, eaq-produced by the decomposition process (1) and by radiolysis of water was scavenged by N20, and converted to OH radicals.18 Both 0- from C10,- and OH from water were scavenged by ethanol.ls e-(e,;)
(4)
lo9 M-'s-') (5)
-
-
e,,
OH(0-)
SCNOH- S C N OH- >5 X lo7s-' 6.8 X lo9 M-' s-l S C N SCN- (SCN);
+
the presenceof02 through (8).19 Thus, theabsorptionof hydrated electron does not interfere with reading the absorptionof (SCN)2-. H radicals are also scavenged by 0 2 through (9).19.9
+
+ N 2 0 -OH
+ ethanol
-
-
9.1
X
(13)
lo9 M-' s-'
ethanol radical 1.9 x io9 M-' s-I(i.2 x io9 M-I
s-I)
(14)
H ethanol ethanol radical 1.7 X lo7 M-I s-' (15) However, C103and C102 radicals do not react with ethanol, which was confirmedby the preliminary experiment. When thechlorate solution containing 0.2 M ethanol was irradiated by electron
Domae et al.
192 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
overall reaction. This value is considered the rate constant of reaction 4, which was confirmed by the computer simulation taking reactions 4 and 7 into consideration. The rate constant of reaction 18 was estimated from the decay rate of C103 radical.
+
L8
-O 0
3
6
9
12
[SCN’] / mM Figure 4. Dependence of the formation rate of (SCN)2- on the concentration of SCN-.
pulses, the decay of the transient absorption due to the C103and C102 radicals was not observed during the time scale of 10 ~ s . Thus, SCN- is oxidized only by C103,and the yield of (SCN)zis represented by the following equation.
-
+
C10, Fez+ Fe3+ C10; 7 .O X 10’ M-‘ s-’ ( 18) Experimental results show lower reactivity of Cl00 radical than NO3, SO4-,and HzP04 (HP04-, P042-) radicals. The decay O f C103radicals was not observed when sodiumchlorate solutions containing ethanol, acetone, chloride ion, or nitrite ion were irradiated. It seems that C103 radicals are not reactive toward organic compounds. Although the C103 radical can oxidize SCN-, it cannot oxidize C1-. EO(SCN/SCN-) and Eo(CI/Cl-) are reported as 1.66 and 2.41 V vs NHE, respectively.20 Thus, Eo(C103/C103-)seems to be placed between 1.66 and 2.41 V vs NHE. There has been an estimation of redox potential of C103 radical in the aqueous solution, l?/(c103/c103-) = 2.1 V vs NHE” which is consistent with our experimental results.
Acknowledgment. We thank Mr. T. Ueda and Mr. T. Kobayashi for their assistance with the experiments. The work was supported in part by a Grant-in-Aid for Scientific Research (No. B-04453163), The Ministry of Education, Science and Culture, Japanese Government.
G((SCNJ-) =f,G(C103) (16) Experimental results are shown as filled squares in Figure 3. By extrapolating this experimental result to& = 1, the yield of C103 was evaluated as 1.5.
References and Notes
G(C103) = 1.5 (17) From eqs 12 and 17, the yields of C103and C102 were determined as 1.5 and 1.0, respectively. The absorption spectrum shown as filled circles in Figure 1 is proved to be a superposition of absorption spectra of C103 and C102 radicals, and the yields of C103 and ClOz radicals are evaluated. The absorption spectrum and the molar absorption coefficient of C 1 0 ~radicals in the aqueous solution are k n 0 ~ n . l ~ Thus, the absorption spectrum of C103radicals was obtained by subtracting the product of the yield and the molar absorption coefficient of C102 radicals (shown as filled triangles in Figure 1) from the absorption spectrum shown as filled circles in Figure 1. The result is shown as filled squares in Figure 1, which is the absorption spectrum of C103 radical in the aqueous solution. 3.3. Reactivity. The rate constants of some reactions of C103 radical with additives are evaluated by fitting the decay curve of C103radicals or the formation curve of the products by the reaction of C103radical with additive through first-order kinetics. Figure 4 shows the relation between the formation rate of (SCN)2- and the concentration of SCN- as an additive. The formation rate in Figure 4 increases linearly with the concentration of SCN-, whereas (SCN)z- is formed not by a single-step reaction but through successive reactions (reactions 4 and 7). From the slope in Figure 4, 3.4 X lo9 M-I s-1 is obtained as a rate constant for
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