RADIATION (%EMISTRY
OF
31
AQUEOUSSOLUTIONS
On the Contribution of H Atoms to GHain the Radiation Chemistry of Aqueous Solutions y E. Peled, U. Mirski, and G. Czapski* Department of Physical Chem‘iStTY, The Hebrew Univeraity, Jerusalem, Israel
(Received July 30,1970)
Fuhliea:twn Costs borne completely by the Journal of Physical Chemistry
was determined in neutral and in 1 M NaOH air-saturated solutions as a function of [Na-1. Et was found that N3- reduces Ga2 from 0.43 at lo-* M N3- to 0.157 at 5 M N3- in neutral solutions and from 0.37 at 0.5 M NI- to 0.21 at 4 M Na- in alkaline solutions. Through pulse radiolysis studies it was found that the value of the rate constant kea,+Ns- was 5 1.5 X lo8 M-’ sec-I and that Nt- had no effect on Ge,,. In competition experiments between N3- and isopropyl alcohol, kH+Ns- = 2.7 X 109 M-l sec-l was obtained. From Hz OHthe decrease of Ga, by efficient electron scavengers it was concluded that the reaction H e,, in spurs makes a major contribution to GHz. This conclusion together with other experimental results makes necessary the assumption that in addition to a distribution in the number of radicals in various spurs there is a distribution of concentrations of radicals in each kind of spur. The “condensed spurs” (spurs with higher initial concentration of radicals) have an important contribution to Gae.
+
It is generally accepted that in the y radiolysis of aqueous solutions, molecular hydrogen (GH2)is produced as a result of the recombination of radicals in spurs. Recently, Schware’ has reviewed and compared the results of the computer calculations made by the application of the spur diffusion model and most of the available experimental yesults. By assuming a primary source of molecular hydrogen yield (GI%?= 0.15) and hydrogen atoms yield (GHO = 0.6),he showed that no real disagreement exists between the model and the experimental results. The precursors of the remainder of the: molecular hydrogen yield were assumed to be e&, and H. These precursors mainly react in the spurs by reactions 1 and 2. ea,
+ e,,
+ 20H-
+HZ
H 3- ea, -+ Hz
+ OH-
(1) (2)
A small fraction of H2 (except in very concentrated acid rsolutions) is produced in the reaction
H+H+Hz
+ H++
+
yield of Ha has never been measured, but calculations using the diffusion model show that the contribution of reaction 2 is important. It was recently suggested’ that reaction 2 may be even more important in “condensed spurs” where the initial concentrations of radicals are higher than the average. In this investigation we have tried to determine the importance of reaction 2 in the formation of the molecular hydrogen. This has been done by measuring GHz as a function of azide concentration. Azide was chosen as it reacts very fast with H atoms but rather slowly with eaqs
Experimental Section Materials. Triple-distilled water was used for all solutions. NaOH and isopropyl alcohol were of analytical grade. NaN3 (BDH) was used without further purification. Dosimetry and Irradiation Source. The irradiations were carried out in a Csl37 y source. A Fricke do-
(3)
Other precursors of the molecular hydrogen, such as HsO,E H-,Y e:iciled water m o l e c ~ l e s ,and ~ dry electrons16were suggested. The effect of electron scavengers on G H ~provides good evidence thtit e,, is one of the main precursors of the molecular hydrogen, probably through reactions 1 and 2.6J The IrI atoms yield(G H) arises at least partially in spurs through reaction 4.
e,,
-
H
(4)
This was shown by the effect of H+ scavengers on GH.8,Y The relative contribution of reactions 1 and 2 t o the
* On sabbatical leave a t Radiation Research Laboratories, Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pa. 15213. (1) H. A. Schwarz, J . Phys. Chem., 73, 1928 (1969). (2) T. J. Sworski, Advan. Chem. Ser., No. 50 (1965). (3) M. Faragi and J. Desalos, Int. J . Radiat. Phys. Chem., 1, 335 (1969). (4) M. Anbar, 9. Gnttmann, and G. Stein, J. Chem. Phys., 34, 703 (1961). (5) W. H. Hamill, J. Phys. Chem., 73, 1341 (1969). (6) E. Hayon, Nature (London), 194, 737 (1962). (7) E. Peled and G. Czapski, J . Phys. Chem., 74, 2903 (1970). (8) A. Appleby, “The Chemistry of Ionization and Excitation,” G. R. A. Johnson and G. Scholes, Ed., Taylor and Francis, London, 1967. (9) (a) G. Czapski and E. Peled, unpublished results. (b) (keaq+El)p means that kea,+, is at the proper ionic strength ( p ) . The Journal of PhysicaZ Ch,emistry,Vol. 76,No. 1 , 1971
E. PELED, U.MIRSKI,AND G.CZAPBKI
32 simeter was used t o determine the dose rates taking G(Fe8+> = 15.5. The absorbed dose in concentrated solutions was corrected according to the increase in electron density of the solutions. The dose rates were either 200 or 80 rads/min approximately. Total doses used were 1.1 X 1OI8 t o 3.8 X 10l8eV/cm*. Q(H2) Detenainalion. All solutions were irradiated in 10-cm3 syringes and the gas products were determined by gas chromatography. Details of this technique have been described elsewhere.’O
15
!c
-
2 10 0
5 I A
Results The E$ect of Ns- on GH~. The molecular hydrogen yield was measured in irradiated aerated azide solutions. The presence of oxygen (air) prevents the reactions e,, -t- e,, -c HSand e,, -I- H20-+ H in the bulk of the solut,ion while this low 0 2 concentration hardly affects GHe. GHz values were calculated from the slopes of linear yield vs. dose plots, each from 5 to 11points. A . ~ a s i 6 Xolutions. : QHz values were measured in solutions containing 1 M NaOH and Various azide concen trations. The results are summarized in Table I.
Table I : The Dependence of Gaz on Azide Concentration in 1 M NaOH Aerated Solutions
U 3
g
5
E v
10
20 Dose
30
40
ev c X le-’’
Figure 1, The molecular hydrogen yield as a function of dose: 0,10-2 M NaNa; 0 , 0.5 M NaN3; 0, 4 M NaNs; A, 4 M NaN3 4-1 M NaOH.
Table I1 : The Dependence of Gaz on Azide Concentration in Aerated Neutral Solutions [NaNsl, M
0.5 I
2 4
0.39 0.366 0.31
0.245
10-3 10-2 0.1 0.2
0.37 0.33 0.27 0.21
Ga, values are corrected for the increase of the electron density in the solutions.
The Journal of Phg8ica.l Chemistry, Vol. 76, N o . 1 , 1971
0.43 0.42
0.38 0.38 0.35
0.5 1
a
B . Neutral Solutions. In neutral solutions the molecular hydrogen yields were found t o be linear with dose (for all azide concentrations), but a t concentrations above 0.5 M NaN3, positive intercepts were observed in yield vs. dose plots. These intercepts increased with the increase in azide concentrations as can be seen in Figure 1. It was reported” that in the photochemistry of solutions of high azide concentrations ( [N3-]> 1 M ) Hz is formed (in N3- solutions of lower concentration no Hz was found). A possible source of the hydrogen is the reaction with IT3- of an intermediate formed during the irradiation. We tried to scavenge this unknown intermediate using several scavengers (S = 0.1 M ethanol, IOv4 M Br-, or 0.05 M I-) but without any success. As we could not avoid the positive intercepts in the concentrated solutions, we calculated GEz values from the yield vs. dose plots (Figure 1). The results are summarized in Table TI.. Determinafioia of 7 C H + ~ $ - . The value of ICH+N~- was determined using the competition method. G(H2)
G H ~
0.31
0.30
0.27
0.25
0.19
0.17 0.157
2 4 5
0.18
‘CHZvalues are corrected for the increase of the electron density in the solutions. values in irradiated argon-saturated azide solutions containing isopropyl alcohol were measured. In addition, these solutions contained 5 X M NOs- to prevent the reactions ea, e,, 3 Hz and e,, iHzO -+ H in the bulk. The experimental results are given in Table 111. The competition reactions are
+
CH3CHOHCHB
NB-
+ H --+ Hz + CH&OHCH,
+ H +products ( f
E,)
(5) (6)
Thus G(H2) should obey the equation
(10) G. Czapski and E. Peled, Israel J. Chem., 6 , 421 (1968). (11) E. Burak, Ph.D. Thesis, The Hebrew University, Jerusalem,
Israel.
or
Figure 2 shows a piot of l/(G(Ht)- GHJas a function of [Np-]/[RQR]. From the slope of this plot and sec-l,I2 we obtained ks = assuming k6 -- 5 X lo7 2.7 X 109M-I sec-IL. The intercept yields GH e0.75. Table 111: The Dependence of C(R2) on the Ratio [Ns-] /[Isopropyl Alcohol] in Irradiated Argon-Saturated Solutions of 10-3 M Aside and Isopropyl Alcohol. All Solutions Contained 6 >< 10-4 M Nos-; Total Dose ~ ~ 3 0 , 0 0Rads 0 [ROHl, M
0.5 0.2 0.125 0.08 0.0625 0.05
[Na -I/ [ROH]
G(Hz)
2 x 10-* 5 x 10-8 8 >< 1.25 X 1.6 x 10-2 2 x 10-2
1.18 1.05 0.98 0.92 0.86 0.82
_ _ _ _ . _ _ I I _ . -
Determination of k e S P + ~ and , - the E$ect of N3- on Geaq-* We measured k o a P + ~using 8the pulse radidysis technique and following the decay of e,, in an argon-saturated 1 Ng- solution. We found that e,, reacts very slowly with N3-: k e a s + ~ a5 1.5 X 106 A!-1 sec-l and that 1 M N3- did not change the initial absorbance of ea,-. This would indicate that Ns- reacts neither with e,, nor with its precursors.
iscussion The results show quite clearly that the addition of N3- causes a decresse in G K z (in the presence of 5 M NI- GH*is decreased t o about one-third of its original value). This effect shows that Ns- interferes with the formation of the molecular hydrogen. It is generally accepted that most of GBHais formed in the spurs, through reactions 1-3. However, quite recently Hamill6suggested that dry electrons are the precursors of Hz. Recently some poigsible supporting evidence for the existence of ellrywa,3 f o ~ n d . ~ ~It?was ' ~ shown that the apparent ratios of the rate constant ol H+ ea, to those of eaowith H202,acetone, Cd2+,and NO3- decrease as d l these eoncentrations increase. This behavior could ii the precursor of e,,, and therefore if3 edry. Such interpretation assumes that edry reacts with all the above-mentioned scavengers of eaQexcept with 1X+. The effect of N3- on GHz could be due either t o t h e reaction of Nt- with the dry electrons or t o some pirocesses competing with the formation of in the spurs. The role and existence of dry electrons in aqueous ated solutioais are not yet well established. , I 3 et al , showed that eaq- is formed within less
+
I 10
I
___L____
15
20
Figure 2. l/(G(XXn) - 0.45) as a function of [Na-]/[isopropyl alcohol], The results were taken from Table 111,
than 10-11 see. So, if edryis the precursor of e,, and of other species, BS suggested by Hamill, then these species must be formed in less than 10-l1 sec. We can rule out, however, the possibility that N8affects GHe due to scavenging of the dry electrons. We found that 1 M NI(-has no effect on the initial yield of e,,. Furthermore, at the end of B 100-nsec pulse, the absorption due to e,, is unaltered by the addition of 1 M N8- in air-free solutions. Should the dry electrons not be the precursors of ea, but form Hz directly, the reaction of edry with azide cannot be ruled out. However, such properties of edry appear quite arbitrary and improbable. (Such an assumption would lead us to conclude that less than 30% of G H would ~ be formed in spur reactions, contrary to earlier determinations.) Thus we come to the conclusion that the effect of Nson GH) must be due to the reaction of this ion with species in the spurs. Therefore, we will consider the reactions of Ns- with e,,, OH, H+, and B atoms and the possible effects of these reactions on G H ~ . The slow reaction rate of e,, with azide excludes any effect of Na- on reactions involving e,, in the spurs. , the reaction esq i- NB- is (Even in 5 M Ns-, T ~ / of -10-1 sec, longer by more than one order of magnitude than the spur's lifetime.) The OH radical reacts with KS-to yield the NI with k ~ ~ = + 6.5 ~ ~X - lo9 M--' sec-1a12 This reaction may affect GHzonly indirectly according to the properties of the NS radical. When Na- scavenges OH in the spurs, the spurs originally c ~ n t a i n ~only ~ g e,,, H, Q changed into spurs containing Na instead of the 8x3 or at least part of the OH'S. I n this case, instead of OF in (12) M. Anbar and P. Neta, Int. J. A p p l . R a d h t . Isotop., 18, 493 (1967). (13) R. K.Wolf, M. d. Bronskill, and J. W. Hunt, submitted for
publication.
(14) G . Caapski and E. Peled, unpublished results. Tho Souriud of Phvsiurl Ch~heneistrv,Val. 76, N o . 1, 19Y.l
E. PELED, U.MIRBKI,AND G. CZAPSKI
34
108
107
1010
109
kH+NjX
[N;]
Or
(ke.,.s.)ppx
[s]
Figure 3. OB2 as a function of (keeq+s)p.g x [SIand as a function of k = + N 3 - :< [NS-] (where S is hydrated electron scavengers): 0, NaN8 1 M NaOH; A, NaNz, neutral solutions; solid line, e,q scavengers? [ (keas+s)p.p is the rate constant of the reaction of eaq S which was measured in the proper ionic strength.]
+
+
+ +
+
addition to the reactions H OH, e,, OH, and OH OH, one has to consider the reactions H N3, ea, N3, OH N,, and NB N3. If Na radicals react with e,, or H atoms faster than the ON radicals, then the azide may decrease GH?, but in the case where Na radicals react with e,, or H atoms slower than OH radicals, N3- may increase G H 2 It is indeed difficult to gauge such an effect without computer calculations on the diffusion model, but is seems improbable that replacing OH with N3 in the reactions with H and ea, could bring about a decrease of GHz down to onethird of its value as found in the absence of a8ide. Another reason for the azide’s effect on GH%might be its reaction with H+ in the spurs. As HN3 is a rather weak acid (pK = 4.7) and the initial [H+] in the spur is high, azide may scavenge H + ions and consequently decrease the amount of reaction 4 in the spur. Such an effect does not explain our results as OH- (a much more effective H+ scavenger) reduced G H z in 1 M NaOH only by about 10-150jo.1681e Figure 3 shows that N8- is less efficient in decreasing G H ~in 1 NaOH than in neutral solutions. This lower efficiency may be due to lower yields of H atoms formed through reaction 4 in alkaline solutions, as OH-- scavenges most of the H+ ions. This effect has been confirmed e~perimentally.~>g Figure 3 shows that the plots of G H z as a function of (ke,q+s)P.PX [SIgb(S being an e,, scavenger) and as a function of ~ H + z J ~X- IN8-1 produce two curves, having the same shape. The azide curve lies above the e,, scavenger curve. Several reasons may be responsible for the difference in these two curves. 1. This difference may indicate that in the spurs the H atoms have w narrower initial distribution than e,,, and thus the molecular yield will be more sensitive toward e,, scavengers than toward H atom scavengers. This behavior might indicate that reaction 1 contributes more to GH%than does reaction 3.
+
+
+
+
2. In the spurs, a fraction of the azide ions may react with H+ to yield HNa and, as kH+HNs > [H+]. We conclude from the above discussion that the effect of NI- is due to its reaction with H atoms in the spurs ( k H + N a - = 2.7 X lo9 M-l secI1)). This work confirms Schware’ calculations’ and also our previous suggestion7 that the reaction ea, H is of importance in the formation of molecular hydrogen. We suggested earlier’ that in order t o get a good agreement between many experimental results and the diffusion model it is necessary to assume a distribut’ion of radical concentrations in the various spurs in addition to the distribution of the number of radicals in the spurs. “Normal spurs” are the ones with initial concentrations of radicals about that assumed in previous calculations’ and (‘condensed spurs” are spurs with much higher initial radical concentration than “normal spurs.’’ Our hypothesis that the main part of the molecular hydroH in (‘condensed” gen is formed in the reaction ea, spurs,’ as supported by this work, removes some previous difficulties in interpretation of several experimental results which seemed in direct contradiction to the diffusion model.
+
+
Table IV : The Effect of eaqScavengers on GxPand on the Isotopic Effect in Hz Formation (a(H2)) Ref for [Scavenger], M
1 X 10-3NaBr 1 . 0 c o s o 4 + 10-3NaBr 1.ONiSOd lO-*NaBr 1 . 0 CdSOa f 10- NaBr 0 . 5 CuS04 1 0 + N a B r 1 . 0 N a N 0 3 10-3NaBr
+ +
+
GH?
0.4“ 0.33b 0.29“ 0.24” 0.24“ 0.09’
LY(H2)“
2.153~0.15 1.9rt0.15 1 . 8 f 0.15 1.65 k 0.15 1 . 6 f 0.15 1.5f0.2
a( H d
d d d d e d
a These G values have been calculated assuming GEZ~ = 0.4 in 10-8 M NaBr; see footnote f. Reference 7. ’ For 1:1 H-D aqueous solutions (for other H / D ratios slightly different results were obtained). S. Alfasi, M. Anbar, and D. Meyerstein, unpublkhed results. e C. Lifshitz, Can. J . Chem., 41, 2175 (1963). D. Meyerstein, Ph.D. Thesis, The Hebrew University, Jerusalem, Israel, 1965.
’
We have previously discussed’ the following points. A. The ‘(condensed” spurs may be the source of GHO and GET?,as proposed by f3chwarz.l B. The “condensed” spur model may explain the break of the line in plots of G H z against the cube root of the concentration of eaq- scavengers, as shown by (15) M. J. Bronskill, R. K. Wolf, and J. W. Hunt, J. Phua. Chem., 73, 1175 (1960). (16) E. Hayon, Trans. Faraday Sac., 61, 734 (lq?4).
Hayon,17 MahlmanLn,X8 and Peled, et a1.f i.e., it explains the real diffciulty of scavenging the last remnant of
GI12
6 . Theis otopic effect in the formation of Hz (a(H2)) in thc, ‘‘condensed” spurs should be about 1.6, equal H . 1 9 This value is in reato that of the reaction e,, sonable agreeiirzent with the value Schwarzl used for G H t (a@120)= 1.8) In order to obtain a proper value for the Isotopic efllect in the overall formation of the molecular hydrogen. The “condensed” spur model might also explain the effect of Itbe o ~ , , twtivengers ~on the isotopic effect in Hz formation. I t was found that e,, scavengers decrease &, and, simniltaneuusly, reduce the isotopic effect in €I2 fonmstion, AS shown in Table IV. The increase of the ei3noentrai#ionof e,, scavengers prevents mainly the formation of I T 2 in “normal” spurs; thus the rest of the 11, is fornr,ied im the “condensed” spurs. Therefore the rcxt of Lihc mlolei@ularhydrogen should have an
+
+
isotopic effect similar to that of the reaction ettq H (a = 1.6)l9 which occurs in the “condensed” spurs. The effect of LET on the isotopic &ect in W, formation is in good agreement with the “condensed” spurs model. It has been found that the isotopic effect in Hz formation decreases from 2.219for y radiation t o 1.86 and 1.93 for higher LET radiations, of 10B(n,a)lLi and %i(n,a)T, respectively.2o It could be cxpectcd that the fraction of the reaction eas 4- H (with an isotopic effect of 1.6) would increase with the inarease of the LET. Acknowledgment. I wish to thank Dr.
M. 21. Bansal
for helpful comments on this manuscript. (17) E. Hayon, Nature (London), 194, 737 (1902). (18) K. A. Mahlinan, J . Chem. Phys., 32, 601 (1960). (19) M. Anbar and D. Meyerstein, Trans. Faraday Soc., 6 2 , 2121 (19G6). (20) M. Anbar and D. Meyerstein, ibid., 61, 263 (1965).
Paramagnetic Species Produced by Ultraviolet Irradiation of Lithium, Potassium, Sodium, Magnesium, and Cadmium in
3-
ylpnentaine at 77OK1 by :F. W. Froba and J. E. Willard* Department of Chemistry, University of Wisconsin, Madison, Wisconsin bSY06 Publi‘cation costs assisted by the
(Received June 24, 1970)
U.S. A t o m k Ensrgy Commission and the University of Wisconsin
An esr singlet with characteristics expected for the signal of trapped electrons is observed when Li, K, N a , Mg, or Cd in a 3-methylpentane matrix deposited from the vapor at 77OK is irradiated a t wavelengths in the region of 250 nm. This is flanked by lines from another species which grow to the photostationary state and (decayfrom it with a half-life of less than 0.2 sec, in parallel with the singlet. The flanking lines may be due to a cationic species with which the electron recombines. A photosensitized decomposit>ionof the matrix to produce relatively stable free radicals also occurs in the case of sodium. For all the metals the photoionization threshold is lower than the gas phase ionization potential and (with the possible exception of Mg) higher than the photoelectric work function. The short trapping lifetime of the electron and the narrowness of the esr singlet indicate that the available electron trapping sites in the matrix deposited from the vapor are weaker than those in a matrix formed by cooling liquid 3-methylpentane to 77’K.
Introduction Electrons produced in hydrocarbon glasses at low temperxt,u,uresby ionizing radiation 01 by photoionization of clrgarlie amines can be trapped in the matrix in substantial yields observable2 by esr and optical spectronletry. T’&e efficiency of electron scavengers in reducing trapping yields suggests that the electrons
often travel hundreds of molecular diameters before being trapped, while decay kinetics and photobleaching quantum yields of the trapped electrons indicate that each eventually reacts with a predestined radical or (1) This work has been supported in part by the U. s. Atomic Energy Commission under Contract AT(ll-1)-1715 and by the w. F. Vila8 Trust of the University of Wisconsin.
The Journal of Physical Chemistry, Val. Y6, Wo. 1 1971
