STUDIES ON
THE
MOLECULAR HYDROGEN FORMATION
place only from much higher energy levels. The higher excited tripIet state which is concluded by us to undergo dissociation may be estimated to have a range of energy of 7.3 to 6.0 eV (3.3 eV for the lowest triplet state and 2.7 to 4.0 eV for the second transition in our experimental condition), which is much higher than its
2903 dissociation energy, but is short of its ionization energy, ca. 8 eV estimated from anisole’s 8.20 eV.23 The mechanism concluded in the present paper, therefore, seems reasonable from the energetic point of view. (23) K. Watanabe, J. Chem. Phys., 26, 642 (1957).
Studies on the Molecular Hydrogen Formation (GH,) in the Radiation Chemistry of Aqueous Solutions by E. Peled and G.Czapski Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel
(Received November 17,1969)
Gaa in the y radiolysis of air-free 0.01, 0.1, and 1 ill solutions of various cations and anions were measured,
The rate constants of most of these ions with eaqat different ionic strength were directly determined by the pulse radiolysis technique. The effect of ionic strength on spurs reactions and on scavenging efficiency was studied. Difficulties previously observed, concerning the lack of correlation between ken,+ [SIand the scavenging efficiency of GH*were shown to be mainly due to ionic strength effects on ke,,+s. Hence it is concluded H and e,, eaq. It is suggested that that ea, is the main precursor of GH*through the reactions: e,, GoH, and GoH (which were proposed by Schwarz to be formed directly) result from spurs which are characterized by initially higher radical concentrations.
+
The irradiation of many aqueous systems has shown that hydrogen, with a yield of about 0.45, is formed as a molecular pr0duct.l When the concentration of some of the solutes in such systems is increased, a decrease in the molecular yield may be observed. This behavior is attributed to partial scavenging of the precursors of the molecular hydrogen by the solutes. It is generally accepted that molecular hydrogen is formed in the spurs following the recombination of its precursors.’ The detailed mechanism of the formation of HZraises several questions. (1) Which are the precursors? (2) Which are the exact spur recombination reactions responsible for the molecular yield? (3) Is the entire molecular yield due only to spur recombination reactions, and is its total scavenging possible? During the very early stages of the study of radiation chemistry of aqueous solutions, it was assumed that OH and H radicals were formed in spurs so that the origin of molecular Hz was attributed to the following reaction‘ H+H+Hz (1) Later, however, it was shown that most of the reducing species are e,, rather than H atoms and other reactions were assumed. e,,
+ H --+ Hz + OH-
(2)
+
and e,,
+ ,e,
+H z
+ 20H-
(3) I n addition, H atoms are formed a t least partially in the spurs through eag
+ H+
---t
H
(4)
Sworski2 suggested the existence of a short-lived Ha0 in the spurs, rather than H or e,, and this species is the precursor of H2. Accordingly, Hz would be formed by the reaction
H30
+ H30
Hz
+ 2Hz0
(5) Other suggested sources for Hz were excited water molecules* or hydride ions4 H-
+
--f
HzO* --t Hz HzO +Hz OH-
+
(6)
(7 )
(1) (a) J. W. T. Spinks and R. J. Woods, “An Introduction to Radiation Chemistry,” Wiley, New York, N. Y., 1964; (b) A. 0. Allen, “The Radiation Chemistry of Water and Aqueous Solutions,” Van Nostrand, Princeton, N. J., 1961. (2) T. J. Sworski, Advance in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965. (3) M. Anbar, S. Guttmann, and G. Stein, J. Chem. Phys., 34, 703 (1961). (4) M. Faragi and J . Desalos, Int. J. Radiat. Phvs. Chem., 1, 335 (1969).
The Journal of Physical Chemistry, Val. 7 4 , No. 16, 1970
E. PELED AND G. CZAPSKI
2904 Hayon6 suggested that reaction 3 is the main source of H2, with some possible additional contribution from reactions 1 and 2. His conclusion was based on the which exhibits a plot of G(HJ as a function of break in the straight line. Furthermore, Hayon and Moreau6 claimed a good agreement between lc,,,,, and the reduction of Gaz a t low scavenger concentrations while better correlation was found for the rate constants ~ H + Sat high solute concentrations. According to Faragi and D e s a l o ~ there ,~ is no correlation between k,,,+ and the efficiency of the decrease of GHz. I n order to bring a little more light to the mechanism of formation of molecular hydrogen and to determine the source of G H ~we , have taken up once again the study of the effect of certain scavengers on GHa.
Experimental Section Materials. All solutions were prepared with tripledistilled mater. Materials were all of analytical grade and were used without further purification. Dosimetry. A Fricke dosimeter served to determine the dose rates, taking G(Fea+) = 15.5. The absorbed dose in concentrated solutions was corrected according to the relative electron density of the solutions. G(H2) Determination. Irradiations were carried out in a 13'Cs y source. The dose rates were either 200 or 80 radslmin. Total doses amounted to 25,000-88,000 rads. All solutions were argon-saturated, and irradiated in 10-cm3 syringes. Gas products were determined by gas chromatography, the detailed method of which was described earlier.' All the G(H2) yields were found t o be linear with dose, and the G values are averages of 4-10 measurements. Determination of Ice,,+,. The rate constants of e,, with different scavengers as a function of ionic strength ( p ) , were determined by pulse radiolysis. A Varian V-7715 linear accelerator gave 5-MeV electron pulses, with 200 mA current. The pulse length was 0.1-0.15 psec. The irradiation cell, made of Spectrosil, was 4 em long. The analyzing light was from a 150-W xenon arc passing three times through the cell. Details of the experimental assembly are given
Results and Discussion Molecular hydrogen yields were measured in irradiated 0.01, 0.1, and 1M aqueous solutions of various solutes. The G(H2)values were corrected according to the electron density of the solutions. Br- (lod3M ) was added to all solutions in order to protect the molecular hydrogen from attack of OH radicals. When silver salts were used, M NO2was added instead of bromide. Effect of Cations on GH~. Table IDand Figure 1show the effect of various cations (which contained M NOz-) on G H ~ . We found that the M NO,- or The Journal of Physical Chemhtry, Vol. 7qr No. 16, 1970
t 0.1
c
0.0
io*
10'
I?'
( k r q t c )y.o'[s]
10"
10"
Figure 1. Gn2as a function of keas+s[S]for cationic scavengers. Values of keasia are taken for p = 0, from Table I: H, CuSO4; L, CdSO4; J, Co(ClO&; K, NiSO4; L, ZnSOl; M, 0,1M; 0,OL MnSO4; N, AgC104; P, Cu(C10,)~;-$jiM;
4,
M.
~ a Function of Different Cations Table I: G H as Solutes, M
1 . 0 cuso4 1 . 0 CdSO4 1.0co(c104)~ 1 . 0 NiSOl I . 0 ZnSOl 1 . 0 MnSO4 0 . 1 CU(C1Od)z 0 . 1 CdSOc 0.01 AgClO4 0.01 c u s o c 0.01 NiSO4 0.01 ZnSOd 0.01 C~(C104)z 1 . 0 ZnSO4 c l.OMnSO1c
UH*
0.21 0.26 0.27 0.30 0.39 0 48 0.27 0.34 0.33 0.36 0.41 0.435 0.35 0.75 0.70
(keas+s)p-Qva M - 1 see -1
4 . 5 X 10'0 6 . 4 X 1010 9 . 5 X lo9 2 . 2 x 1010 1 . 5 x 1096 (4-8) X 10Tb 4 . 5 X 10" 6 . 4 X 10'0 3 . 2 X 10'0 4 . 5 X 1O'O 2 . 2 X 10'0 1.5 X logb 4 . 5 X 10" 1 . 5 x 109* (4-8) X
kH+,,b
M-1sec-1
6 X 108
