y RADIOLYSIS OF METHANOL SOLUTIONS CONTAINING NITROUSOXIDE
4245
Light-Induced and RadiationJnduced Reactions in Methanol. I. Radiolysis of Solutions Containing Nitrous Oxide
by Warren V. Sherman' Smeq h'uclear Research Centre, Yavne, Israel
(Received April 4 , 1967)
A study has been made of the gaseous products of the Co60 y radiolysis of methanol solutions containing nitrous oxide alone and in the presence of a number of solutes. The results with organic solutes are consistent with simple competition between nitrous oxide and the solute for solvated electrons, and an estimate is made of the relative reactivities of these solutes. The effect of sulfuric acid on product yields was also studied. While the latter depressed the nitrogen yield efficiently, the results cannot be interpreted in terms of a simple competition between nitrous oxide and the hydrogen ion for the precursor of nitrogen. A chain reaction was observed in the radiolysis of solutions made alkaline with potassium hydroxide, with considerably enhanced yields of all three gaseous products (hydrogen, nitrogen, and methane). The chain length was found to be dose rate dependent. Inhibition of the chain was brought about by the addition of small concentrations of nitrobenzene, benzophenone, or carbon tetrachloride, while the presence of benzene even in high concentration had little effect. Catalysis of the chain reaction was observed in the presence of acetone. The mechanism of the chain reaction is discussed, and it is concluded that there is evidence for both the solvated electron and the methoxyl radical anion acting as chain carriers.
Introduction There is extensive evidence that solvated electrons and hydrogen atoms play important roles in radiationinduced reactions occurring in methanol and methanol solutions.2 It has been shown that though nitrous oxide is very reactive toward electrons produced in watera,' and in 2-propan01,~it reacts relatively slowly with hydrogen atoms. The study of the radiolysis of methanol solutions containing nitrous oxide would therefore appear to be a promising means of distinguishing the reactions of the solvated electron with the aim of looking a t the influence of this solvent on its reactivity toward a range of solutes. During the course of the present work, two reports on the radiolysis of methanol solutions containing nitrous oxide have a p ~ e a r e d , ~and J a comparison is made between these data and the present results. Experimental Section Methanol (Fisher spectroanalyzed) was refluxed with dinitrophenylhydrazine and sulfuric acid for 24 hr and
subsequently distilled through a 2-m helix-packed column. Both operations were carried out under an atmosphere of nitrogen, and only the middle third of the distillate was retained for use. Methanol thus purified gave G(HJ in the range 4.95-5.01; the mean value of six determinations was 4.98. Nitrous oxide (Matheson) was purified by three trap-to-trap distillations on the vacuum line with the rejection of the first and last quarter of each cycle. The gas was then stored in a bulb on the vacuum line and pumped at liquid air tem~
~
~~
(1) The Radiation Laboratory, University of Notre Dame, Notre Dame, Ind. 46556. (2) H. A. Schwarz, Ann. Rev. Phya. Chem., 16, 347 (1965). (3) F. 5. Dainton and D. B. Peterson, Proc. Roy. SOC.(London), A267, 443 (1962). (4) J. Jortner, M. Ottolenghi, and G. Stein, J. Phgs. Chem., 66, 2037 (1962). (5) W.V. Sherman, ibid., 70, 667 (1966). (6) H. Seki and M. Imamura, Bull. Chem. SOC.Japan, 38, 1229 (1965). (7) J. Teply and A. Habersbergerova, 2nd Tihany Symposium on Radiation Chemistry, Budapest, Hungary, 1966, Preprint 0/30.
Volume 71, Number IS December 1967
WARRENV. SHERMAN
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5
I}
I
I
I
Table I: The
Radiolysis of Methanolic Solutions Containing Benzene and Nitrous Oxide. Product Yields As a Function of Benzene Concentration"
4
0.50 0.50 1.0 1.0 1.0 1.0 1.0 5.0 5.0 5.0 5.0
3
d 2
0.20 0.40 0.20 0.40 0.60 0.80 1.0 0.40 0.60 0.80 1.0
Dose rate, 2.0 X ml-1.
3.05 2.69 3.09 2.59 1.76 1.91 1.93 2.57 2.64 2.10 1.80 ev ml-1 hr-1;
0.48 0.38 0.85 0.69 0.62 0.50 0.42 1.19 1.09 1.00 0.90
0.24 0.31 0.30 0.24 0.24 0.26 0.23 0.13 0.37 0.17 0.23
dose, 1.0 X 1019 ev
Table I1 : The y Radiolysis of Methanolic Solutions of Nitrobenzene and Nitrous Oxide. Product Yields As a Function of Nitrobenzene Concentration" "
II
0
10-8
10-
10-1
Ni0, M Figure 1. The y radiolysis of methanol solutions containing nitrous oxide. Product yields as a function of nitrous oxide concentration. Dose rate, 2.0 X 10'0 ev ml-1 hr-1; dose, 1.0 X 10'8 ev d-l: 0, Hz;0,N2; A, CHd.
perature for a t least 30 min prior to use to ensure complete freedom from oxygen. All other materials were of reagent grade and were used without further purification. The procedure for preparation and irradiation of the samples and analysis of gaseous products was essentially as described previously.6
Results 1. Solutions Containing Nitrous Oxide Alone. The sole radiolytic products gaseous at liquid air temperature were hydrogen, nitrogen, and methane. The 100-ev yields are plotted as a function of nitrous oxide concentration in Figure 1. At concentrations up to M, G(N2)N AG(H2), while above this concentration G(N2)became progressively greater than AG(H2). The effect of total dose on yields was checked in the cases of the 5 X and 5 X M solutions. No dose dependence was found in the range 5 X 10lato 6 X 1019 ev m1-I. Contrary to previous reports, no carbon monoxide could be identified in the radiolysis of pure methanol,8 and none was noted in the radiolysis of solutions containing nitrous oxide.
1.0 1.0 1.0 1.0 50 50 50 50
" Dose
0.20 0.40 0.60 1.0 10 20 30 50
4.02 3.88 3.72 3.74 3.55 2.97 2.78 2.74
rate, 2.0 X l O I 9 ev ml-1 hr-1;
0.74 0.58 0.39 0.32 1.31 0.97 0.70 0.47 dose, 1.0
0.34 0.25 0.23 0.21 0.20 0.14 0.20 0.16
x
1019 ev
d-1.
6. Solutions Containing Nitrous Oxide and a n Organic Solute. A study was made of the effect of benzene and nitrobenzene on the yields of gaseous products (Tables I and 11). A less comprehensive study of the effect of a number of other organic compounds is set out in Table 111. It is possible to classify these compounds into two groups by the efficiency with which they depress the nitrogen yields: group I, compounds which significantly depressed G(N2) when present in concentrations comparable to that of nitrous oxide (nitrobenzene, carbon tetrachloride, benzonitrile, acetophenone, acetone, benzyl chloride, chlorobenzene) ; group 11, compounds which required concentrations greatly in excess of that of nitrous oxide in order to de(8) M. Imamura, s. U. Choi, and N. N. Lichtin, J . Am. Chem. SOC., 85, 3664 (1963), and references cited therein.
7
RADIOLYSIS OF METHANOL SOLUTIONS CONTAINING NITROUS OXIDE
Table I11 : The
y
4247
Radiolysis of Methanolic Solutions of Nitrous Oxide. Effect of Organic Solutes"
Solute
Toluene Fluorobenzene Fluorobeneene Chlorobenzene Chlorobenzene Acetophenone Acetophenone Benzonitrile Benzonitrile Benzyl chloride Benzyl chloride Acetone Acetone Carbon tetrachloride Carbon tetrachloride
500 100 500 1.0 50 1.0 50 1.0 50 1.5 50 1.0 50 1.0 50
0.50 1.0 5.0 10 50 1.0 50 1.0 50 1.0 50 1.0 50 1.0 50
Dose rate, 2.0 X lo'@ev ml-l hr-1; dose, 1.0 X lo1@ ev ml-1.
Table IV : The y Radiolysis of Methanolic Solutions Containing Sulfuric Acid and Nitrous Oxide. Product Yields As a Function of Concentration"
kN,ob
1.0 x 1.0 x 1.0 x 1.0 x 1.0 x 1.0 5.0 x 1.0x 2.0 x 5.0 x 5.0 X 1.0 x 2.0 X 1.0 x 1.0 x
10-6 10-4 10-3 10-2 10-1 10-4 10-3 10-3 10-8
loe3 10-2 lod2 10-l 10-8
...
... ... ...
...
... 1 . 0x 10-3 1 . 0 x 10-8 1 . 0 x 10-3
1.0 x 5.0 X 5.0 x 5.0 X 5.0 x 1.0 x
10-3 10-2 10-2 10-2 10-1
4.92 5.55 5.70 5.90 6.25 6.35 5.28 5.30 5.80 5.90 5.18 5.25 5.25 5.90 3.86
... ...
... ...
... ... 0.30 0.22 0.16 0.09 1.35 1.07 0.84 0.40 2.48
0.41 0.29 0.27 0.29 0.33 0.34 0.32 0.38 0.36 0.24 0.22 0.34 0.21 0.33 0.36
...
... ...
... ... ... 8.00 5.92 4.33 2.93 8.10 6.45 4.86 2.64 11.0
" Dose rate, 2.0
X 101@evml-1hr-1; dose, 1.0 X 1010 ev ml-1. * Calculated from expression D'.
press significantly G(N2) (beneene, toluene, fluorobenzene). 3. Solutions Containing Sulfuric Acid. The addition of sulfuric acid to methanol brought about 5t progressive increase in G(H2)(Table IV). The addition of sulfuric acid to solutions containing nitrous oxide decreased G(N2) and increased G(H2)(Table IV). In all experiments no significant change in the yield of methane was distinguishable. 4. Alkaline Solutions of Nitrous Oxide. Product
2.06 3.54 2.26 4.14 3.14 3.84 2.77 3.65 2.86 4.02 2.89 4.00 3.35 5.19 3.26
0.29 0.39 0.33 0.31 0.30 0.20 0.20 0.30 0.16 0.31 0.30 0.34 0.33 0.35 0.24
0.55 0.85 1.03 0.71 2.18 0.50 0.85 0.46 0.80 0.65 1.43 0.74 1.31 0.20 0.48
5.9 x 5.8 x 5.9 x 9.3 x 9.7 x 1.9 1.8 2.1 2.0 7.5 x 7.0 x 8.3 x 8.2 x 6.3 4.0
10-4 10-3 10-3 10-2 10-2
10-1 10-l 10-l 10-1
Calculated from expression D'.
yields were determined as a function of the concentrations of potassium hydroxide and nitrous oxide (Figures 2 and 3). The effect of total dose on product yields was determined in the case of a solution containing 5 X M nitrous oxide and 1.0 X lo-' M potassium hydroxide. The yields of hydrogen and nitrogen were found to remain essentially constant over the dose range 5.5 X lo'* to 5.5 X 10'9 evml-', while G(CH4)was found to be 0.67 and 0.64 for doses of 5.5 X lo1*and 1.1 X 10lg ev ml-', and 1.63 for doses in the range 2.2 X 10lgto 5.5 X 10'9 ev ml-l. The presence of a dose rate effect was checked by measurements a t three dose rates, 3.9 X lo'*, 2.0 X 10'9, and 6.0 X 1019ev ml-' hr-l. It was found that for a given solution the yields of all three products decreased in proportion to the reciprocal of the square root of the dose rate (Figure 4). The
Table V : The y Radiolysis of Methanolic Solutions Containing Potassium Hydroxide and Nitrous Oxide. Effect of Solutes on Product Yields"
Solute
None Benzene Benzene Nitrobenzene Nitrobenzene Carbon tetrachloride Acetone
[Solute], mM
G(H2
G(Nz)
C(CH3
1.0 100 0.10 1.0 5.0
21.0 20.5 10.0 4.85 3.66 4.80
101 100 50.5 7.42 3.30 3.68
3.1 3.1 1.7 0.43 0.24 0.42
5.0
34.8
126
1.3
...
[KOH] = 0.1 M ; [NZO] = 0.2 M. Dose rate = 2.0 X 1 O l 9 ev ml-1 hr-1; total dose = 1.0 X lo1@ ev ml-1.
Volume 71, Number 13 December 1967
WARRENV. SHERMAN
4248
30
r
r
10 a
I
I
1
102
rj
10
I
10
1
10-1 10-1
lo-'
10-1
1
[NzOl, M . 0 0
10-1
10-1
10-1
[KOHI, M .
Figure 2. The y radiolysis of methanol solutions containing potassium hydroxide and nitrous oxide. Product yields aa a function of potassium hydroxide concentration, [NzO] = 5 x 10-3 M ; dose rate, 2.0 X l O l 9 ev ml-l hr-1; dose, 1.0 x 1010 ev d-l: 0, H2; 0,Na; A, CHI.
effect of the presence of several organic solutes on the chain was also studied (Table V).
Discussion 1 , Neutral Methanol Solutions. The rate constants for the reaction of hydrogen atoms with methanol and nitrous oxide have been determined in aqueous solution as 1.7 X lo6 and 1.2 X lo4M-' sec-l, respectively.QJO If it may be assumed that the ratio of these two rate constants does not alter appreciably when the solvent is changed from water to methanol, the competition between nitrous oxide and the solvent for hydrogen atoms will be unimportant at all nitrous oxide concentrations. Moreover, it would require a change in the ratio ~(H+N~o)/~(H+cH~oH) of about three orders of magnitude before the presence of 0.1 M nitrous oxide could compete successfully for only 10% of the hydrogen atom yield. Hence, based on this kinetic argument it may be concluded that the decrease in G(H2) with increasing nitrous oxide concentration (Figure 1) is not due to a process involving hydrogen atoms. The first absorption band of methanol, beginning a t The Journal of Physical Chemistry
Figure 3. The y radiolysis of methanol solutions containing potassium hydroxide and nitrous oxide. Product yields as a function of nitrous oxide concentration. [KOH] = 10-1 M; dose rate, 2.0 X 1010 ev ml-l hr-1; dose, 1.0 X 1019 ev ml-1: 0, Ha; 0,Nz; A, CHa.
about 200 nm, is structureless and dissociative," and there is no evidence for an excited state of methanol which is sufficiently long-lived t o be able to transfer energy t o nitrous oxide. Similarly, the ionization potential of nitrous oxide is considerably greater than that of methanol12 so that positive-charge transfer is thermodynamically unfavorable and cannot be the mechanism of nitrogen formation. It has been shown that the solvated electron produced in pulse-irradiated methanol has a sufficiently long lifetime to undergo reaction with a solute present at low concentration.13J4 The results in Figure 1 are therefore consistent with electron attachment by nitrous oxide (reaction l ) competing with the other pro(9) J. Rabani, J . Am. Chem. Soc., 84, 868 (1962); L. M. Dorfman, I. A. Taub, and R. E. BUhler, J . Chem. Phys., 36, 3051 (1962); G.Scholes and M. Simic, J . Phys. Chem., 68, 1731,1738 (1964). (10) F. S. Dainton and 8. A. Sills, Proc. Chem. SOC.,223 (1962). (11) J. Hagege, S. Leach, and C . Vermeil, J . Chim. Phys., 62, 736 (1965), and references therein. (12) K. Watanabe, T. Nakayama, and J. Mottl, J . Quant. Spectr. Radiative Transfer, 2, 369 (1962). (13) 1. A. Taub, M. C. Sauer, and L. M. Dorfman, Discussions Faraday SOC.,36, 206 (1963). (14) I. A. Taub, D. A. Harter, M. C. Sauer, and L. M. Dorfman, J . Chem. Phys., 41, 979 (1964).
y
RADIOLYSIS OF METHANOL SOLUTIONS CONTAINING KITROUS OXIDE
10
8
6
E!
0, 0
4
2
0 0
1 2 (Dose rate)
3 4 5 X 10’0, mll/* hr”’ e@.
6
Figure 4. The y radiolysis of methanol solutions containing potassium hydroxide and nitrous oxide. Product yields as a function of dose rate. Dose, 1.0 X l O I 8 ev rn1-I: 0, H1; 0, Nf; A, CH4.
4249
is the number of nitrogen molecules produced per electron scavenged by a nitrous oxide molecule, and ki[Xi] is the rate at which electrons disappear by reaction with other species (e.g., solvent molecule, positive center). The plot of 1/G(N2) us. l/[nT20] for concenM conforms well to trations of nitrous oxide up to expression A (Figure 5 ) . From the intercept, aG(e - sol) = 1.7; and from the slope, ZkPIXi]/kl = 5.5 x 10-4 M . The concentration at which the 1/G(n’2) vs. 1/[N2O] plot begins to deviate from a straight line corresponds approximately to the inflection in the hydrogen yield plot. A rationalization which is consistent with these observations is that up to -1 X M , nitrous oxide reacts with homogeneously distributed solvated electrons which are a precursor of the hydrogen yield. As the nitrous oxide concentration is increased above -1 X M , the further decrease in G(H2) corresponds to the reaction of nitrous oxide with a second precursor of hydrogen. It was assumed previously5 that the latter are electrons which,’in the absence of a solute which can attach electrons, do not escape the radiation spurs but are recaptured by the parent ions. 2.5
d
cesses for the disappearance of the solvated electron. e-sol
+ N20
--f
(N20-) +N2
+ 0-
(1)
The presence cif a solute which is a good electron acceptor will depress the nitrogen yield by competing with nitrous oxide for the available electrons. This is the case with the organic compounds of group I (cf. Results, section 2) which have been shown to be reactive toward the hydrated electron,15 while compounds of group I1 which react relatively slowly with the hydrated electron16have little effect on G(N2) unless present in high Concentration. Furthermore, the order of efficiency with which a given concentration of a particular compound depresses G(K2) is precisely that expected from its relative reactivity toward the hydrated electron. The depression of G(K2) by sulfuric acid is similarly consis tent with an electron-attachment process by hydrogen ions. If the yield of nitrogen may be assumed proportional to the number of solvated electrons scavenged by nitrous oxide, then application of homogeneous steadystate kinetics gives expression A for the nitrogen yield in a solution containing nitrous oxide alone, where cr
2.0
2
h
1.5
P” 1.0
0.5
0
2
4
6
(l/[NzO]) X lo-’, M-1.
Figure 5. The y radiolysis of methanol solutions containing nitrous oxide. Nitrogen yields LW a function of nitrous oxide concentration. Dose rate, 2.0 X 1 0 1 9 ev ml-1 hr-1; dose, 1.0 X 1019 ev ml-1. (15) M. Anbar and P. Neta, Intern. J . Appl. Radiation Isotopes, 16, 227 (1965); Israel Atomic Energy Commission Report IA-1079
(1966).
Volume 71, Number IS December 1967
WARRENV. SHERMAN
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The decrease in the hydrogen yield, AG(Hz), corresponding to the plateau in the G(Hz) us. [NzO] plot (Figure 1) was 1.1. This value agrees well with the yield of solvated electrons (1.1) determined by the pulse-radiolysis technique by Dorfman and his coworkers.16 It also agrees with the yield of solvated electrons “readily scavengeable” by Ni2+ and Co2+ salts (1.05 f 0.05)17 and differs only slightly from the yield of “electrons which are scavenged by low concentrations of solutes” (1.3) in the study by Baxendale and Mellows of the effect of anthracene and ferric chloride.ls However, the results of the earlier published study of the nitrous oxide-methanol system6 give a value which is significantly different from the present one. Analysis of the hydrogen and nitrogen yields for the nitrous oxide concentration range 0-5 X M indicate that G(e-sol) = 2.1. Recently, Teply and Habersbergerova have presented a study of the effect of nitrous oxide and oxygen on the radiolysis of methanol7 and concluded that G(e-s,,l) = 1.05, which is in good agreement with the present work. The hydrogen yield from pure methanol as determined by these workers was G(H& = 4.48, and attention was drawn to the interesting fact that while widely differing values for G(H& have been determined (EI.~,~?’* 5.26,17 4.4Sj7 and 4.98 in the present work) the calculated yields of G(eeso1) are practically identical apart from one exception.6 If we accept the conclusion that for nitrous oxide M the hydrogen yield is concentrations up to diminished by a process involving the capture of solvated electrons by nitrous oxide, then it is necessary to explain why G(N2) > aG(HZ) a t nitrous oxide concentrations in the range 10-2-10-3 M. Since it has been shown in gas-phase worklo that 0- can decompose nitrous oxide to nitrogen, this could be a possible explanation. Another alternative is that not all of the solvated electrons scavenged by nitrous oxide would have led to hydrogen in the absence of nitrous oxide. This is possible if nitrous oxide was in fact scavenging electrons in the spurs, and if these electrons do not lead to hydrogen with a 1 : l correspondence. While this type of process may be important a t high concentrations,5 it would seem to be highly improbable that it could have much importance a t such low concentrations. The disappearance of the solvated electron in pure methanol is consistent with its reaction with a solvent molecule20 Hence, replacing zkl[X1] by kz[CH20H] we have ICz/ki = 2.2 X which is in reasonable agreement The Journal of Physical Chemistry
with the value obtained by Teply and Habersbergerova (2.5 X 10-6).7 Assuming that kl has the same value as in water (5.6 X lo9 2M-I sec-l 21), then ICz = 1.2 X 105 M-’ sec-l. Since the impurity level of the methanol has not been established conclusively, this value for kz should be taken as an upper limit. The foregoing estimate for kz indicates that the lifetime of the electron in pure methanol is 0.3 psec. This is almost an order of magnitude smaller than Dorfman’s value.14 The reason for this inconsistency is not clear. A higher impurity level in the methanol used in the present study might be the cause, but, since the purification procedures appear to have been essentially indentical, it must be considered unlikely that the resulting impurity concentrations were so different as to have such a large effect on the lifetime. It is noteworthy, however, that Dorfman and co-~orkers’~ found that the disappearance of the electron in pure methanol does not correspond to simple order kinetics, while steady-state scavenger studies, such as the present one, indicate that the kinetics of the disappearance of the electron in the presence of a scavenger may be treated as competition between two first-order reactions (one involving the scavenger, and one a species present in pure methanol). The high radiation intensities involved in pulse-radiolysis studies favor high-order reactions, and while this could be the reason for the difference in kinetics it would be expected that the lifetime of a radiolytic intermediate obtained under these conditions would be shorter, rather than longer, than that resulting from lower radiation intensities. Competition between nitrous oxide and a second solute, S, for homogeneously distributed electrons
can be expressed by
Combining (B) with (A) we have (16) M. C. Sauer, S. Arai, and L. hf. Dorfman, J . Chem. Phys., 42, 708 (1965); L. M. Dorfman in “Solvated Electron,” Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 36. (17) E. Hayon and M. Moreau, J. Phys. Chem., 69, 4053 (1965). (18) J. H. Baxendale and F. W. Mellows, J. A m . Chem. Soc., 8 3 , 4720 (1961). (19) B. P. Burtt and J. Henis, J. Chem. Phys., 41, 1510 (1964). (20) F. S. Dainton, J. P. Keene, T . J. Kemp, G. A. Salmon, and J. Teply, Proc. Chem. Soc., 265 (1964). (21) J. P. Keene, Rudiatwn Res.,,?2, 1 (1964); E. J. Hart and E. M.
Fielden in “Solvated Electron, Advances in Chemistry Series, NO. 50, American Chemical Society, Washington, D. C., 1965, p 253.
y
RADIOLYSIS OF METHANOL SOLUTIONS CONTAINING NITROUSOXIDE
0
4251
0.1
0.2
0.3
I
I
0.4
0.5
and AG(Nz) k3 (SI - G(Nz) zkt[Xtl h[N201
+
(D)
where AG(N2) -- G(S2)o - G(Nz). The data of Tables I and I1 are plotted in Figures 6 and 7 and may be seen to give satisfactory straight-line plots in accord with expression C. The reactivity relative to nitrous oxide, ~ N , O = k3/k1, calculated from these plots is 2.3 X and 4.0 for benzene and nitrobenzene, respectively. In the case of nitrobenzene it was possible to check the validity of (C) at a concentration of nitrous oxide (5 X M ) where it may be assumed that scavenging of nonhomogeneously distributed electrons within spurs may be important. Here also it may be seen that the results conform to expression C. Actually, at nitrous oxide concentraM , Zk,[X,] becomes tions greater than about 1
3.
I
I
I
I
01
1
0.2
I
0.6 0.8 [Nitrobenzene], M X 10-4. 0.4
I
1.0
Figure 7. Nitrogen yields as a function of the concentration of nitrobenzene in the y radiolysis of methanol solutions containing nitrobenzene and nitrous oxide. Dose rate, 2.0 X lo1*ev ml-l hr-1; dose, 1.0 X 1019 ev ml-1: 0,[NzO] = lO-'M; 0, [NzO] = 5 X 10-2 M .
2.
2.
negligible compared with k,[NzO] and (C) and (D) become
-
i?
u'
> 1.
AG(N2) ---
G(N2) 1
0
I
0
0.2
I
I
0.4 0.6 [Benzene], M .
I
0.8
I
1.0
Figure 6. Nitrogen yields as a function of the concentration of benzene in the 7 radiolysis of methanol solutions containing benzene and nitrous oxide. Dose rate, 2.0 X 1010 ev ml-1 hr-1; dose, 1.0 X 1010ev ml-l: 0 top, [NtO] = 5 X 10-4M; middle, [N20] = 10-3M; bottom, [NZO] = 5 x 10-3~.
- ka[SI ki [hTzO3
(D '1
The latter expression is that used as a measure of reactivity in the previous studies of 2-propanol5 and cyclohexane.22 The ~ N * Ovalues for a number of organic compounds are calculated in the last column of Table I11 using expression D'. With the exception of the relatively unreactive toluene, it was possible to check the effect of concentration on ~ N ~ o Only . in the case of carbon tetrachloride is an appreciable change in the calculated found on varying the solute concentrations. (22) W.
V. Sherman, J. Chem. Soc., Sect. A , 599 (1966). Volume 71, Number IS December 1967
WARREN V. SHERMAN
4252
I n a plot of k N I 0 values in methanol against the corresponding data for the hydrated electron in aqueous solution (Figure 8) all of the compounds studied lie close to a single straight line with a slope of approximately unity. This observation of essentially identical relative rate constants for the electron in water and methanol is consistent with the previously noted agreement between the reactivity of oxygen and benzyl chloride in these two solvents.’6 The reactivity of the monosubstituted benzenes conform to a linear Hammett papa,a plot, with p = 4.7. This p value is essentially identical with that determined by Anbar and Hart (4.8) for the hydrated electr0n.~3 The significance of this result has been discussed p r e v i o u ~ l y . ~ ~ 2 . Acid Methanol Solutions. The progressive increase in G(Hz) with increasing acidity has been noted previously.’@ It was attributed to the scavenging of electrons in spurs by hydrogen ions competing with electron capture by positive ions which does not lead to hydrogen formation. However, the observation of a significant increase in G(Hz) on addition of only lod4 M acid would require a spur lifetime as long as about lo-’ sec, assuming that e-sol H + is diffusion controlled. This is somewhat larger than the currently accepted estimates of spur lifetimes. For a given concentration of nitrous oxide, the progressive decrease in G(N2)with acid concentration is consistent with competition between nitrous oxide and the hydrogen ion for a common reactive species, namely, the solvated electron. However, analysis of the data using expression D’ leads to apparent rate constants which are not independent of the initial solute concentrations (cf. last column of Table IV). For a given concentration of nitrous oxide the apparent rate constant decreases with acidity, while for a given acid concentration it increases with nitrous oxide concentration. On this basis it is not possible to rationalize the behavior of sulfuric acid solutions in terms of the previously suggested mechanism of simple competition between nitrous oxide and hydrogen ions for solvated electrons. While a progressive increase in the calculated k N J 0 could result from an increase in the second ionization of sulfuric acid (HS04- -+ H + Sod2-) the effect is too great, and furthermore it would not account for the dependence of the calculated kx20 on nitrous oxide concentration. A decrease in kxzo with increasing ionic strength, p, is predicted by the Debye-Huckel equation, log (klk”) = 2AZ,Z~(p1/’/(l P ” ~ ) ) ,for the reaction between species of opposite charge in a medium of high dielectric c0nstant.~5 For a given temperature the constant A is inversely proportional to the dielectric
+
+
+
T h e Journal of Physical Chemistry
1.01
I
I
I NITROBENZENE
F
9’
BENZONITRILE
FLUOROBENZENE
-2.0
BENZENE TOLUENE
-3.0
1
I
-2.0
I
I
-1.0
0
(Log ~ N ~ O ) C H ~ O H .
Figure 8. Plot of log k ~ values ~ o for the electron in methanol us. log kNzo values for the electron in water.
constant on the solvent, and, on the basis of the latter for methanol being 33,26A = 1.2. Hence, for the reacH + in methanol, the Debye-Huckel treattion e-aol ment predicts
+
+
Plots of log kxZO vs. pl’z/(~ p’”) for solutions initially containing and 5 X M nitrous oxide give satisfactory straight lines (Figure 9). (For the calculation of p it is assumed that only the first ionization of sulfuric acid is important.) The slope in the case of 5 X M nitrous oxide is -2.5 which is in good agreement with the value predicted by (E). However, for M nitrous oxide the slope is much greater (-8.0) and not in accord with (E). It is significant, and somewhat surprising, that agreement of the data with expression E is good for the nitrous oxide concentration (5 X M ) where some spur scavenging may occur, while it is poor at low concentration M ) where homogeneous kinetics should obtain. ~~
(23) M. Anbar and E. J. Hart, J. Am. C h e n . SOC.,86, 5633 (1964). (24) W. V. Sherman, ibid., 88, 1567 (1966). (25) S. W. Benson, “The Foundations of Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y.,1960,Chapter XV. (26) F. Buckley and A. A. Maryott, National Bureau of Standards Circular 583,U. 5. Government Printing O5ce, Washington, D. C., 1958.
y
RADIOLYSIS OF METHANOL SOLUTIONS CONTAINING XITROUS OXIDE
I
I
1.1
I
I
4253
The observation (Table V) that the reaction was inhibited by solutes which are good electron acceptors (nitrobenzene, carbon tetrachloride), while relatively insensitive to a solute which is reactive toward neutral free radicals but a poor electron acceptor (benzene) is consistent with this assumption. The linear dependence of yields on the reciprocal of the square root of the dose rate (Figure 4) is consist,ent with a chain reaction involving a termination process which is second-order with respect to the chain ~arrier.~' It is proposed that the following sequence of reactions for the chain formation of molecular nitrogen is in accord with the foregoing observations : initiation
1.0
0.9
0.8 0
c
Y
bo
s 0.7
CH30H --MY-) e-sol 0.6
+ H . + R + other products
where R is any species which can undergo a hydrogenabstraction reaction with the solvent
+ CHBOH+Hz + CH20H
He
0.5
I
+ CHiOH-RH
R
0.4 0
1.5
1.0
0.5 [P1'2/(1
+
P'91
x
2.0
I
2.5
10.
Figure 9. Debye-Hiickel plots for methanol solutions containing sulfuric acid and nitrous oxide: 0,initial concentration of NnO = 10-8 M ; D, initial concentration of NzO = 5 x 10-2 M .
+ CH30-
--f
+I 6 H 2 0 - + CH30H I1
and/or
I
+ CH30- +HCHO + e-sol
propagation An analogy between the sulfuric acid data and previous results14 may be noted a t this point. If, as suggested,'* only the first dissociation of sulfuric acid is important in methanol, the apparent k N 2 0 for the hydrogen ion a t infinite dilution obtained from the approximately identical' intercepts of the two plots in Figure 9 is about 10. This is significantly greater than kN20 found in water (4.1).15 It is interesting to observe that the absolute rate constant for the reaction between the electron and hydrogen ion in methanol obtained by Dorfman and co-workers (3.9 X 1O1O 2CI-l ~ e c - ~ )was l ~ also significantly greater than the corresponding value for the hydrated electron (2.3 X 10'O M-' ~ e c - l ) . ' ~Two nonionized solutes were also studied by Dorfman-oxygen and benzyl chloride. Here, it may be noted, the absolute rate constants were found to be identical with those in water. 8. Alkaline Methanol Solutions. The magnitude of the progressive increase in yields of all three gaseous products with nitrous oxide concentration (Figures 2 and 3) is consistent with a chain reaction involving nitrous oxide in a propagation step. Since the chain was only observed under alkaline conditions, it may be assumed that the chain carrier is an anionic species.
I1
+ N20 +HCHO + + 05 2
and/or e-sol
0-
+ NzO +N2 + 0-
+ CHIOH +OH- + I
termination 211 4CH2(0)CH202and/or
211
+ CHIOH +2CH30- + HCHO
and/or 2e-,,1 +H2
+ 2CH30-
A chain propagation step analagous to (7) was originally suggested by Scholes, Simic, and Weiss2*for the radiation-induced chain decomposition of nitrous oxide in an alkaline aqueous solution containing 2-propanol. (27) F. S. Dainton, "Chain Reactions," biethuen and Go., Ltd., London, 1956,Chapter IV. (28) G. Scholes, M. Simic, and J. J. Weiss, Discussions Faraday SOC.,
36, 214 (1963).
Volume 71. Number 19 December 1967
4254
The data on the chain reaction observed in irradiated alkaline 2-propanol containing nitrous oxide were also found to be consistent with this type of charge transfer from the alcohol radical anion to nitrous 0xide.~9 Dainton and Fowles have written reaction 7 as a propagating step in their study of the light-induced chain oxidation of methanol by nitrous oxide in alkaline aqueous solution.30 If I is in fact the precursor of the chain carrier, it should be possible to observe a similar chain reaction in an alkaline methanol solution containing nitrous oxide in the absence of ionizing radiation by introducing any reactive species capable of abstracting the carbinol hydrogen atom of the solvent and thereby generating radical I. t-Butoxy radicals are produced in the photolysis of di-t-butyl peroxide,31 and the abstraction of the carbinol hydrogen of methanol by this radical is an energetically favorable process. When a deaerated alkaline methanol solution containing both nitrous oxide and dit-butyl peroxide, contained in a Pyrex ampoule, was exposed to light from a high-pressure mercury lamp formation of hydrogen, nitrogen, and methane was observed.32 The yields were of sufficient magnitude to indicate a chain reaction. The relative yields of the three chain products were similar to that found in the present study. Inhibition of all three products occurred on addition of relatively small concentrations of carbon tetrachloride or nitrobenzene, while the yields were unaffected by benzene. All of these observations therefore strongly support the proposed mechanism for the mode of initiation of the y-ray-induced reaction. The proposal of the solvated electron, formed by reaction 6a, as an alternate chain carrier is required in order to explain the formation of hydrogen as a chain product. The reaction may be expected to be exothermic, anti, moreover, is loosely analogous to the reaction of hydrogen atoms with hydroxyl ions in aqueous solution for which considerable kinetic33 and spectrophotometric evidence34 exists. The propagation step for the formation of hydrogen could then be the first-order disappearance of the solvated electron, reaction 2 ; while the second-order termination mocess required by the dose-rate dependence is reaction lo' It be that the latter reaction is somewhat questionable since Dorfman was unable to observe the bimolecular react,ion between two solvated electrons in methanol in his pulse-radiation In view Of the uncertainty concerning the relative importance of 1 and e-aol as chain carriers, no detailed mathematical analysis of the results is warranted a t the present time. Additional evidence for the occurrence of reaction Ga is provided by recent photochemical experiment^.^^ The Journal of Physical Chemistry
WARRENV. SHERMAN
Dilute solutions of di-t-butyl peroxide in alkaline methanol contained in a Pyrex ampoule were exposed to light from a medium-pressure mercury lamp under anaerobic conditions. The observation of the evolution of molecular hydrogen is in accord with radical I undergoing reaction 6a. It is pertinent to compare the chain reaction in irradiated methanol with that observed previously in 2propan01.~9 First, in the latter system the yields of nitrogen and methane were considerably larger. At nitrous oxide concentrations in the range 2 X 1X M it was possible to observe an enhanced hydrogen yield, but at higher concentrations if a significant yield of hydrogen occurred, it was less than -1% of the nitrogen yield and not experimentally o b ~ e r v a b l e . ~ ~ The possibility of the solvated electron, generated by a reaction analogous to Ba, being a chain carrier was not considered previously, but could provide a suitable source of the hydrogen yield. On the basis of the mechanism involving the solvated electron as chain carrier, the ratio G(Nz)/G(Hz) will be dependent on the ratio of the rate constants for reactions 1 and 2. The greater G(N2)/G(H2) ratio observed for the reaction in 2-propan01~~ is therefore consistent with the observation that the half-life of solvated electrons generated in pure 2-propanol is significantly longer than in pure methan01.l~ The increase in yields of hydrogen and nitrogen on the addition of acetone is noteworthy. Acetone could compete with nitrous oxide for the charge on anion I and for the available solvated electrons. The 2-propoxide radical anion (111) thus formed can then act as a chain carrier according to the mechanism suggested pre~iously.~9~~6 The enhancement of G(H2) and G(N2) would then be consistent with the chain-propagation step involving I11 being more eficient than that involving 11. A mechanism in which reaction 1 was the sole propagation step would not be consistent with the catalytic effect of acetone. Preliminary results with the di-t-butyl peroxide photoinitiated reaction also indicate catalysis by acetone. (29) W. V, Sherman, J . Phys. Chem., 71, 1095 (1967). (30) F. S. Dainton and P. Fowles, Proc. Roll. SOC.(London), A287. 312 (1965). (31) L. hf. Dorfman and 2. W. Salsberg, J . Am. Chem. Boc., 73, 255 (1951). (32) W. v, Sherman. Chem. Commun,, 790 (1966). (33) J. Jortner and J. Rabani, J . Phys. Chem., 66, 2078 (1962): S. Nahari and J. Rabani, ibid., 67, 1609 (1963). (34) hi. S. Matheson, Radiation Res. Suppl., 4, 74 (1964): 11.1. S. Matheson and J. Rabani, J. Phys. Chem., 69, 1324 (1965). (35) Previously unpublished work carried out at the Radiation Laboratorv. " , Universitv of Notre Dame. (36) w. 17. Sherman, J . Am. Chem. SOC.,8 9 , 1302 (1967).
THESELF-DIFFUSION OF OXYGEN IN MAGNETITE
The yield of methane was diminished by all of the solutes listed in Table V. Since they are all good radical scavengers, it would appear likely that the methyl radical is the precursor of methane. In the 2-propanol system it was suggested that the mechanism of methane formation could be the addition of 0- to acetone to give an unstable intermediate which decomposed to methyl radical and an acetate ion. The analogous reaction in methanol would be the addition of 0- to formaldehyde, and it is unlikely that the intermediate thus
4255
formed would be a precursor of methyl radicals or methane. The mode of formation of methane remains unclear, but since it results from a chain reaction which is apparently subject to second-order termination, it seems likely that the chain carrier involved in the formation of hydrogen and nitrogen is also concerned in the formation of methane. Acknowledgment. The experimental portion of this work was carried out during the tenure of an Israel Atomic Energy Commission Fellowship.
The Self-Diffusion of Oxygen in Magnetite. Techniques for Sampling and Isotopic Analysis of Micro Quantities of Water
by J. E. Castle and P. L. Surman Central Electricity Research Laboratory, Leatherhead, Surrey, England Accepted and Transmitted by The Faraday Society
(April 6,1967)
A new technique for the sampling and isotopic analysis of micro quantities of water is described. The method is applied to the investigation of oxygen self-diffusion in magnetic using a continuous sampling method. The self-diffusion coefficient of oxygen in magnetite over the temperature range 302-550" can be expressed as
D
=
3.2(*1.6) X
*
exp( -17,OOO 1650) RT
The very small value for D makes it unlikely that oxygen solid-state diffusion is a r a t e controlling step in the corrosion of mild steel by water or steam.
Introduction A feature common to the oxidation of steel in water, steam, and alkaline solutions a t temperatures below 570" is the formation of a magnetite layer, adjacent to the metal, by what is apparently oxygen diffusion. Although the coefficient for oxygen diffusion in magnetite is unknown, there are many indications that it will be small, particularly a t t~heselow temperatures. A useful guide to its likely value comes from data for nickel-chromium spinel which has the same closepacked oxygen structure. Kingery, Hill, and Nelson1
report values for the temperature range 1200-1550" which extrapolate to cm2/sec a t 450". Also Himmel, Mehl, and Birchenal12 give data for iron in magnetite which extrapolates to 10-l6 cm2/sec at 450". Thus, using the observation of Kingery, et al., that the oxygen diffusion coefficient was about one order of magnitude less than that for cations, the value (1) W. D. Kingery, D. C. Hill, and R. P. Nelson, J . Am. Ceram. Soc., 43, 473 (1960). (2) L. Himmel, R. T. Mehl, and C. E. Birchenall, J . Metccls, 5,827 (1953).
Volume 71, Number 13 December 1967
