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Radiation Sensitized Chain Reactions of gas with the above assumptions if the lamp is very close to the films. (29) S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, Discuss Faraday SOC.,36, 193 (1963). (30) (a) J. K. Thomas, S.Gordon, and E. J. Hart, J. Phys. Chem., 68, 1524 (1964); (b) S.Gordon, private communication.
(31) M. Anbar, M. Bambenek, and A. B. Ross, NaN. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 43 (1973). (32) D. Meisel and G. Czapski, J. Phys. Chem., 79, 1503 (1975). (33) NOTEADDEDIN PROOF:In further studies currently underway using nitrogen laser flash photolysis eaq-was directly observed. J. Rabani, M. Ottolengi, and U. Lachish, private communication.
Radiation Sensitized Chain Reactions. Aqueous Nitrous Oxide and 2-Propanol' T. G. Ryan and G. R. Freeman* Chemistry Department, University of Alberta, Edmonton, Alberta, Canada, T6G 2G2 (Received February 1, 1977) Publication costs assisted by the University of Alberta
The radiolysis of aqueous solutions containing 2-propanoland nitrous oxide were examined in the temperature range 513-583 K. The yields of nitrogen and acetone at 573 K were G(N2)= G(acetone) = 1100. The high yields are explained by a radiation induced chain process. The proposed mechanism involves the transfer of an oxygen atom from NzO to a C(CH3)ZOH radical producing .OC(CH&OH. The supporting evidence for the mechanism is a linear dependence of G(Nz)and G((CH&CO) on the square root of both the NzO and the 2-propanol concentrations, a linear dependence of the G values on D-'I2 for large dose rates, and the same Arrhenius parameters for both G(Nz) and G((CH3),CO).
Introduction The radiolysis of aqueous solutions of methanol containing nitrate ion has been studied by Allan at room temperature.2 A reaction between .CH20H and NO?producing formaldehyde, NOz- and OH- in equal proportions was proposed. Mellows3has suggested a reaction between N 2 0 and .CHzOH to explain high yields of nitrogen from radiolysis of methanol vapor containing N 2 0 a t 20 "C. The reaction was believed to yield OH radicals and equal amounts of nitrogen and formaldehyde. Since OH radicals can react with methanol to produce CHzOH, high yields of nitrogen are produced. Baxendale4 has proposed that the reaction between N 2 0 and .CHzOH could account for high yields of N2 obtained from the radiolysis of gaseous mixtures of water, methanol, and NzO at 120 "C. The present work leads to.the conclusion that reaction between N20and the (CH3)&OH radical occurs above 470 K in aqueous solutions of 2-propanol and N20. Experimental Section Materials. The water used was triply distilled, the first distillation being from acid dichromate, the second from alkaline permanganate, and the third from a flask with no additive. Research Grade nitrous oxide (Matheson Co.) was freed of nitrogen dioxide by bubbling through three concentrated potassium hydroxide solutions. It was then passed through a 30-cm column of Drierite and thoroughly degassed by freeze-pump-thaw and distillation cycles under vacuum. The 2-propanol was obtained from Fisher Scientific Co. (Spectro Grade), acetone (Analytical Grade) from Mallinckrodt Canada Ltd., and salicylaldehyde (Reagent Grade) from Fisher Scientific Co.; Linde Molecular Sieves 5A and helium (99.995%) were from Union Carbide Corp. Drierite was from Hammond Drierite Co., and potassium hydroxide from Baker Chemical Co. Sample Preparation. Five milliliters of water was introduced into the sample cell (8.5-mL capacity) and degassed using conventional vacuum techniques, Appropriate amounts of alcohol and nitrous oxide were then added, using extreme care to ensure removal of nitrogen
and oxygen. Most irradiations were carried out at temperatures in excess of 470 K. At these high temperatures, the combined vapor pressure of the components (>20 atm) greatly exceeded the breaking limit of the seals on the glass cells. The cells were therefore enclosed in a steel pressure vessel containing water, then positioned in a portable heating apparatus5 and heated to the required temperature using a two-mode controller obtained from APD Instruments. The exact temperature was determined using an Helwett-Packard 3420A differential voltmeter. After irradiation the pressure vessel was allowed to cool to room temperature, and the sample cells were removed. Irradiation. The samples were normally irradiated in a 6oCoGamma Cell 220 (Atomic Energy of Canada Ltd.), to a dose of 1.4 X 1017eV/g. The dose rate determined by Fricke dosimetry was 1.4 X l O I 7 eV/g min. A 6oCo source in an irradiation cave was used for the dose rate dependence study over the range 0.5 to 4.8 X 1017eV/g min and total dose from 0.25 to 1.4 X 1017eV/g. Product Analysis. Nitrogen was extracted by a vacuum distillation through two cold traps at 77 K, collected and measured in a Macleod-Toepler apparatus, and analyzed using gas chromatography. A 2.5-m Molecular Sieves 5A gas chromatography column was used at 298 K, with helium carrier gas and a Gow Mac thermal conductivity detector. Acetone was determined using salicylaldehyde.6 The method was calibrated with standard samples. Results Nitrogen and acetone were the principal products found from the radiolysis of water containing nitrous oxide and 2-propanol at elevated temperatures. At 573 K, with [(CH&CHOH] = 0.3 M and [N20]= 8.7 mM the yields were G(NJ = G[(CH3),CO] = 1100. 1. Effect of Nitrous Oxide Concentration. The Ostwald solubility coefficient for N 2 0 in water at 573 K has not been measured. However measurements have been made to a temperature of 433 K,' and by analogy with the solubility of other gases at high temperatures, it is possible to estimate that the Ostwald solubility coefficient for N20 in water at 573 K is between 0.3 and 1.4. With the The Journal of Physical Chemistty, Voi. 8 I, No. 15, 1977
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T. G. Ryan and G. R. Freeman
3000
2000 2000
I
I i” A i D-’/~(Io’~eV/g min)-”’
Figure 1. Product yields plotted against the square root of the nitrous G(N2); oxide molarity at 573 K. [(CH3)&HOH] = 0.30 f 0.03 M: (0) (A) G(acetone). The full curve was calculated from eq 11 and 12 using the rate constant values given in Table 11.
Figure 3. Product yields plotted against (dose rate)-’”. [N20] = 8.7 f 0.4 mM, [(CH&CHOH] = 0.30 f 0.03 M, T = 573 K: (0) G(N2); (A) G((CH&COI.
TABLE I: Effect of Adding Acetone at 573 K
---
[(CH3),- [(CH,),CHOH], CO], [N,O], Dose, M M mM lO”eV/g 0.02 0.00 3.8 1.40 0.02 0.02 3.8 1.34 0.30 0.00 3.8 1.34 0.30 0.01 3.8 1.34 0.30 0.30 3.8 1.34 0.30 0.00 3.8 1.34 0.00 0.02 3.8 1.34 0.30 0.30 3.8 0.00 0.02 0.02 3.8 0.00 0.00 0.02 3.8 0.00
G(N,) 194 280 1050 1020 430 1050 2Q
45b 8b
a The blank (zero dose) sample yield was not substracted from this. The amount of nitrogen obtained from the zero dose sample was equivalent to this G value at 1.34 x l o ” eV/g.
”0
0.1
0.2
2/[2
0.3
0.4
0.5
0.6
TABLE II: Rate Constant Values (L/mol s) at 573 K
- PROPANOL]
Figure 2. Product yields plotted against the square root of the 2-propanol molarity at 573 K. [N,O] = 8.7 f 0.4 mM: (0) G(N& (A) G(acetone). The full curve was calculated from eq 15 and 16 using the rate constant values given in Table 11, with ks = 1 X lo* Llmol s. The dashed curves show the effect of assuming k 8 = 1 X l o 7 or 1 X l o 9 L/mol s.
technique used, a change in the coefficient from 0.3 to 1.4 would produce a twofold change in the N20 concentration. A change from 0.3 to m would produce a 2.6-fold change in the N 2 0 concentration. A value of 0.7 was assigned to the coefficient to calculate the N20 concentrations. Figure 1 displays the yields of nitrogen and acetone from irradiations at 573 K and 0.30 f 0.03 M 2-propanol. The nitrous oxide concentration was varied from 0.24 to 12.7 mM and G(N,) values were in the range 83 to 1370. The acetone yields were equal to those of nitrogen within experimental error and varied from 138 to 1150. Blank sample yields of nitrogen and acetone were between 10 and 50 at 573 K and appropriate blank yields have been subtracted from all values reported in this work. 2. Effect of 2-Propanol Concentration. Figure 2 contains results from the irradiation of samples containing 8.7 f 0.4 mM nitrous oxide and concentrations of 2propanol between 0.18 and 300 f 30 mM. The irradiation The Journal of Physical Chemistry, Vol. 81, No. 15, 1977
k3 k4 k6
k, k,
2,9 X
lo6
8.7 x 103
1 x 10’OQ 1 x 10’Oa
