RADIOLYSIS OF NITROUS OXIDESATURATED SOLUTIONS
3983
Radiolysis of Nitrous Oxide Saturated Solutions: Effect of Sodium Nitrate, 2-Propanol, and Sodium Formate’
by H. A. Mahlman Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee (Received July 13, 1966)
The 60Co y radiolysis of aqueous NZO-saturated NaN03 solutions has given the yield of reducing radicals not producing molecular hydrogen as 2.95 and k(NO8 Red)/k(NzO Red) as 1.44. Hydrogen yields observed in the 6oCoy radiolysis of aqueous NzO-saturated 2-propanol solutions are interpreted as arising from (1) the GE, from the radiolysis of NzO-saturated water and determined to be 0.34, (2) the H atom abstraction of hydrogen from the organic solute and evaluated as GH = 0.61, and (3) a “direct effect” equal to 0.54[2-propanol]. Similar treatment of the sodium formate data ‘evaluatesGH = 0.57 and the “direct effect” as 0.16 [sodium formate]. The simultaneously measured nitrogen yields are constant, indicating that the H atoms are not reducing NzO to Nz in aqueous N20saturated solutions.
+
Introduction I n the 6oCoy radiolysis of neutral or basic aqueous solutions, chemical and physical evidence has identified the major reducing species as the solvated electron.2 When aqueous solutions containing dissolved N20 are irradiated, the hydrated electron has been shown to react rapidly with NzO to produce N2.3 However, H atoms have also been shown to be produced in the radiolysis of aqueous solution^^^^ and that they can react with NzO in aqueous solution to form Nz.6 It has been suggested that the G(N2) observed in the radiolysis of NzO-saturated aqueous solutions may represent the sum of the solvated electron and the H atom yields.’ The work reported in this paper was undertaken to determine the extent of the H atom reaction with NzO dissolved in aqueous solution. Also reported is a redetermination of the data representing the competition between NzO and NO3- for reducing radicals.E The addition of 2-propanol or formate ions, which are good H atom scavenger^,^ to NzO-saturated aqueous solutions would evaluate the question concerning the H atoms. I n the course of this investigation, G(H2) was observed to increase as a function of the added organic solute concentration. The G(Hz) was attributed to three sources: (1) the molecular hydrogen produced during the radiolysis of NtO-saturated water, (2) the H atom abstraction of hydrogen from the or-
+
ganic solute, and (3) a “direct action effect” on the dissolved 2-propanol .and formate ions that was directly proportional to the organic solute concentration.
Experimental Section All NaN03 solutions were prepared from Baker and Adamson reagent grade NaN08 with triply distilled water. The pH of these solutions was 4.8-5.2. Baker and Adamson KBr and Na2S04 were used without further purification. Appropriate amounts of Matheson Spectroquality reagent 2-propanol were added to NzOsaturated water to obtain the desired alcohol concentrations. These solutions were in the pH range 5.86.5. The sodium formate solutions were prepared (1) Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp. (2) E. J. Hart, Ann. Rev. Nucl. Sei., 15 (1965); Science, 146, 19 (1964). (3) F. S. Dainton and D. B. Peterson, Nature, 186, 878 (1960); Proc. Roy. SOC. (London), A267, 443 (1962). (4) J. T. Allan and G . Scholes, Nature, 187, 218 (1960). (5) C. Lifshita, Can. J. Chem., 40, 1903 (1962). (6) G. Caapski and J. Jortner, Nature, 188, 50 (1960). (7) D. Head and D. C. Walker, ibid., 207, 517 (1965). (8) A. M. Koulkes-Pujo and H. A. Mahlman, Compt. Rend., 259,788 (1964). (9) A. J. Swallow, “Radiation Chemistry of Organic Compounds,” Pergamon Press Inc., New York, N. Y., 1960; M. Anbar and P. Neta, Intern. J . A p p l . Radiation Isotopes, 16, 227 (1965).
Volume 70, Number 12 December 1966
3984
H. A. MAHLMAN
tl$
VAPOR SATURATED N20
I
c- 3 ! !
SAMPLE IRRADIATION CELL 30cc SYRINGE
mined spectrophotometrically a t 3050 A utilizing the molar extinction coefficient of 2240 at 25”.13 All yields were calculated on the basis of total energy absorbed by the solution and the 100-ev yields are reported as G (product).
Results Assuming that H atoms do not contribute to the G(N2) in N2O-saturated aqueous solutions (which shall be proven later), the competition between N2O and NO3- was considered to be for one type of reducing radical. The competitive reactions of N 2 0 and NO,given by
GAS SATURATION CELL Figure 1. Gas saturation and irradiation cells.
with Baker’s reagent grade crystals and were in the pH range 7.6-8.5. Matheson reagent grade N20, especially low in nitrogen content, was used to deaerate and saturate each individual sample solution. The gas saturation cell and sample irradiation cell are illustrated in Figure 1. The irradiation cell, similar to that used by Hart and coworkers,1° was a 30-cc B and D syringe with a capillary tube terminating with a 5/20 ground-glass male joint. After thoroughly rinsing the syringe with N20-saturated solution, a 25-cc sample was withdrawn and the exit was capped. Upon completion of the irradiation the cap was removed and a 2-cc portion of sample was discarded. The irradiation cell was then placed on a vacuum line for analysis utilizing a 5/20 female groundglass joint with a mercury seal. A sample aliquot, about 20 cc, was introduced into the vacuum system followed successively by expansion of the gases to approximately a 500-cc volume, freezing the aqueous solution with COS slush and the NzO in a liquid Nz trap. The permanent gases, N2, Hz, and 0 2 , were then analyzed by the micro techniques previously described.“ The exact amount of sample used in each analysis was determined by weight after the gas analysis. The volume of N20 dissolved in the NaN03 solutions was measured and the molarity was calculated to M at 24” and 750 mm pressure. This be 2.4 X is in good agreement with the molarity calculated from Henry’s constant.12 The W o y ray source used for the irradiations had a dose rate of 1.45 X 10l8ev ml-l min-1 as determined by ferrous oxidation in an air-saturated 0.4 M &So4 solution assuming 15.60 molecules of ferrous are oxidized per 100 ev.13 The ferric ion concentration was deterThe Journal of Physical Chemistry
N20 NO3-
+ Red +N2 + . . .
+ Red -+
products other than N2
(1) (2)
may be kinetically expressed by
The straight line drawn through the data points in Figure 2 represents the least-squares fit and has a calculated intercept of 0.313 and a slope of 0.451. Thus the intercept corresponds to G(N2) = 3.19 in N20saturated water in agreement with other aut h o r ~ . l6 ~ From ~ ~ ~the ~ slope ~ ~ ~to -intercept ratio, the ratio of reaction rate constants k2/kl is calculated to be 1.44. Not plotted in Figure 2 is the G(N2) = 0.033 determined at a N03-/N20 ratio of 52, which is in reasonable agreement with that calculated from the data at lower N03-/Nz0 ratios (G(N2) = 0.042). When 0.02 or 0.166 M NazS04 was added to a N20saturated 0.02 M NaN03 aqueous solution, the observed nitrogen yields were 1.50 and 1.37, respectively. These nitrogen yields are much different from the G(N2) predicted if the rate constant is sensitive to A comparison of the changes in ionic (10) E. J. Hart, S. Gordon, and D. A. Hutchison, J . Am. Chem. Soc., 75, 6165 (1953). (11) J. W.Boyle and H. A. Mahlman, Nucl. Sci. Eng., 2,492 (1957). (12) “Handbook of Chemistry and Physics,” 44th ed, Chemical Rubber Publishing Co., Cleveland, Ohio, 1962-1963, p 1709. (13) C. J. Hochanadel and J. A. Ghormley, J . Chem. Phys., 21, 880 (1953). (14) J. T . Allan and C. M. Beck, J . Am. Chem. Soc., 86, 1483 (1964). (15) G.Scholes and M. Simic, J . Phys. Chem., 68, 1731 (1964). (16) G. V. Buxton and F. S. Dainton, Proc. Roy. Soc. (London), A287, 437 (1965). (17) S. E. Benson, “The Foundations of Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y.,1960,p 525. (18) G.Czapski and H. A. Schwarz, J . Phys. Chem., 66, 471 (1962). (19) E.Collinson, F.S. Dainton, D. R., Smith, and S. Tazuke, PTOC. Chem. Soc., 140 (1962). (20) P. J. Coyle, F. S. Dainton, and S. R. Logan, ibid., 219 (1964).
3985
RADIOLYSIS OF NITROUS OXIDESATURATED SOLUTIONS
+ 0.54[2-propanol]
(11)
+ O.lG[sodium formate]
(111)
G(H2) = 0.95 G(Hz) = 0.91
Table I : Nitrogen and Hydrogen Yields from NzO-Saturated Solutions -HzO-NzO-CHaCHOHCHs-KBr-
12-Propanol], M
.2
t
0
I
1
1
1
1
.2
.4
.6
.8
1.0
NO;/N20
I 1.2
I
1.4 RATIO
I f,6
I 1.8
I
1 I
2.0 2.2
Figure 2. Variation of the G(N2) w a function of the NOs-/N20 ratio in NZO-saturated aqueous solution.
average G(N2) = 1.43 observed in the presence of added N@S04and the G(N2) = 1.42 observed in the absence of N@S04 indicates that there is no significant effect attributable to increased ionic strength. The nitrogen yields observed in the radiolysis of N2Osaturated 2-propanol solutions are given in Table I. These nitrogen yields were calculated from the best values determined from a plot of doses vs. molecules of product and after correction of the data for nitrogen blanks. The constancy of the nitrogen yields is readily apparent, with excellent agreement shown beM tween N20-saturated water, NzO-saturated KBr solution, N20-saturated aqueous 2-propanol solutions, and with the calculated intercept of the N20NO3--Hz0 solution data given in Figure 2. The G(N2) observed from the N20-saturated aqueous sodium formate solutions (also given in Table I) are approximately constant but somewhat higher. Since these G(N2) are single determinations uncorrected for nitrogen blanks, attention was focused on the observation that no nitrogen yields were less than those observed for 2-propanol additions and, therefore, no significance was attached to the absolute magnitude of the yield. It has recently been suggested21 that high nitrogen yields observed in N20-saturated aqueous formate solutions were due to the reduction of N20 by formate radical. It was observed, however, that when NzO gas containing an especially low Nz content was used to saturate the sodium formate solutions, approximately the same nitrogen yields were observed as in the N20-saturated2-propanol solutions. The hydrogen yields, tabulated in Table I, are illustrated in Figure 3, where the linear dependency of G(H2) on the organic solute concentration is evident. A leastsquares analysis of these data gave the equations
O(10-8MKBr PH 5) 0 (no KBr) 0.0013 0.013 0.13 0.50 1.18 3.08 3.98 5.00 5.90
-NzO-HzO-HCOONa-KBr[HCOONa], M C(Nd C(Hd
G(Nd'
G(H9
3.07
0.34
0.5
3.75b
1.00
3.03 3.03 3.06 3.21 3.17 3.12 2.95
... ... ...
1.0 1.5 2.0 3.0 4.0 5.0
3.7Sb 3.18' 3.49* 3.60b 3.06' 3.05'
1.05 1.16 1.21 1.45 1.56 1.69
0.99 1.20 1.65 2.59 3.16 3.62 3.72
...
3.04 3.02
Av 3.07
'
a Corrected for Nz blank. Single determinations, uncorrected for NZ blank. The G(N2) are greater than the G(N2) measured in the NzO-saturated aqueous 2-propanol solutions. New tank of NzO with negligible NZ content. The G(N2) are about equal to those measured in the NnO-saturated aqueous 2-propanol solutions.
Discussion Sodium Nitrate-Nitrous Oxide-Water Solutions. Baxendale, et al.,22and Gordon, et ~ l . 3utilizing , ~ pulsedradiolysis techniques, have determined the reaction rate constants for electrons reacting with various solutes. From their data, one can calculate the ratio of rate constants k2/k1 to be 1.46 and 1.27, respectively. Appleby, Scholes, and SimicZ4have obtained k2/kl = 1.17 by a chemical method. These ratios are to be compared to the ratio kz/kl = 1.44 calculated in this paper. Some might consider fortuitous the excellent agreement between the pulsed-radiolysis experiments, which were determined at low ionic strength, and this work, where the ionic strength was allowed to vary from 0.01 to 0.05. Considering the BrZnsted-Bjerrum theory of ionic reactions and the extended Debye-Huckel theory of electrolytes1' one would predict that the ratio k2/k1 determined in this paper would be too large. Increas(21) G. Scholes and M. Simic,
J. Phys. Chem., 68, 1738 (1964). (22) J. H. Baxendale, et ul., Nature, 201, 468 (1964). (23) G. 6. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, Discussions Faraday SOC.,36, 193 (1963). (24) A. Appleby, G. Scholes, and M. Simic, J . Am. Chem. SOC.,8 5 , 3891 (1963).
Volume 70,Number 1.8 December 1966
H. A. MAHLMAN
3986
I
1
I
I
0
I
I
I
I
SODIUM FORMATE
I
I
2.0 3.0 4.0 5.0 CONCENTRATION, moles/liter
1.0
I
I
6.0
Figure 3. Hydrogen yields as a function of 2-propanol and sodium formate concentration in N20-saturated aqueous solution,
ing the ionic strength would favor the reaction of a negatively charged reducing radical with the negatively charged nitrat’eion as given by
IC
log - = 1.022,2bP1”/(1 IC0
+
P1’2)
However, when the ionic strength of an NzO-saturated 0.02 M NaN08 solution was altered by the addition of 0.02 or 0.166 M NazSO4, no significant change in G(N2) was noted when compared with G(N2) observed in NZOsaturated 0.02 M NaN03 solution. I n fact, the average G(NZ) determined in the solutions containing the KazSO4 additions was 1.43 compared to 1.42 in the absence Of N&S04. The applicability of eq IV has been demonstrated in the radiolysis of dilute solution^;^^^^^^ however, at higher scavenging concentrations and ionic strengths the “constant” (1.02) has been observed to decrease.20 Furthermore, the absence of an ionic strength effect has been reportedlg for a 0.01 M scavenging solute having a reaction rate constant about 1O’O M - l sec-l. These variations in eq IV have been attributed to the reaction of the solvated electron in a time shorter than that necessary to establish an ionic atmosphere. Thus the absence of an ionic strength effect noted in the radiolysis of NzO-saturated 0.02 M NaN03 solutions containing 0.02 or 0.166 M Na2S04 may be attributed to the fast reaction of nitrate ions with solvated electrons, which also has a reaction rate constant about 1Olo M-‘ sec-’. Consequently, agreement could be expected between the ratio kz/kl determined in pulsed radiolysis studies at low ionic strengths and that determined in this work a t much greater ionic strengths. It should be emphasized that the G(N2) = 3.19 reported in this paper represents the total number of species reacting with NzO to produce Nz. Under no The J o u d of Physical ChemiatTy
circumstances should it be construed to mean that the yield of reducing radicals in the bulk of the solution is 3.19. Three sources are considered to be contributing to the G(N2), namely: (1) the reducing radical yield in the bulk of the solution that reacts with N 2 0 ; (2) an undetermined number of reducing radicals or other reducing species that “normally” are considered to disappear by back reactions to re-form water but in the presence of a high NzO concentration now react with the solute; and (3) the reaction of molecular hydrogen precursors with XzO. I n a deaerated aqueous M KBr solution, the G(H2) = 0.46,25while in an identical solution saturated with N20, the G(HJ = 0.34. Thus the molecular hyM N 2 0 is 0.12. If it drogen suppression by 2.4 X is assumed that for each hydrogen molecule suppressed, two molecules of Nz are formed,14 the contribution to the G(X2) manifested by suppression of the molecular hydrogen is 0.24. In the NaNOrN20-H20 system, it may be concluded that exclusive of the molecular hydrogen suppression, the radicals reducing KzO to IT2 have a yield of 2.95 in an aqueous solution of 2.4 X M N20. From the average Nz yields observed in the 2-propanol-nitrous oxide-water system (see Table I), the yield of radicals reducing NzO to Nz is 2.81 after correction for the molecular hydrogen suppression. Thus these two systems give reducing radical yields that are in good agreement with the G,,,- = 2.85 determined by Czapski and AllenUz6 A comparison of the data reported in this paper for aqueous NzO-saturated NaN03 solutions with that previously reported* is reasonably good. The modified method employed herein for deaeration and NzO saturation is considered to be more reliable and reproducible as reflected by the precision of Figure 2. Use of irradiation cells without free space above the solution may also be important. 2-Propanol-Nitrous Oxide-Water Solutions. I n addition to solvated electrons, H atoms are a 6oCoy-radiolysis product in aqueous solutions. Since H atoms can also react with NzO to form Sp, the extent of this possible contribution to the observed G(N2) was investigated. When an aliphatic alcohol such as 2-propanol is irradiated in dilute aqueous solution, molecular hydrogen has been shown to be formed by hydrogen atoms abstracting hydrogen from the alcoho1.4,5,14,24127--37 Therefore, in NzO-saturated aqueous (25) H. A. Mahlman and J. W. Boyle, J . Chem. Phys., 27, 1434 (1957). (26) G. Ceapski and A. 0. Allen, J . Phys. Chem., 66, 262 (1962). (27) P. Kelley and 1%. Smith, J . Chem. SOC.,1487 (1961). (28) J. Rabani and G. Stein, J. Chem. Phys., 37, 1865 (1962).
RADIOLYSIS OF NITROUS OXIDESATURATED SOLUTIONS
solutions containing 2-propanol, any competition between NzO and 2-propanol for H atoms will be reflected by a decreasing K2 yield and an increasing HZ yield. As previously mentioned, the data in Table I show that G(N2) is unaltered even when the 2-propanol concentration is increased to 5.9 M . Thus it is concluded that the H atoms do not contribute to G(N2). G(Nz) observed in the radiolysis of NzO-saturated aqueous 2-propanol solutions is in excellent agreement with M G(N2) determined in NzO-saturated water and KBr and the intercept calculated in the NO3--NzOH 2 0 system (Figure 2) and precludes an H atom contribution to the G(N2). The absence of an H atom contribution to G(Nz) in N20-saturated water or solutions may be explained by the rapid reaction of the H atoms with the radiolysis products HzOzand Oz. The formation of molecular hydrogen in NzO saturated aqueous solution of 2-propanol is illustrated in Figure 3 and is expressed by eq 11. The G(H2) is considered to be a net yield from three sources: (1) the molecular hydrogen formed from the radiolysis of water, (2) the abstraction of hydrogen from the 2propanol by H atoms, and (3) the “direct action” yield of hydrogen which appears to be linearly proportional to the %propanol concentration. The intercept value, G(Hz) = 0.95, is calculated from the measured hydrogen yields in concentrated 2-propanol solutions, where all of the H atoms have reacted with the alcohol. Since the “direct effect’’ is zero at the intercept (zero 2-propanol concentration), the G(H2) at this point is assumed to be a composite of the molecular hydrogen formed by radiolysis of the water plus the molecular hydrogen formed by H atoms reacting with the alcohol. Since the G His~ determined to M KBr solution, the be 0.34 in an N20-saturated GH is calculated to be 0.61. The “direct effect’’ has a concentration dependency of 0.54 [Bpropanol]. Nitrous Oxide-Sodium Formate- Water Solutions. I n the 6oC:o y radiolysis of sodium formate or formic acid solutions, molecular hydrogen is also produced by H atom abstracting hydrogen from the organic sol~ t e , 4 J , ~ 1 ~ 2The ~,~ hydrogen ~ - ~ ~ yields produced during the radiolysis of N2O-saturated aqueous sodium formate solutions are tabulated in Table I, illustrated in Figure 3, and represented by eq 111. Using the same interpretation as employed for the 2-propanol solutions, the G H is calculated to be 0.57 and the “direct effect” as 0.16[sodium formate]. The H Atom Yield. These determinations of GH are tabulated and compared in Table I1 with the determinations of other authors. Although the solutions used in this work contained two solutes, NzO and 2-propanol or N 2 0 and formate ion, the GH is not determined by
3987
Table I1 : Yields of GH in Aqueous Organic Solutions Authors
System
GH
Allan and Beck14 Allan and Scholes4 Allana7 Anbar and Meyersteinas Hayon and Allendo Kelley and Smith27 Rabani and Stein2*
2-Propanol-nitrous oxide 2-Propanol-acetone Methanol-sodium nitrate 2-Propanol-acetone Chloroacetate 2-Propanol-Na 2-Propanol-acetone Sodium formate-acetoneferricyanide Sodium formate-oxygen 2-Propanol-nitrous oxide 2-Propanol-Cu f2-Nz0 Concentrated 2-propanolnitrous oxide Concentrated sodium formate-nitrous oxide
0.60 0.60
Scholes and Simic41 Scholes and Simic21 This work
0.45
0.62 0.80 0.30
0.55 0.75 0.74 0.66 0.61 0.57
competition between the solutes for the H atom. Thus a possible source of error was eliminated. If the reaction rate constants of two competitive solutes differ greatly, a high concentration of the solute with the lower rate constant may be required to obtain analytically significant differences in the monitored product. Thus a “direct effect” may insidiously enter to give results that are too large. For example, if the hydrogen yields observed in the competitive study of Orsaturated aqueous sodium formate solutions40are corrected for the “direct effect” on the sodium formate, the calculated GH is lowered about 20% and is then in excellent agreement with that determined in this work.
Acknowledgment. The author wishes to acknowledge discussions of this work with J. W. Boyle, C. J. Hochanadel, P. S. Rudolph, and T. J. Sworski. (29) J. T. Allan, M. C. Robinson, and G. Scholes, PTOC. Chem. SOC., 381 (1962). (30) J. Rabani, J. Am. Chem. Soc., 84, 868 (1962). (31) J. Rabani, J. Phys. Chem., 66, 361 (1962). (32) G. Czapski, J. Rabani, and G. Stein, Trans. Faraday Soc., 58, 2160 (1962). Chem. SOC.,23 (1964). (33) M. Anbar and D. Meyerstein, PTOC. (34) M. Anbar and D. Meyerstein, J. Phys. Chem., 68, 1713 (1964). (35) M. Anbar and D. Meyerstein, ibid., 68, 3184 (1964). (36) C. Lifshitz and G. Stein, J. Chem. SOC.,3811 (1964). (37) J. T. Allan, J. Phys. Chem., 68, 2697 (1964). (38) H. Fricke, E. J. Hart, and H. P. Smith, J. Chem. Phys., 6, 229 (1938). (39) D. Smithies and E. J. Hart, J. Am. Chem. Soc., 82, 4775 (1960). (40) E. Hayon and A. 0. Allen, J. Phys. Chem., 6 5 , 2181 (1961). (41) G. Scholes and M. Simic, Nature, 199, 276 (1963). (42) S. Nehari and J. Rabani, J. Phys. Chem., 67, 1609 (1963). (43) G. Scholes and M. Simic, ibid., 68, 2697 (1964). (44) E. J. Hart, Radiation Res. Suppl., 4, 74 (1964). (45) E. Hayon, Trans. Faraday Soc., 61, 734 (1965).
Volume 70. Number 12 December 1966
