111. L. HYDER
1858
approach to the catalyst surface through a-complex adsorption suggests that the main cause for the faster aromatic exchange is due to a substitution process. The dissociative r-complex substitution mechanism is further supported by the observed “complete” and “severe” ortho deactivation effects.
Acknowledgment. The authors thank the Australian
Institute of Nuclear Science and Engineering for assistance with the purchase of the heavy water, the New South Wales State Cancer Council for the use of their facilities, and Commander J. Mason for instrumentation advice. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
The Radiolysis of Aqueous Nitrate Solutions’
by M. L. Hyder Savannah River Laboratory, E. I . du Pont de Nemours and Company, Aiken, South Carolina (Received November $6, 1964)
The production of nitrite from neutral and alkaline solutions of sodium nitrate by Coeo y-radiation was measured over a range of nitrate concentrations from to 4.0 M . The production of 0 2 from these solutions at pH 13 was also measured, and the origin of the 0 2 was determined by labeling the water with 0’8. The effect of scavengers, particularly 0 2 and I-, on the nitrate reduction was determined. From these studies, and from comparisons with ultraviolet photolysis and with previously published work, the radiolysis of NO3- in neutral solutions was concluded to involve three effects: (1) reduction of NO3by electrons and H atoms, (2) direct excitation by radiation, and (3) an effect possibly due to excited water molecules or spur reactions. In alkaline solutions an additional contribution is found, probably from the reaction of 0-. The data allow a determination of G,,,- 2 3.8 and G-H~O 4.5 at pH 13.
Introduction Interest in the chemical behavior of aqueous nitrate solutions undergoing radiolysis derives both from the use of such solutions in handling highly radioactive materials and from the inconsistent results of published studies On this system* Data from various investigators are often in disagreement and have led to some i,e.,that OH radiunusual and differing conc~usions, cals react with nitrate,2 that large yields Of active excited water are produced by radiolysis,a or that in moderately concentrated solutions the conventional free-radica1 becomes As an example, conilicting values for GNO*have been reThe Journal of Phys&?izl chemistry
ported. At pH 7, values of GO,of 2-34J and or 5-6av6 have been found, and values ranging up to 6 have also been found in alkaline solution^.^^^ These (1) Work performed under Contract No. AT(07-2)-1 with the U. S. Atomic Energy Commission. (2) (a) G. E. Challenger and B. J. Masters, J. Am. Chem. SOC.,77, 1063 (1955); (b) A. 0.Allen, “The Radiation Chemistry of Water and Aqueous solutions,” D. Van Nostrand c o . , Inc., Princeton, N. J., 1961, p. 101. (3) M. A. Proskurnin and V. A. Sharpatyi, Russ. J. Phys. Chem., 34, 1009 (1960). (4) J. BednLI and S. LukitE, Collection Czech. Chem. Commun., 29, 341 (1964). (5) V. A. Sharpatyi, V. D. Orekhov, and Ya. Karpov, Nukleonika, 5, 12 (1959).
RADIOLYSIS OF AQIJEOUS NITRATESOLUTIONS
1859
the resulting solutions was less than M . 01* observations are difficult to reconcile with established water (1.6%) was obtained from the Oak Ridge Nameasurements of radical production? Reported values tional Laboratory and was filtered and distilled beof and of are also in disagreement, but it fore using. Alkaline solutions were prepared and has been established that H2, 02,Hz02, and N02are the only significant products of radioly~is.~,~'stored in an argon atmosphere to avoid contamination by C02. In addition, oxygen was removed from the Other published information on nitrate radiolysis, samples by extensive argon purging or by freezing, including studies which gave evidence of a direct depumping under vacuum, and thawing, followed by composition of nitrate by radiationEand measurements irradiation under vacuum. Oxygen-saturated samples of the effect of nitrate on the radiolysis of other were prepared by purging with oxygen at 1 atm. for ions,2a,8v12 is also difficult to reconcile with many of the approximately 20 min. previously cited reports. These precautions sufficed to make the data reThe work reported here was performed (1) to reproducible to a reasonable degree. The maximum solve the discrepancies in the literature and (2) to error of the reported values of GNO%was estimated to determine the reactions involved in the y-radiolysis be less than =klO%. The actual fluctuations were of nitrates. For this purpose, an extensive series of generally much lower, except for some cases that will measurements of nitrite production was made over a to 4 M and be noted. In alkaline solutions it was possible to obrange of nitrate concentrations from tain dose vs. NOz- production curves that were close from pH 7 to 13. Particular emphasis was placed on the alkaline solutions because (1) relatively few to linear a t the doses used; in neutral solutions, howradiolysis studies have been made in such solutions ever, the data represent lower limits as the accumulating nitrite appeared to be removed by a secondary reaction and ( 2 ) most of the unusually high radiation yields a t very low concentrations. In the presence of Iof nitrite had been reported a t high pH. The effects or 02 scavengers linear slopes were obtained. All of radical scavengers on nitrite yields were investigated-particularly I- and 0 2 , but also Br- and H202. measurements of NO2- production were made a t total The formation of the other radiation products was also absorbed doses close to lo4 rads. No effect of dose rate was observed in the range 1ojto lo6rads/hr. studied; in particular, the origin of the O2 formed in The analyses for NO2- were made by automatic alkaline nitrate solutions was determined by irradiacolorimetric methods described by Britt. l 3 This tion of solutions made from Ol8-enriched water, as was done by R4ahlman8 in neutral solutions. Studies method was accurate to *5% in the range of a few were also made of the photochemical decomposition of parts of nitrite per million parts of solution. When nitrate. materials were present that interfered with the analytical method, such as peroxide or iodide, appropriate Experimental standards were prepared and analyzed in order to make All y-irradiations were made with a Co60 source prothe necessary corrections. HzOz was analyzed by a viding a maximum dose rate of 1.8 X lo6 rads/hr. method developed in this laboratory. l4 Irradiations were made a t or near this dose rate, except Measurements of the gaseous products were made on for those studies where the effect of dose rate was of samples that had been outgassed and sealed under primary importance. Dosimetry was carried out with vacuum; the gases which were recovered after irradiathe Fricke dosimeter, assuming a G value for Fe3+ tion were measured with a McLeod gauge, and their production of 15.6. Sample positioning methods fixed the precision of the dose a t *2%. Irradiations (6) A. M. Kabakchi, V. A. Gramolin, and V. M. Erokhin, Proceedings of the First All-Union Conference on Radiation Chemistry, Moscow, with ultraviolet light were made with unfiltered mercury 1957, 11, Radiation Chemistry of Aqueous Solutions (Inorganic and lamps or mercury-xenon lamps. The data were corOrganic Systems), 1959, p. 45. rected for variations in absorbed dose with solution (7) A. 0. Allen, Radiation Res. Suppl., 4, 54 (1964). (8) H. A. Mahlman, J . Phys. Chem., 67, 1466 (1963). concentration. (9) N. A. B a a , V. I. Medvedovskii, A. A. Revina, and B. D. BituiSolutions were prepared from triple-distilled water, kov, Proceedings of the First All-Union Conference on Radiation boiled to remove co%and Premirradiated to destroy Chemistry, Moscow, 1957, 11, Radiation Chemistry of Aqueous organic materials, and from reagent grade LiNO, Solutions (Inorganic and Organic Systems), 1959, p. 39. (10) J. BednBI, Collection Czech. Chem. Commun., 27, 2204 (1962). Hz02,NaOH, 1\TaBr, KI, and NaN03. The last three were recrystallized from triple-distilled water. Na2C03 (11) w. Boy1e and A. MaMmani Nucl. En#., 2, 492 (1957). was removed from NaoH by precipitation from a (12) T. J. Sworski, J . Am. Chem. Soc., 77, 4689 (1955). saturated NaOH solution and in "e cases by Pre(13) R. D. Britt, Jr., Anal. Chem., 34, 1728 (1962). cipitation of BaC03; the concentration of cos2-in (14) E. K. Dukes and M. L. Hyder, ibid., 36, 1689 (1964). G
~
~
0
~
~
7
~
~
~
J*
Volume 69, Number 6 June 1966
M. L. HYDER
1860
chemical and isotopic compositions were determined by mass spectrometry. Samples required about lo7 rads in order to generate enough gaseous products for analysis ; radiolytically produced nitrite could be measured after only about lo4rads.
Oxygen-Free Solutions
6.0
-
,.
pH7 pH I3
0
I
I 5.0 N
8
-u 4.0 Y,
Results Repeated determinations were made of the production of NOz- in neutral and alkaline nitrate solutions. Some of the data, covering the range of nitrate concentration from lo-, to 4 M and pH 7 to 13, are shown in Figures 1 and 2. These data are from the most carefully prepared samples and represent the trends observed in sets of samples prepared from the same stock solutions. There was some variation among sets. In particular, there was a pronounced tendency for the data to scatter in the concentration range 0.1 to 1 M NO,- in the alkaline solutions, where the slope of the curve is the greatest (Figure 1). The general trend, however, was quite reproducible. The G values shown in these figures agree with some of the previous work; that at pH 7 is in rough agreement with ref. 4 and 5 and is consistent with established radical yields. The G value a t pH 13 confirms the unexpectedly high yields of nitrite previously rep~rted.~pjFigure 2 shows the dependence of the nitrite yield on pH; the yield rises above pH 10, which is consistent with data reported in ref. 5 . The effect of radical scavengers on the nitrite yields was investigated to establish which radicals were involved in the reduction. The effect of oxygen (shown in Figure 3) decreases the nitrite yield to a low value which is approximately independent of pH. I- and Br- were added to the solution to scavenge the OH radical and to eliminate its reactions; however, the product of the reaction of Br- and OH reacted with NO2-, making it difficult to draw any conclusions. Results obtained in the presence of 0.10 M I- are shown in Figure 3; in both alkaline and neutral solutions GNO*-is increased to the same high value by I-. The addition of Hz02to scavenge OH was also investigated. However, even lowaM H202 interfered with the analysis for nitrite, producing low results. In a few scouting experiments, very little effect of HzOz on the nitrite yield was observed. We have also measured the hydrogen peroxide production in deaerated nitrate solutions. Our measurements in neutral solutions are consistent with those of o ~the range 0.6Bednitr' and LukitE14showing G H ~ in 1.0; however, a t pH 13, Ga202was found to be less than 0.05. .Uthough peroxide is unstable in these solutions, our analyses were made within 5 min. or less following irradiation. By adding small amounts of The Journal of Physical Chemistry
0
E
3.0
$2.0
0
I .o 0
I
2
1 ( ( 1 1 1
1
1
1
s
1
1
/
1
t
I
, ,
t , , , , ,
1 1 , 1 1
5.0
4.0 >
8
0
2 3.0
-P p
2.0
1-
p
W
1.0
'6
7
9
8
II
10
12
13
I
PH Figure 2. Variation of Gyo2- with pH, in 1.0 M solutions of NaNOs.
V"
---Solutions conloininqneilhsrO~nor1-lFromFig.1) ---.Oaygm-Frcc Solutions containing10pH13 8 p H 7 5.0 -Oxygen-Saturated Solulions ~ n o l 7 , ~ k OpH13 e p H 7 */.,
.'.
50 4.oi._.___.-.- ! w---*
_---
i
3'0 (pH =13)-#-----
,1
2.0
---
---_-5
,/
j
#/'*
/--r
~
(*--e
e
9p e
.
0
0
0.0%
H202to the solution before irradiation or by saturating the system with O2 to produce it during irradiation, we confirmed that this species would be detected if
1861
RADIOLYSIS OF AQUEOUS NITRATESOLUTIONS
reaction between 0 2 and water a t high pH; since, formed. Therefore, we conclude that no significant however, we find the 018/016 ratio in the 0 2 to vary quantity of peroxide is produced a t pH 13 in deaerated widely and reproducibly with nitrate concentration, solutions. it does not seem to be occurring here. Either the Experiments with 01*labeling, similar to those by oxygen escapes to the relatively large void space in ;\!tahlman,*were performed a t pH 13 to determine the the vessel quickly, or else the chain is broken by nitrate. origin of the O2 produced by radiolysis. In these Since 0- was postulated as the chain initiator, and we solutions O2 is the only oxidized product in apprehave evidence for reaction of this species with nitrate, ciable quantity. At the high dose rates required to the latter case is probable. accumulate sufficient oxygen for mass spectrometric We also carried out some photochemical studies in analysis, the considerable back reaction between the an attempt to characterize the "direct effect" of radiaaccumulating oxygen and nitrite in both neutral and tion on dissolved nitrate ion. It had been shown by alkaline solution precluded the direct determination of Cultrera and FerrarilB that ultraviolet light decomGO,. However, the 018/016 ratio was determined, poses nitrate to nitrite. Reasoning that the mechaand the total amount of O2 formed was calculated by nisms of decomposition by radiolytic or photolytic exstoichiometry (1) from GNO,- determined at low doses citation should be similar, we irradiated nitrate soluand (2) from published values of GH,. The results tions containing radical scavengers with light from a of these measurements, shown in Table I, indicate that mercury lamp. Absolute quantum yields were not most 0 2 originates from water in dilute solutions, with determined, but care was taken to reproduce irradiation an increasing fraction originating with nitrate at higher time and intensity. Increasing pH was found to innitrate concentrations; this result will be discussed in detail later. Hart, Gordon, and H u t c h i s ~ n ~ ~crease the yield of nitrite, as previously reported16; Br- had less effect, and O2 virtually none (see Table have discovered a radiation-induced chain exchange 11). In separate irradiations it was found that no H20z could be detected in the alkaline solutionseither it was never formed, or it was efficiently photoTable I : Production of 02 from NO3- in Solutions Containing 018-Enriched HIO" lyzed. The 0 2 produced originated mainly from nitrate ion (see Table I). Fraction
NOa-, Salt
% O'afound of 0, in 0, from NOa-
M
NaN03
0.12
NaN03
0.50
-,-Irradiations 1.4gb 0.03 1.63
1.45 1.50
0.09
Go,.
Go2 from
calcd.
Nos-
2.0
0.06
2.35
0.21
Table 11: Production of Nitrite by the Ultraviolet Irradiation of Aqueous Nitrate Solution Relative yield of nitrite, arbitrary units Wz purge 02 purge
Composition of sample
NaN03
1.0
1.36 1.38
0.16
2.40
0.38
NaN03
2.5
1.16 1.21
0.30
2.35
0.70
NaN03
4.0
1.07 1.06
0.39
2.30
0.90
LiN03
0.25
1.60 1.47
0.05
2.20
0.11
LiNO,
1.0
1.34 1.35
0.19
2.40
0.44
LiN03
2.0
1.19 1.21
0.29
2.35
0.67
NaN03
1.0
...
...
Ultraviolet irradiations 0.54 0.76 0.53
" Original concentration of 0 1 8 in water was 1.6%; samples were 0.10 M in NaOH. Results of duplicate experiments.
1.OMNaN03 l.OMNaN03,O.lOMNaOH 1.0 M NaN03,O.lO M NaOH, 0.10 M NaBr a
38 68
32 62
81,83"
80,71"
Results of duplicate experiments.
Discussion The basic question to be answered in the study of nitrate radiolysis is whether the behavior of this system can be explained by a theory of free-radical intermediates alone or whether additional processes are involved. The radical theory predicts that in dilute (15) E. J. Hart, S. Gordon, and D. A. Hutchison, J. A m , Chem. SOC., 75, 6165 (1953). (16) R. Cultrera and G. Ferrari, Ann. Chim. (Rome), 47, 1321 (1957).
Volume 69, Number 6 June 1966
M. L. HYDER
1862
solutions the reduction of nitrate” should course
HzO Nos-
+ ea,-
ea,-, H, OH, etc.
+ 20H- +
+NOzaq
or
+ H +N02,q + OH+ H2O + NOz- + Nos- + 2H+
Nos2N0,,,
(3)
The result of these reactions would be 0.5 molecule of NO2- per solvated electron or H atom. Thus, these - to 0.5reactions could account for values of G N O ~up GFHZOor between 2 and 2.5. An additional process must be found to account for the greater amount of NO2- found in alkaline solutions. Four additional processes that may occur during radiolysis of nitrate solutions have been proposed : (a) the excited water theories of Proskurnin and Sharpatyi,3,18(b) the theory of Bed& and L u k Q 4 which suggests preferential radiolysis of the solute in concentrated solutions, (c) Mahlman’ss hypothesis of direct excitation of nitrate ion, and (d) suggestions of The nature the reaction of OH radicals with of these processes and their possible importance are briefly described in the following paragraphs. The “Excited Water” Explanation, Proskurnin and S h a r p a t ~ ihave ~ ? ~suggested ~ that excited water molecules are responsible for the high yields of nitrite. Their reasoning is based on the hypothesis that the initial processes in the radiolysis of water should be the same in the liquid and vapor phases although the reported values of G--H20 are about 4.5’ and 11.719in the two phases. It is suggested that the difference is due to excited water molecules, normally unreactive in the liquid phase, which react with concentrated nitrate. The excited water molecules are postulated (1) to be primarily radical pairs trapped by the solvent cage and (2) to undergo typical radical reactions. Several arguments against this theory may be presented. There are theoretical objections to the basic hypothesis that the primary radiolytic processes should be the same regardless of statez0; this is particularly true when considering very concentrated ionic solutions. Also, it is not clear why only nitrate and perhaps a few other materials1* should show a high reactivity toward the excited water that is not observed in other irradiated systems.21 In addition, some of the results of Proskurnin and Sharpatyi could not be duplicated in this and other investigation^,^^^ including in particular their reported value of G N O ~ approaching (5 at pH 7. This theory therefore appears somewhat tenuous. The Journal of Physical Chemistry
Bedndi and Lukdc”s Theory. The theory of Bednitr and LukBE4 is based on the nature of the radiolysis process in concentrated solutions. They reason that in sufficiently concentrated solutions the initial excitation would be delocalized enough for the solute to be involved in a large fraction of the primary excitations. They therefore propose that in concentrated solutions, reactions 1-3 should be replaced by a series of reactions including
(H20, NOS-)
-m+
N03aq*
N%aq* +NO2aq
Nos-
+ eaq-
+0
+ 0 +NO2- + 0 + HzO +20H
0 2
(4) (5) (6)
(7) From their experiments, Bedn6i: and Luk61: derived from reaction 4 a maximum G = 3.04 for very high nitrate concentrations. They also postulated the formation of excited water, which undergoes the reaction
H2O*
+ Nos-
---f
H202
+ NO2-
(8)
The G value for the formation of H20* from their data analysis is in the range 1.2 to 1.5. This theory would account for the production of very large amounts of nitrite in the radiolysis of concentrated solutions. The maximum GNO,- would be G(o) G(s),which in 0.5G(z) 0.5G(~ 0.5Go) the limit of very concentrated solutions would be equal to 2G(4) G(8) 7.6. The theory does not provide for a dependence of GNO,- on pH. These authors believe that the direct effect of Mahlman,8 described next, is not due to the direct excitation of nitrate by radiation but rather to a competition between reactions 6 and 7 . The Direct Egect. Mahlmans observed that in neutral nitrate solutions 0 2 was produced from the nitrate ion, in an amount roughly proportional to the nitrate concentration. This he assigned to the direct excitation of the nitrate by radiation. The “direct” GO, measured by Mahlman is, however, only of the order of tenths, so that this “direct effect” cannot by itself account for the very high nitrite yields observed. Reactions of OH Radicals. The suggestion that OH radicals might react with nitrate ion2 could also ac-
+ +
+
+
+
(17) The amount of solvation of the hypothesized “NOn” intermediate is unknown; to distinguish this species from the gaseous molecule it is denoted Nola, in the subsequent discussion. (18) V. A. Sharpatyi, Russ. Chem. Rev., 30, 5, 279 (1961). (19) R. F. Firestone, J . Am. Chem. Soc., 79, 5593 (1957). (20) U. Fano, “Comparative Effects of Radiation,” John Wiley and Sons, Inc., New York, N. Y . , 1960, p. 14. (21) M. S. Matheson, Radiation Res. Suppl., 4, 1 (1964).
RADIOLYSIS OF AQUEOUS NITRATESOLUTIONS
1863
count for a high value of G o z - . If the OH radicals could serve as one-electron reducing agents, then the nitrite yield might be approximately doubled. Implications from Radical Scavenging Studies. The results obtained in this work with radical scavengers, particularly O2 and I-, are to be examined with respect to the preceding theories. As shown in Figure 3, the presence of O2 during the irradiation drastically lowers the GNO~-.Generally, such an effect is attributed to the reaction eaq-
+ 02
+0 2 -
+ aq.
(9)
which removes the solvated electrons and thereby prevents reaction 2. However, it is now known that the reaction constants for eaq- with 0 2 and with Nosare very nearly the same.' Therefore, the drastic reduction of G N O ~produced by M 0 2 in much more concentrated nitrate solutions would not be expected. If the 0 2 is not reacting with the primary species, then it is evidently reacting with the intermediate, "NOzaq,"to oxidize it back to nitrate. It can be inferred that NOzaqis oxidized equally well at all concentrations of nitrate by M 0 2 . In the is decreased by this process more dilute solutions 60,almost to zero, which indicates very efficient oxidation of NOzaq. However, in the more concentrated solutions for which the value of G N O ~is- appreciable even in the presence of oxygen, some of the nitrite is evidently produced by processes in which Nos,, is not an intermediate. Reactions 6 and 8 are of this type, and a limit can be set on the extent to which such reactions are involved. The studies of G N O ~in - the presence of I- provided additional information. I- is known to undergo the reactionz2
I-
+ OH -+ OH- + I
(10)
found in the presence of I-; in view of the high concentration of reducing agent present, it is probably equal to this value. These data allow some quantitative calculations of radical yields. G o z - formed with I2present, shown as the upper curve in Figure 3, includes both NO2- formed from NOzsqand production of that nitrite which is unaffected by radical scavengers and therefore does not derive from NO%,, (lower curve in Figure 3). By subtraction of these two curves, a measure of G N O and ~ ~ thus ~ of G ( H + ~ , ~ - ) may be obtained; this subtraction is shown in Figure 4. G N O ~ is , ~ seen to rise with nitrate concentration slowly to about 1 M nitrate, above which it drops off sharply to lower values. This is consistent with what is known about the system; MahlmanZ3has shown M NOS- to that G E ~drops from about 0.36 in
