Radiation-Induced Decomposition of Sodium Nitrate
35
Mechanism of the Radiation-Induced Decomposition of Sodium Nitrate. I’ Arnold Frances and Everett R. Johnson’2 Department of Chemical Engineering, University of Maryland, College Park, Maryland 20742 (Received April 5, 7974; Revised Manuscript Received September 9, 7974) Publication costs assisted by the University of Maryland
Experimental evidence is presented to show that the “apparent” increase in nitrite yield when nitrite is determined by the ceric sulfate method as compared to the method of Shinn is due to the presence of a color center and is not due to the presence of radicals such as NOz, NO, NzO3, etc. as has been proposed. The color center is believed due to an 0- or 0 2 - bound to the nitrite ion/or ions formed simultaneously.
The radiation-induced decomposition of the alkali nitrates has been extensively studied3 and i t is generally agreed that the overall reaction is MNO,
_If
MNOz
f
%O,
This paper is concerned primarily with the observation by Cunningham4 that when irradiated NaN03 or KN03 are analyzed for nitrite ion by ceric sulfate reduction, the apparent nitrite yield so determined is appreciably higher than when nitrite is determined by the conventional S h i m 5 method. The increase in apparent nitrite yield found using the ceric method of analysis was attributed by Cunningham to the presence of small amounts of reducing substances other than NOz- such as NO2, NO, 0 2 - , 0-, or OzZ-. Cunningham6-s developed a very complex mechanism, supported by optical and esr studies, for the decomposition of nitrates based on the initial formation of species such as Oz2-, N0z2-, NO3, N032-, N2032- and reactions of these species to give final stable products, such as NO, NOz, Nz04, and NOz-. Kaucic and Maddockg in their studies on the decomposition of NaN03 doped with Ca(N03)z confirmed the apparent increase in nitrite yield observed by Cunningham when the irradiated samples were analyzed by the ceric method as compared to the Shinn method. However, they observed that this apparent increase in the nitrite yield (ANOz-) was only evident in the early stages of the decomposition, i e . , the ratio of the nitrite yield as determined by the ceric method to that determined by the Shinn method at very low absorbed dose was about 2.5, with increasing dose the ratio dropped to about 1. There is, however, serious discrepancy in the literature concerning the optical and esr studies reported by Cunningham and that by others.1°-17 A review of all the papers pertaining to esr studies on irradiated NaN03 indicates that NO, Oz2-, or 0- are not present. Weak resonance spectra have been reported in NaN03 irradiated at room temperature which have been tentatively assigned to either NOz10J3 or Nos, however, these results are not supported by the studies of others.16aJ7 Mass spectrographic analysis of the gas evolved when irradiated NaN03 or KNO3 is heated,4Js-20 or of the gas collected over water after dissolution of the irradiated salt,ZO and by chemical means2I does not show the presence of NO or NOz. Esr studies do reveal the presence of NO2 a t liquid nitrogen t e m p e r a t ~ r e , l l J ~ Jhowever, ~ ~ J ~ only if NOz- is present in the l a t t i ~ e , l ~i.e., ~ J ~if NaN03 is irradiated at 77’K and the esr spectrum examined at this temperature, NOz is not present, however, if the irradiated sample is
first warmed to room temperature then cooled to 77’K and reirradiated, the spectrum of NO2 is observed. This strongly suggests that the nitrite ion as such is not a primary product. The spectrum of NO2 is observed if K N 0 3 is doped with nitrite ion prior to irradiation at 77’K. There is, however, evidence for the presence of N032- a t low temperature~.~~~,~~ As to the optical spectra, P r i n g ~ h e i mhas ~ ~ observed ,~~ a single color band in irradiated NaN03 in contrast to five bands reported by C ~ n n i n g h a m . 2 ~ The fact remains, however, that analysis for nitrite in irradiated NaN03 using the ceric sulfate method gives consistently higher values than the Shinn method, hence there must exist some reducing species which has not as yet been identified. The work reported here is an attempt to elucidate the nature of this reducing substance.
Experimental Section The bulk of the experiments were performed using a cylindrical cobalt-60 source of about 1500 Ci. However, some experiments were performed using a cobalt-60 source whose activity was approximately 25,000 Ci. Nitrite ion was determined by the method of Shinn5 and the ceric sulfate m e t h ~ d In . ~ the Shinn method sulfanilamide is diazotized and coupled with N - (I-naphthy1)ethylenediamine hydrochloride to form a colored azo compound with an absorption maximum a t 545 mp. The molar extinction a t this wavelength was 54,100. In the ceric method an irradiated sample is dissolved directly in 0.4 N HzS04 which is approximately M in ceric ion. The ceric ion concentration was determined a t 330 mp; the molar extinction coefficient at this wavelength was 5,500. Dosimetry was determined by ferrous oxidation using a G value of 15.45 molecules of ferrous ion oxidized per 100 eV absorbed. Ferric ion was determined a t 305 my, and the molar extinction used was 2201. All NaN03 was reagent grade. Results and Discussion More than 100 analyses of nitrite concentration comparing the ceric method and the Shinn method have been performed on irradiated NaN03. The ceric method gave very erratic results but in all cases the nitrite yield as determined by the ceric method was always greater than when determined by the method of Shinn. The difference in nitrite yield, ANOz- (ceric-Shinn), for room temperature irradiations, however, occurred during the initial absorbed The Journaiof PhysicalChernistry, Vol. 79, No. 7, 7975
36
dose only; beyond the initial absorbed dose the difference in NOz- yield as determined by the two methods remained constant (see Figure 1). These results are consistent with those reported by Kaucic and M a d d ~ c k As . ~ indicated in the introductory paragraph, there is no experimental support for the presence of oxides of nitrogen during the decomposition of NaN03. Since the ceric ion is very easily reduced, and ANOz- is relatively small -0.15 wmol/g (for room temperature decompositions), it appeared that the erratic results could possibly be attributed to surface impurities which formed reducing radicals during irradiation. Accordingly, a series of experiments was done with NaN03 which had received a number of different treatments prior to irradiation. These pretreatments consisted of (1)recrystallization from boiling concentrated H N 0 3 with subsequent recrystallization from deionized water, (2) repeated recrystallization from deionized water, (3) baking a t 100' Torr), and (4) combinations of a t high vacuum ( P < these pretreatments. In addition, irradiation was also performed with NaN03 in the presence of about 3 atm pressure of NzO. These results are summarized in Table I. As can be seen there is no apparent difference in the results between the NaN03 used as purchased, the NaN03 irradiated in the presence of NzO, or the NaN03 that had received various pretreatments. Furthermore, the difference in nitrite yields by the two methods cannot be attributed to decomposition of any nitrite formed during irradiation. Pure NaNOz was irradiated to an absorbed dose of 7.5 X 1020 eV/g. Analysis for total nitrite by the two methods gave identical results. In addition, particle size did not cause any change in the difference in yields as determined by ceric and Shinn analyses. Samples of irradiated CsN03 and KNOBwhen analyzed by these two methods gave results roughly comparable to the NaN03 i.e., AN02- -0.1. However, this difference in NO*- was about the maximum observed. In many irradiated samples little or no difference was observed in NO2content using the two methods. It appeared, therefore, that the difference in "apparent" nitrite yield (ANOz-), as determined by the two methods could not be attributed to surface effects, particle size, or prior treatment of the salt. Nor could the difference in yields be attributed to any decomposition products of nitrite ion. An important aspect, however, of the difference in apparent nitrite yields (ANO2-) using these two methods is the fact that the difference increases with decreasing temperature (Figure 2). Samples irradiated at liquid helium temperatures and subsequently warmed to room temperature and analyzed by the two methods gave results (AN02-) roughly 5-10 times greater than those found in samples irradiated a t room temperature to the same absorbed dose. These results indicated that the additional reducing substance may be a trapped electron (or color center). It is well known that defects (trapped electrons and holes) are formed in NaC1, MgO, and Si02 when these substances are irradiated and these color centers can be detected by chemical meandeb consequently samples of NaC1, MgO and Si02 were irradiated to an absorbed dose of about loz1 eV/g and analyzed for the presence of reducing species using ceric sulfate. In addition, samples of Na2S04, which are not known to contain defect centers, were also irradiated and analyzed. The insoluble irradiated MgO and Si02 were simply added to the ceric solution, stirred, and subsequently filtered off. These results reThe Journal of Physical Chemistry, Vol. 79, No. 1, 1975
Arnold Frances and Everett R. Johnson TABLE I: Difference in Apparent Nitrite Yield (&NO,-) in N a N 0 3Irradiated in Different Atmospheres or a f t e r Different P r e t r e a t m e n t s (Absorbed Dose = 1.4 X lozoeV/g) &NOz- (av),
Treatment Normal NaNO8 (irradiated in vacuo or air) Irradiated in 3 atm pressure
woVg
-0.15
of N20
-0.15
Recrystallized from boiling "03
-0.12
Recrystallized from deionized water 1.31
I
-0.14 I
1
1
i
c0b
I
I
I
10
20
30
ABSORBED DOSE, eV/g x
1 1
40
16''
Figure 1. ANOn- (ceric-Shinn) as a function of absorbed dose.
tI
I
I
I
I
I
I
O.O ------04h
50
I 100
I50
I
200
TEMPERATURE,
I
1
1
250
300
350
)
OK
Figure 2. The increase in AN02- (ceric-Shinn) as a function of ternperature (absorbed dose = 1.4 X IOzo eV/g).
ported as ANOz- are summarized in Table 11. As can be seen all the samples with the exception of Na2S04 contained reducing species. MgO and Si02 showed reducing properties if they were simply heated a t 120' in uucuo for about 20 hr without irradiation (the reduction of ceric ion in these latter samples on a per gram basis was greater by a factor of 2-4 than that of the irradiated sample). Most color centers in irradiated ionic crystals are easily bleached by thermal annealing at relatively low temperatures, however, when samples of NaN03 irradiated at room temperature to an absorbed dose of 1.05 X lozo ev/g were annealed a t 100" for 48 hr and subsequently analyzed, AN02- remained essentially unchanged, Le., the equivalent nitrite yield as determined by the two methods remained the same (-0.15 pmol/g) despite annealing. However, when samples of NaN03 similarly irradiated were an-
37
Radiation-Induced Decomposition of Sodium Nitrate TABLE 11: Reducing Properties of Various Irradiated Salts (Absorbed Dose = 10 X IOz9eV/g) Salt NaCl MgO SiOp
Na2S04
LNO, - (equivalent), mol of NOz-/g 0.049 0.52 1.05 0.00
nealed a t 200°, although appreciable thermal decomposition occurred, analysis of these samples by the two methods gives identical results indicating that the substance or substances that gave rise to the difference in equivalent nitrite yields had disappeared during the heating. Subsequently, samples irradiated a t -196' to an absorbed dose of 1.4 X 1020eV/g were annealed a t different temperatures for 18 hr and the annealed samples analyzed by the two methods (see Figure 3). These results are plotted as ANOz(ceric-Shinn) us. temperature. As can be seen there is a rather abrupt decrease in ANOp- a t a temperature of about 160'. The abrupt character of this plot resembles a threshold event, such as a fairly deep electron trap rather than a chemical reaction or some diffusional process. Another group of samples irradiated a t -196' to an absorbed dose of 1.2 X 1020 eV/g was heated a t 200' for varying times (see Figure 4). As can be seen appreciable annealing occurs within the first hour. Irradiated NaN03 (as well as K N 0 3 and CsN03) has a slight yellow color which is intensified if the irradiation is done a t low temperatures. The properties of this color band as observed by Pringsheim appears to have much in common with the reducing species identified chemically as AN02. For example, Pringsheim observed that the color band was bleached when the crystal was heated for 30 min a t 200°, however, the color center was stable at 100'. This is consistent with the observations reported here. Pringsheim observed that a t room temperatuie the color center intensity increases very rapidly with absorbed dose and then levels off, whereas a t -196' the color center intensity rises more slowly with absorbed dose but reaches a value more than twice that found a t room temperature. This is consistent with the chemical observations, i.e., at room temperature SNOz-, after the first few minutes of irradiation, reaches its peak value and then remains relatively constant, whereas a t liquid nitrogen temperatures S N 0 2 appears to reach its peak value more slowly than a t room temperature; however, this value a t -196' is about 3-4 times greater than a t room temperature (see Figure 1). Pringsheim also observed that the color band formed when NaN03 was irradiated a t -196', could be reduced in intensity if the sample were subsequently irradiated a t room temperature. Table I11 shows the decrease in ANOp- when samples of NaN03, irradiated at -196' are subsequently irradiated a t room temperature. Another property observed by Pringsheim was that the color band was readily bleached by light from a high-intensity mercury arc lamp. Samples of NaN03 irradiated a t -196' and ground to a fine mesh were placed in an open beaker and bleached a t liquid nitrogen temperature and at room temperature. These results are summarized in Table IV. As can be seen a substantial reduction in ANOa- was achieved. Complete reduction in ANO2- did not occur because of the conditions of the experiment, Le., lack of penetration of the light plus poor stirring of the powders. In these experiments, however, it was virtually impossible to prevent photochem-
TABLE 111: Decrease in ANOz- of N a N 0 3 Irradiated at 196" (Absorbed Dose = 1.72 X 1020 eV/g) as a Result of Post-Irradiation at R o o m T e m p e r a t u r e aNOz-, mol of NOz -/g
Absorbed dose, eV/g 0.00 2.6 X
0.72 0.37
lozo
TABLE IV: Decrease in AN02- in N a N 0 3 Irradiated at -196" (Absorbed Dose = 1.4 X lozoeV/g) Bleached w i t h Mercury Arc L a m p LNOt-, mol of
Bleaching time, hr
Temp, "C
NOp-/g
0.00 0.5 1.5 4.0 8.0 20.0
- 196 - 196 - 196 25 25 25
0.68 0.45 0.14 0.49 0.34 0.22
330
275
325
35C
375
400
425
450
475
"K
Change in ANOn- as a result of annealing for 18 hr at different temperatures. Figure 3.
0608
I
T
000
I
_____________________________-_
I
-0203'
I
1
I
5
I0
w
. _O_ _ _ -_ J
I
1 I5
I
I
:2
ANVEALING TIME, kCU?S
Figure 4. Change in AN02ature = 200').
as a function of annealing time (temper-
ical decomposition. This is to be expected since the color band a t 330 pm is very close to one of the nitrate bands a t 300 pm. The lack of good reproducibility (at low absorbed doses) using the ceric method is readily explained for the determination of NOn- in these experiments if we assume that we are indeed determining color center concentration in addition to nitrite ion concentration. The production of color centers in NaN03, unlike nitrite ion production, is not quantitatively predictable, Le., the color center concentraThe Journal of Physical Chemistry, Vol 79, No. 1. 1975
30
L. A. Cosby and G. L. Humphrey
tion will vary somewhat with different crystals and hence so will ANOz-. It appears therefore that ANOZ- is a measure of the color center concentration and that the reduction of ceric ion is another chemical method of determining these centers. The question remains, however, as to the nature of the color center. The center described by Pringsheim has its maximum a t 335 mp and cannot reasonably be attributed to either NO2 or NO in the lattice, since the absorption spectra of these molecules when observed in an inert mat r i are~ very ~ far ~ removed ~ ~ from ~ 335 mp. Pringsheim attributes the color center to an electron trapped in the vicinity of the nitrite ion. We do not think that the color center can be an electron trapped in an anion vacancy as are, for example, the color centers observed in the alkali halides. In these latter salts the color centers are easily annealed a t room temperature and below whereas in NaN03 a minimum temperature of about 160' is required. We are inclined to accept the view that the color center is an electron trapped in the vicinity of a nitrite ion; however, it must be very tightly bound and is more likely associated with a relatively stable molecular species. The results of Zeldes and LivingstonlGaclearly indicate that the nitrite ion is the precursor of the NO2 molecule observed in the esr spectra a t low temperatures and that the actual formation of nitrite ion requires some thermal activation, i.e., a t liquid nitrogen temperature the nitrite ion is nut formed but rather some intermediate which, when the irradiated sample is warmed to room temperature, produces nitrite ion. The color center, we believe, is associated with this intermediate. Since the concentration of this species a t room temperature is relatively high, -0.15 pmol/g, it cannot be NOS, N022-, or N03*- since in this concentration range
Kinetics of
cid ~
these species are easily detected by paramagnetic resonance techniques. We suggest that the center is an 0- or an 0 2 - associated with the nitrite ion/or ions formed simultaneously in some sort of a complex.
References and Notes (1) Research performed under the auspices of the U.S. Atomic Energy Commission Contract No. AT(40-1)-3660 To whom correspondence should be addressed. For a general review of nitrate radiolysis see, E. R. Johnson "Radiation Induced Decomposition of Inorganic Molecular Ions," Gordon and Breach, New York, 1970. J. Cunningham, J. Phys. Chem., 67, 1772 (1963). M. E. Shinn, ind. Eng. Chem., Anal. Ed., 13, 33 (1941). J. Cunningham, J. Phys. Chem., 70, 30 (1966). J. Cunningham, h t . J. Phys. Chem. Solids, 23, 843 (1962). J. Cunningham, J. Phys. Chem., 66, 770 (1962). S. Kaucic and A. G. Maddock, Trans. Faraday SOC., 65, 1083 (1969). W. B. Ard, J. Chem. Phys., 23, 1967 (1955). E. Bleany, W. Hayes, and P. M. Liewellyn, Nature (London), 179, 140 (1957). H. Zeldes, "Paramagnetic Resonance," Vol. ii, Academic Press, New York, N.Y., 1963, p 764. D. Shoemaker and E. Boesman, C. R. Acad. Sci., 252, 2099, 2865 (1961). R Adde and P. Petit, C. R. Acad. Sci., 256, 4682 (1963). See also ref 1, p 80. (a) H. Zeldes and R. Livingston, J. Chem. Phys., 35, 563 (1961); 37, 3017 (1962); (b) See W. G. Burns and T.F. Williams, Nafure (London), 175, 1043 (1955); J. G. Rabe, B. Rabe, and A. 0. Allen, J. Phys. Chem., 70, 1098 (1966); E. A. Roja and R. R. Hentz, ibid., 70, 2919 (1966). R. W. Holmberg, personal communication. H. Pogge, Thesis, Stevens Institute of Technology, Hoboken, N.J.; University Microfilm, Ann Arbor, Mich., No. 68-16, 222. E Ladov and E. R. Johnson, J. Amer. Chem. SOC.,91, 7601 (1969). G. Hennig, R . Lees, and M. Matheson, J. Chem. Phys., 21,664 (1953). T. H. Chen and E. R. Johnson, J. Phys. Chem., 66, 2249 (1962). P. Pringsheim, J. Chem. Phys., 23,369 (1955). E. Hutchinson and P. Pringsheim, J. Chem. Phys., 23, 1113 (1955). J. Cunningham, J. Chem. Phys., 41, 3522 (1964). G. W. Robinson. M. McCarthy, and M. C. Keelty, J. Chem. Phys., 27, 972 (1957). H. Brion, C. Moser, and M. Yamazaki, J. Chem. Phys., 30, 673 (1959).
~ in Alkali ~ Halide~ Nlatrices1i2 1
~
~
~
s
L. A. C Q S ~and Y G. L. HMmpRley" Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506 (Received June 12, 1974)
The thermal decomposition of malonic acid dispersed in alkali halide pressed pellets has been studied. The study was made by heating the pellets and observing the changes in absorption of several infrared bands of the reactant and products. The reaction was found to be first order with respect to disappearance of malonic acid with an apparent rate constant of k (seeT1) = 6.0 X 109 exp[(-23,400 f 300)/RT]. The effects of changing the matrix material are discussed.
Introduction
Although several ~ t u d i e s ~ have - ~ been carried out on the kinetics of the thermal decomposition of pure malonic acid in the molten state, none appear to have been reported for the reaction in solid solution. By employing a technique similar to that of Bent and Crawford,s we have made an infrared spectroscopic study of the thermal decomposition of malonic acid in the solid state at, or above, its melting The Journal of Physical Chemistry, Voi. 79, No. 1 . 7975
point. The reactant was incorporated in an alkali halide solid matrix and quantitative kinetic results were obtained by observing the change in infrared absorption of reactant and products as a function of time. Many investigations of the decarboxylation of malonic acid in numerous different solvents have been r e p ~ r t e d From .~ the proposed mechanisms for the reactions in solution it appeared that a study of the reaction in a highly ionic solid medium would be beneficial.
