RADIOLYSIS OF FROZEN SOLUTIONS
2529
the resonance of the CY protons decoupled from the @ protons yielded only three singlets. An analysis based on the above results indicates that the resonance should appear more complex and the calculated and observed spectra are in fair agreement.
Radiolysis of Frozen Solutions. VI.
These results, which are quite similar to those obtained for PMM,16 indicate that only the p protons of PVC in syndiotactic diads are sensitive to the configuration in terms of tetrads and that the other groups may be interpreted in terms of diads and triads.
Electron and Excited Water
Reactions in Nitrate Ices
by Larry Kevan Department of Chemistry, University of Kansas, Lawrence, Kansas
(Received February 21, 1966)
~~
The y radiolysis of frozen nitrate solutions irradiated at 77°K has been studied. Nitrite yields were measured as a function of nitrate concentration, pH, radiation dose, and cation type. The data are compared with epr and aqueous solution results from which we conclude that NOz- is mainly produced by reactions of electrons and of electronically excited water with nitrate. The latter is responsible for the enhanced NOz- yields above pH 11. The results in aqueous and in frozen solutions show very close parallels and strongly imply that the same radiation-produced intermediates are formed and react similarly in both phases.
Introduction In the radiolysis of frozen nitrate solutions at 77°K the NO2 radical is observed directly by epr. Previous studies have shown that NO3- acts as an electron scavenger in frozen solutions.z However, from the effect of electron scavengers and the NO2 yield dependence on nitrate concentration it was proposed' that the trapped NOS arose from reaction of N03- with HzO+. This proposal probably received little serious consideration among aqueous solution radiation chemists. Recently though, the possibility of reactions of H20+in ices has been strongly supported by the studies of Moorthy and we is^.^ I n aqueous solution studies the solvated electron reacts with NO3- eventually to yield NOz-. The aqueous reaction is thought to involve an NO2 intermediate. To clarify the relationship between the reaction of NO,- with e,- (ern- denotes the mobile electron formed in ices) and with HzO+ in
frozen solutions and the aqueous nitrat.e solution results, we have studied the NOZ- yields produced upon melting irradiated, frozen nitrate solutions. A second goal of these chemical studies is to demonstrate the parallelism between aqueous solution radiolysis and frozen solution radiolysis. We further point out that information gained from the two approaches is largely complementary and serves to strengthen some conclusions and clarify others. Experimental Section All chemicals were reagent grade. Water was triply distilled over alkaline permanganate and had pH 6. Oxygenated and nitrogenated solutions were prepared (1) L. Kevan, J . Phgs. Chem., 68, 2590 (1964). (2) L.Kevan, P. N. Moorthy, and J. Weiss, J . Am. Chem. SOC.,86, 771 (1964). ( 3 ) P.N.Moorthy and J. J. Weiss, J. Chem. Phys., 42,3127 (1965).
Volume 70, Number 8 August 1966
LARRYKEVAN
2530
by bubbling the gas through a glass frit for 15 min. Frozen samples were prepared by plunging 5-mm 0.d. tubes of solution into liquid nitrogen. Some tube breakage was encountered; this was minimized by preirradiating the sample tubes. The radiation source geometry required that the tube length be about 10 mm. Therefore, several sample tubes were required for each nitrite analysis. All samples appeared to be polycrystalline. Irradiations were carried out in a liquid nitrogen filled dewar in a Cow y source a t a dose rate of 0.7 Mrad/hr to H20. Fricke dosimetry was used taking G(Fe3+) = 15.5, and decay corrections were made monthly. Room temperature irradiations were carried out in the dewar filled with water. Nitrite analysis was done by spectrophotometry according to Shinn4but modified as follows. The solution was placed in ice prior to addition of sulfanilamide solution after which it was warmed to 40" before the N-(1naphthy1)ethylenediamine dihydrochloride was added. Optical density readings were taken at 546 mk. Beer's law was verified over our analysis range.
is much lower and does not provide observable competition. Cation E$ect. No alkali cation effect on the nitrite yields would be expected in aqueous solutions. This has been verified for 0.5 M solutions.'j However, in frozen solutions a decided cation effect exists. The results are shown in Table I. The dose was 0.11 M a d . All results other than those in Table I are for sodium nitrate. When other salts were added to change the nature of the solution (e.g., pH) the sodium cation was kept common. A few potassium nitrate frozen solutions were also studied and seemed similar to sodium nitrate solutions with respect to concentration and pH except for a shift in the absolute G(N02-) values. Table I: Cation Effect on Nitrite Yields in Nitrate Ices Irradiated a t 77°K to 0.11 Mrad Solution
0.2 M 0.2 M 0.2 M 0.2 M
Results Comparison of Frozen and Aqueous Solutions. To establish a frame of reference a comparison of nitrite yields in frozen and aqueous solutions was made for 0.5 M NaN03 (pH 6) which was deoxygenated by purging with nitrogen. Frozen solutions were irradiated a t 77°K and aqueous solutions at 30" to 0.11 Rlrad. Triplicate samples gave G(NOz-)Bq/G(NO2-)f,o,,n = 1.39 i 0.05 with G(?;OZ-),~ = 2.1. Our aqueous results are in excellent agreement with those recently reported by Hyder for identical solutions.6 Hyder has reviewed earlier work on radiolysis of aqueous nitrate solutions and has discussed the consistencies and inconsistencies therein. For quantitative comparison, we will compare our frozen solution results with the latest aqueous solution results reported by Hyder. Effect of Deoxygenation. Solutions 0.5 M in KN03 were prepared in three ways: as made up, 0 2 saturated, and Nz purged. Six duplicates of each type of solution were irradiated to 0.11 Mrad a t 77°K. The NO2yields from all of the solutions were equal within the experimental deviation of 9%. We conclude that deoxygenation is unnecessary in frozen solutions. Accordingly, the results reported here refer to solutions as made up. This has also been found to be the case in studies on other frozen I n aqueous solutions, O2 saturation depresses the nitrite yield by about 50y0 a t 0.5 M nitratee5 This has been interpreted as competition between 0 2 and a nitrite precursor. In frozen solutions the effective 0 2 concentration The Journal of Physical Chemistry
G(NOz-)
LiNOa NaN03 KNO3 CsNOs
1.1
1.2 0.51 0.72
Concentration Efect. The nitrite yield as a function of nitrate concentration in frozen and aqueous solutions M is shown in Figure 1 and Table 11. Above 3 X nitrate, the nitrite yield increases rather rapidly. The data for aqueous solutions were taken partially from Hyder's Figure 1.6 Several runs of our Table 11: G(N02-) and G(NO2) in ?-Irradiated NaN03 Solutions Q(NOz-)," Nitrate concn, M
1.0 5 x 10-1 3 x 10-1 1 x 10-1 3 x 10-2 1 x 10-2 3 x 10-3 1 x 10-3 a
0.1 Mrad.
25'
aqueous soln
2.5c
...
2.25 2.OC 1.72 1.6' 1.49 1.4'
G(NOZ-),~ 77OK frozen soln
1.7 1.5 1.3 1.0 0.60 0.48 0.40 0.31
See ref 1, 0.04 Mrad.
C(NO*),~ 77OK frozen soln
0.23 0.21 0.20 0.16 0.13 0.096
...
0.032
See ref 9, Figure 1.
(4) M.B. Shinn, I d . Eng. Chem., Anal. Ed., 13, 33 (1941). (5) M. L. H y d e r , J. Phya. Chem., 69, 1858,4425 (1965). (6) H. A. Mahlman and G. K. Schweitaer, J. Inorg. Nucl. Chem., 5, 213 (1958).
RADIOLYSIS OF FROZENSOLUTIONS
2.4
-
2.1
-
1.8
-
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IN
z 0
-.I2
W
0.9
- .08
-
- .04
I
a3 10-2
~ x I O - ~
10-1
I
3 x 10-1
00 .00 1.0
C NaN031
Figure 1. NOz- and NOz yields us. solute concentration in r-irradiated NaNOs solutions: X, G(N02-1 in aqueous solutions ( 2 5 O , 0.1 Mrad); A, G(N02-) in frozen solutions (77OK, 0.1 Mrad); 0, '/zG(NOz-) in aqueous solutions ( 2 5 O , 0.03 Mrad) with 10-2 M added methano1;g 0, G(NO2) in frozen solutions (77"K, 0.04 Mrad).1
own were also made to obtain more points. Both the aqueous solution and frozen solution nitrite yields M. For show a change in slope a t about 3 X comparison, the trapped NO2 yield observed by epr' is also shown in Figure 1 but a different scale is used. The trapped NOz yield shows no significant change in to 1.0 M NaN03, although it does slope from show a change in slope at lower concentrations.1 p H Efect. The effect of pH over the range from 0 to 13 on the nitrite yields is displayed in Figure 2. Also displayed are previous results on aqueous solutions. Alkaline pH values were obtained with added NaOH to In acid NaN03 and acid pH values with added "03. solutions of pH
