2697
RADIOLYSKS OF AQUEOUS XETHANOL-SODIUM NITRATE: SOLUTIONS
The Radiolysis of Aqueous Methanol-Sodium Nitrate Solutions’
by J. T. Allan Radiation Research Laboratories, Mellon Institute, Pittsburgh, Pennsylvania (Received April 16, 1964)
Radiation chemical studies have been carried out on deaerated, aerated, and oxygenated aqueous methanol-sodium nitrate solutions. The reduction of nitrate to nitrite by hydrated electrons has been shown to proceed via the intermediate forniation of the radicalion X03-2. I n oxygenated solutions, the competition between 0 2 and Nos- for the hydrated electrons was investigated; the ratio of rate constants was found to be ke,,-+o,/ ke,d+No8- = 1.15 f 0.2. Nitrite ions were produced according to H+ NOS OH- and 2x02 HzO + 2H+ NOSSOS-, and G(NOZ-) = ‘/’2G(eaq-). In the deaerated system, nitrite yields were determined by conipetition reactions involving ?703-2, the organic radicals CHzOH, and hydrogen ions, the predominant process being + CHzO NOzOH-; thus G(??O,-) was approximately equal to CHzOH G(eaq-). The product yields have been accounted for on the basis of the initial production of two reducing species, eaq- and H, in the aqueous alcohol solutions. The yield of hydrated electrons in the bulk of a neutral solution was found to be G(e,,-) = 2.80 f 0.15 for both oxygenated and deaerated solutions.
+
+
+
+
+
Introduction Recent experimental evidence obtained from radiation chemical studies of dilute aqueous solutions has indicated that two fornis of reducing radicals are present in the irradiated solutions. The two species, which have widely different reactivities and/or modes of reaction with specific solutes’ have been identified with the hydrated electron and the hydrogen atom. The origiii and respective yields of these two reducing radicals were discussed in a prior communication dealing with the radiolysis of deaerated aqueous alcohol solutions containing NZO.z It was noted that, under neutral conditions of pH, the electron yield in the bulk of the solution was G(e,,-) = 2.80‘ f 0.10 and was independent of X20concentration up to -2 x M. The second reducing species was produced in a yield of G(H) = 0.60 rt 0.05. The present study is a continuation of the abovc work and was undertaken in order to determine the yields of the reducing radicals in aqueous alcohol solutioris containing oxygen. Nitrate was’ used in place of NZO since it seemed possible on the basis of a recent) investigations that the hydrated electron could react with NO3- in a manner analgous to the electron plus NzO reaction. Subsequent experiments showed that this
+
+
.--)-
+
was not the case. The radiation chemistry of aqueoui3 alcohol-nitrate solutions was therefore investigated in more detail.
Experimental Solutions were made up with triply distilled water, the pH being 5.9 rf: 0.2. Other pH values were obtained using either sulfuric acid or sodium hydroxide. Oxygen-saturated solutions were prepared by bubbling inedicinal oxygen through the solutions, the gas having first been filtered through a liquid nitrogen trap and washed with triply distilled water. Degassed samples were obtained by use of the standard freezing-pumping techniques. Irradiation vessels were trcated with a 1: 1 sulfuric-nitric acid mixture, washed and filled with triply distilled water, preirradiated, and rinsed thoroughly with triply distilled water prior to use. Baker Analyacd sodium nitrate and sodiuin nitrite and Fisher “certified” niethanol and 2-propanol reagents werie used without additional purification. Irradiations were carricd out with ‘j0Co y-rays or (1) This work was supported, in part, by the U. S. Atomic Energy Commission. (2) J. T. Allan and C. M. Beck, J . Am. Chem. Soc., 86, 1483 (1964). ( 3 ) A. Appleby, G. Scholes, and A I . Simio, ibid., 85, 3891 (1963).
Volume 68, Number 9
September, 196.4
J. T. ALLAX
2698
with 2.5-Mev. electrons from a Van de Graaff accelerator. The dose rates of the cobalt-GO sources (3.25 X 1 O I 6 and 18.7 X 10l6e.v. g.-I min.-') were determined by ferrous ion oxidation in an air-saturated 0.4 X H2S04-10-3 M F e S 0 ~ 1 0 - ~ M NaCl solution taking G(FelI1) = 15.5. Sitrite mas determined spectrophotometrically after reaction with sulfanilic acid and a-naphthylamine hydrochloride. The latter compound was decolorized prior to making up the reagenL4 Hydrogen peroxide was determined by either the titanium sulfate5 and/or the iodide technique.6 Formaldehyde was quantitatively determined as its 2,4-dinitrophenylhydrazorie according to the method described by Johnson and Scholes.' The red color which results after treatment with alkali was measured a t 4300 8. on a Cary recording spectrophotometer (Model 14), which enabled the optical density of the solution to be determined at the time of mixing. Acetone was estimated by the method developed by Berntsson.s Gaseous products were collected by means of a Toepler pump after passing through two traps cooled to -78 and -196", respectively. Analysis of the samples was then carried out by mass spectrometry.
Results Deaerated, aerated, and oxygen-saturated (1 atm.) aqueous solutions containing an aliphatic alcohol (methanol or 2-propanol) were irradiated in the presence of sodium nitrate. Unless otherwise stated, the product yields were a linear function of the radiation dose. Doses of up to 2 x 10I5 e.v./g. were normally employed, the range being reduced by a factor of ten for solutions containing less than 116 oxygen or nitrate. G values were obtained from yield-dose dependencies representing a t least four determinations. Rolutions Containing 02. The products which were
HCHO NO2
H2Oz
0L-j-J
,&.''*
, 10-3
I
10-2
N03- Concent ration
I
I
lo-'
4
(MI
Figure 1. Dependence of the yields of NO*-, HzOz, and J4 C H 2 0 on sodium nitrate concentration in oxygenated CH30H-NaXO3 solutions irradiated a t neutral p H : 0 , 10+ M 2-propanol-NaSOs solutions.
The Journal of Physical Chemistry'
determined in the y-irradiated CH30H-P\TaXO3-O2 system were formaldehyde, hydrogen peroxide, and sodium nitrite. S o dependence of the product yields on dose rate was observed for neutral oxygenated 10-2 31 CI'I3OH-KaSO3 solutions irradiated with yrays or 2.5-Mev. electrons. Figure 1 shows the dependence of the yields of nitrite, hydrogen peroxide, and formaldehyde as a function of M methanol KaXOs concentration in 02-saturated solutions irradiated at neutral pH. Replacing methanol by 2-propanol produced no marked change in G(X0, -) . Formaldehyde yields were constant within the limits of evperimental error; G(HCH0) = 2.95 f 0.15. The dependencies of the nitrite and hydrogen peroxide yields on methanol concentration and on pH are summarized in Table I.
Table I : Dependence of G(NOz-) and G(H202) on pH and Methanol Concentration in 02-saturated NaN03 Solutions NOaconcn , PH
,M
Neutral Seutral Xeutral Seutral 3 50 -13
lo-' lo-' lo-' 2 X 10-2 lo-' 2 x 10-2
CHaOH concn , M
lo-' 5 X lo-'
10-1
G(NOz-)
2 2 2 1 2 0
05fO 15 30 45 i 0 20 f 0 75 f O
G(Hz0z)
1
1 73fO 1 1 80
05 1 05
2 00 1 80 2 30
Figure 2 gives the dependence of the nitrite and hydrogen peroxide yields as a function of nitrate concentration in aerated (25") loF2M methanol solutions a t neutral pH. The NO2- yiclds are considerably greater than those obtained for the corresponding 02-saturated solutions. However, both G(XOz-) dependencies exhibit the plateau in the region of 2 X loL2M NaT\TOI and the subsequent increase at higher nitrate concentrations. The yields of hydrogen peroxide in oxygenated and aerated 10-2 174 methanol solutions were equal, Q(H202) = 3.4 i 0.1. Deaeiated Solutions. The products determined in deaerated CH30H-Na?J03 solutions were formal(4) F. D. Snell and C. T. Snell, "Colorimetric Methods of Analysis," D. Van Nostrand Co., New York, N. Y., 1961, Chapter 11, p. 802. (5) G. M. Eisenberg, Ind. Eng. Chem., Anal. Ed., 15, 327 (1943). (6) A. 0. Allen, C. J. Hochanadel, J. A. Ghormley, and T. W. Davis, J . Phys. Chem., 56, 575 (1952). (7) G. R . A. Johnson and G. Scholes, Analyst, 7 9 , 217 (1954). (8) S. Berntsson, Anal. Chem., 2 8 , 1337 (1956).
2699
RADIOLYSIS OF AQUEOUSMETHANOL-SODIUM NITRATESOLUTIONS
Table I11 : HZand HzOz Yields in Deaerated 10-2 M CHaOH-NaN03 Solutions a t Neutral pH Nitrate concn.. Radiation
0 10-s 2X 2 x 10-1 10-3
BOCo ?-rays 60Co ?-rays
, O
L
10-~ 10-2 NO3- Concentrotion ( M I
W 10- .
I
I
io-’
1
Figure 2. Dependence of the yields of Not- and HZOZon sodium nitrate concentration in aerated l o + M CHaOHN a N 0 3 solutions irradiated with WCo 7-rays a t neutral pH [dose rate 3.25 X 10l6e.v. g.-I rnin.-l].
G(H8)
M
COCO ?-rays @“Io7-rays 2.5-Mev. electrons ( 0 . 5 x 10-6 amp.)
G(Hz0n)
0.92 f 0.05 0.90 0.50 f 0 . 0 5 0.68 0.60 0.40 0.72 0.90 0.61
Table IV: Dependence of G(N02-) on pH in Deaerated 10-I M CHaOH-IO-’ A4 NaNO, Solutions
dehyde, sodium nitrite, hydrogen, and hydrogen peroxide. The results compiled in Table I1 demonstrate the dependencies of G(N02-) and G(CH20) on the methanol and sodium nitrate concentrations in deaerated CH30H-STaN03 solutions a t neutral pH.
3.50 5.80 10.65 -13
2.80 3.30 3.48 3.75
Discussion Table 11: Dependence of G(N02-) and of G(CH20) on the Nitrate and Methanol Concentrations in Deaerated CH80H-NaN03 Solutions a t Neutral pH CHsOH concn..
NaNOa concn., M
1M
10-2 10-2 10-2 10-2 10-2 10-2 10-2 10-2 10-2 10-1 10-1 lo-’
4 2 5 2 4 2 2
x
10-4 10-3
x x
10-3
10-8 10-2 x 10-2 x 10-2 10-1 x 10-1 10-3 x 10-2 10-2
2.5-Mev. electrons (0.5 X
G(NOa-)
2.30 f 0 . 1 2.40 2.40 2.60 2.73 2.80 3.02 3.25 3.45 2.50 2.85 2.60”
G(CHe0)
... 2.50 i0 . 1 5 2.50
... 2.60
... 2.73
... 2.80
...
Although the radiation chemistry of dilute aqueous nitrate solutions has been studied quite extensively, the mechanism of the free radical reactions is far from clear. Rakh, et aL9 found the niajor reaction products in X-irradiated neutral oxygen-free sodiuin nitrate solutions to be nitrite, hydrogen peroxide, hydrogen, and oxygen. Yields of G(902-) = 1.24, G(H202) = 1.60, G(H2) = 0.5, and G(O2) = 0.07 were reported for 0.1 M NaN03 solutions. On increasing the nitrate concentration, G(N02-) increased and G(H2) decreased in a manner which indicated that the production of nitrite resulted from a primary reaction of “H” atonis wit8h nitrate
2.85
“H”
...
amp.).
--+
NO2
+ OH-
(1)
the NO2 then giving nitrite via 2x02
Substituting 2-propanol for methanol in solutions containing A4 NaN03 did not influence the observed nitrite yield. The hydrogen and hydrogen peroxide yields were determined for loW2M CH8OH-NaNO3 solutions a t various nitrate concentrations. The results are given in Table 111. The effect of pH on the nitrite yields in deaerated lo-’ Af CH3OH-10-’ M NaN03 solutions is illustrated by the data in Table IV.
+ NO3-
+ H20 --+2H+ + NOz- + NO,-
(2)
in a yield approximately equal to 1/2G(“H’’). According to current ideas, G(“H”) is composed largely of the electron yield, which implies therefore that reaction 1 is predominantly eaq-
+ SO3-
--+
NO2
+ 20H-
(3)
(9) N. A. Bakh, V. I. Medvedovskii, A. A. Revina, and V. D. Bityukov, Proceedings of 1st All-Union Conference on Radiation Chemistry, Moscow, 1957,Consultants Bureau, New York, N. Y . , 1959, p. 39.
Volume 68, Number 9
September, lo64
2700
J. T. ALLAN
Recently, Appleby, et aLJ3in a study of the y-radiolysis of deaerated aqueous 2-propanol-nitrate solutions measured nitrite yields of G(NOz-) = 2.52 and G(N02-) = 2.85 a t sodium nitrate concentrations of 1 0 - ~N and lop2?.I, respectively. These results show that the reaction of hydrated electrons vith sodium nitrate leads to an equivalent yield of nitrite, possibly according to ea,-
+ SOa- -+KOz- + OH- + OH
(4)
Such a process would be analogous to the reaction of electrons with nitrous oxide and would likewise predict a marked increase in the organic product yield in
eaq-
+ X20 -z
+
Nz OH-
+ OH
-
+ r\'O3-'
+
(CHa)&O
+
1 0 2 -
+ OH-
(6)
has been tentatively identified The radical ion with one of the individual electron paramagnetic resonance patterns obtained in e.p.r. studies of the radical species produced in irradiated K S 0 3crystals. l 1 The above mechanism may be verified by measuring the formaldehyde yields produced in deaerated neutral methanol- sodium nitrate solutions. I n methanol solutions containing no added nitrate G(HCH0) 0.3, since the predominant reaction of the organic free radicals is dimerization to ethylene glycol1oand not disproportionation to a carbonyl product and the parent alcohol as in the case of the organic radicals formed from 2-propanol. Accordingly therefore, a reaction analogoiis to (6) would predict a marked increase in G(HCH0) for methanol solutions containing nitrate.
-
The Journal of Physical Chemistry
HzO -,-+
(5)
going from the 2-propanol to the 2-propanol-nitrate system. 2,10 The latter system was reinvestigated in this laboratory. The nitrite yields obtained by Appleby, et al., were confirmed and the acetone yield in l o + M 2-propan01-10-~ M sodium nitrate solutions was found to be G(acetone) 3, in good agreement with the value obtained for 2-propanolM 9 , O solutions.2 Preliminary data (Fig. 1) on the nitrite yields in the 2-propanol-NaN03-02 system showed, however, that G(SO2-) was approximately equal to l/zG(e,,-). These observations were thus in accord with those made earlier by Bakh, et al. ,4 further consideration of the mechanisms which could account for these apparently contradictory observations suggested that, in the deaerated system, reaction of the organic free radicals with some intermediate nitrate species could lead to the same stoichiometry as that predicted by reaction 4, e.g. (CH3)zC-OH
That such is the case is evident from the results given in Table 11. In most of the subsequent investigations methanol was used as the OH radical scavenger. Solutions Containing Oxygen. The results presented in Fig. 1 have been interpreted on the basis of two reaction mechanisms, one of which is prevalent throughout the entire nitrate concentration range and the second of which beccmes important only at nitrate concentrations greater than -2 X M. For neutral oxygen-saturated solutions containing 2 X lo-' 111 i\-aS03or less, the reaction products may be accounted for in terms of the reactions ea,-,
H , OH, H202,H2
+ CHsOH --+ CHZOH + H20 CH2OH + --+ OzCHzOH H + --+ HO2 S H + + 202CHzOH +2CH20 + H202 + OH
0 2
0 2
02-
0 2
2OzCHzOH -+CHzO OzCHtOH
+ HCOOH + HzO +
+ HOz(Oz-) + CH2O
+ 0%
esq-
+ HzO2 +
(8) (9)
(10) (11)
0 2
(12)
0 2
(13)
(14)
+0 2 -
+ YO3- +S03-2 + H + + NO2 + OH2N02 + H20 2H+ + + Noseaq-
---j
(7)
5 0 2 -
(15) (16) (17)
Reaction 12 has been introduced in order to account for the small amounts of formic acid produced in oxygenated aqueous methanol solutions12 and for the lack of stoichiometrical balance in the system. Thus the yield of hydrogen peroxide in CHaOH-02 solutions should be G(Hz02) = GM(Hz02)
+ '/z[G(OH) + G(e,,-)
+ G(H)] = 3.70 *
The measured yield, G(H2O2) = 3.4 0.1, indicates that approxiniately G = 0.4 of the OH radicals are accounted for by reactions 8, 9, and 12. S o attempt was made to measure the formic acid yields. The increase in G(N02-) and the decrease in G(H202) with increasing nitrate concentration is largely deter~~~
(10) G. Scholes, M. Simic, and J. J. Weiss, 36th Annual Discussions of the Faraday Society, University of Notre Dame, September, 1963. (11) J. Cunningham, J . Phys. Chem., 66, 779 (1962). (12) J. Lyon. Ph.D. Thesis, University of Durham, England, 1960.
RADIOLYSIS O F AQUEOUSIIETHANOL-SODIUM -.
-
mined by the competition between reactions 14 and 15. In addition, there will be a slight contribution t o the nitrite yield a t concentrations greater than 2 X A', NaN03 as a result of scavenging of those electrons in the spurs which normally contribute to the molecular hydrogen yield1 uta esq-
+ eaq-
--+
BZ
270 1
S I T R A T E SOLUTIONS
(18)
This contribution, which aniounts to a maximum of G(NOz-) = 0.2 a t XaX03concentrations >2 X lop2X , may be calculstted from the nitrite and hydrogen yields obtained for deaerated CH30H--?;aN03 solutions (Tables I1 and IIL). I n terms of reaction kinetics the yield of nitrite is then
H
+ NO,-
--j-
XOz
+ OH-
(19)
I n aerated solutions the measured nitrite and hydrogen peroxide yields (Fig. 2) are inexplicable on the basis of reactions 7 to 17 alone. Nitrite yields were greater and HzOz yields lower than predicted and deviations from the calculated values increased with increasing nitrate concenlxation. These observations may be accounted for in ternis of reactions 7 t o I7 together with CHzOH
+ SOa-'
--+-
CHZO
+ SOz- + OH-
(20)
which will conipete with reactions 9 and 16. Values of AG(N02-) and aG(HZO2)due to reaction 20 should thus be equal and will reach a maximum when all the hydrated electrons react via reaction 1.5. From Fig. 1 and 2 , AG(XOz-),., = 0.45 f 0.10 and AG(H~O~),,, = -0.30 0.10. If reaction 20 made any significant contribution to the nitrite yields in oxygen-saturated solutions, G (SOZ-) would exhibit a hydrogen ion concentration dependence. Under conditions such that scavenging of electrons by hydrogen ions did not occur, i . e . , 10-1 M NaX03, identical nitrite yields were obtained for solutions of PI-I3.50 and pH -6 (Table I). The reactions of the radicals in solutions containing 0.1 M KaOH, i . e . , under conditions where reaction 16 is effectively eliminated, are not clear. I n oxygenated 2 X l o F 2M NaN03-10-2 A// CHsOH solutions, G(S02-) decreased to 0.75 and G(H202)increased by 0.30 on addition of the S a O H (Table I). Substituting 2-propanol for methanol in the solutions gave G(NOt-) = 0.50 and an increase of G = 0.30 in the yields of acetone and peroxide. These results are best explained on the basis of the reaction
*
where X is the contribution to G(1\;02-) from scavenging of reaction 18 and G(e,,-) represents the yield of hydrated elecirons in the bulk of the solution. Rearranging the (aboveexpression we obtain
The nitrite yields shown in Fig. 1 nere plotted accorcling to eq. B 011 the assumption that oxygen concentration was a constant factor in these experiments. A linear relationship was obtained which yielded a value of kld/klj = 1.15 -f 0.2 and a value for the yield of electrons in the bulk of the solution of G(ea,-) = 2.80 + 0.15. The calculated decrease, from G(Hz02) = 3.40, in the yield of hydrogen peroxide as a function of nitrate concentration was determined for solutions containing M !VaN03 using the expression up to
NO,-'
+ CHSOH + NOz-
The calculated yields of G(HZOz), Fig. 1 (dotted line), agree well with the experiniental data. At higher nitrate concentrations the peroxide yields fall below the limiting value of G(H202) = 2.0 predicted by the above mechanism. 'Thus G(H202) = 1.7 f. 0.1 in the range 10-1 to 5 X 10 -I JI NaS03. This additional loss of HzOzmay be associated with reaction 12 since a t the higher nitrate concentrations reaction 13 is virtually eliminated. Any contribution from competition of reaction 19 with reaction 10 will be negligible since klo/lclg = 2000.1a
+ CHzOH + OH-
(21)
the nitrite yield being lower in the case of the predominantly ionized 2-propanol. This reaction may also contribute to the observed dependence of G(K02-) on methanol concentration a t pH 5.9 (Table I), Reaction 21 accounts for G(N03-2) 0.7 in the alkaline solutions; thus approxiniately G(X'03+) = 2.6 f 0.2 of the radicals participate in reactions leading to products other than nitrite. I n solutions containing NaN03 in concentrations :>2 X 10+ M , additional nitrite yields were obtained by a mechanism other than the one just described. This
-
(13) A. 0. Allen, Conference on Radiation Chemistry, Gatlinberg, Tenn., M a y , 1963.
Volume 68, .%'umber 9
September, 1964
J. T. ALLAN
2702
mechanism has not been deduced. The following information is, however, relevant to the interpretation : (a) the increase in*G(K02-) was unaffected by either the presence or absence of oxygen (Table V j or by increasing the concentrations of inethanol and hydrogen ions (Table I) ; (b) there were no net changes in either G(HCH0) or G(HzOz)comparable to the increase in G(N02-) ; (c) the magnitude of the effect at any specific nitrate concentration was essentially the same in the neutral CH30H-NaK03systems as in the Ce4+-n'aN03HZSO4and Ce4+-NaX03-T1 + -HzS04systems studied by RIahlman (Table V).14 Table V :
Increase in G(N02-) from Mechanism 2 Ce4 *NaNOa-
--- C H ~ O H - N ~ N Osystems-S
NaKOa concn.,
Oxygen-
Evacu-
&SO&
1M
ated
Aerated
ated
systems
10-1
0.45 f. 0.10 0.80 1.05
0.55 i: 0.10 0.80
0.45 f 0.10 0.70
0 . 4 0 f. 0.10 0 . 70 1.05
2 X lo-' 5 x 10-1
The above criteria appear to be incompatible with the suggestion made by Challenger and I\Iasters15 and by Kustin'e that the mechanism involves OH radical scavenging by nitrate ions. The latter author has interpreted the data obtained with the Ce4+-SaS03 systems by proposing the competition
+ OH --+Ce4+ + OHNos- + OH NO3 + OHCe3+
--+-
n'os
--+-
NOz f 0 . ~ 5 0 ~
(22)
(23) (24)
I n the CH30H-XaK03-02 system an analogous competition between methanol and nitrate ion for OH radicals would result in a decrease of the formaldehyde and hydrogen peroxide yields equal to 2AG(N02-) and AG ( S O ? - ) ,respectively. Deaerated C H 3 0 H - N a N 0 3 Solutions. In the absence of 0 2 , the hydrated electrons will react solely with nitrate ions according to reaction 15 and the OH and H radicals will yield CHZOHradicals via reactions 8 and 25, respectively.
H
+ CH3OH +CHZOH + Hz
(25)
The nitrite ion and formaldehyde yields in solutions containing less than -2 X X NaN03 (Table 11) are then attributed to the competing processes 2CHzOH ---j. (CHZOH), 2CHtOH +CHZO The Journal of Physical Chemistry
+ CH3OH
(26)
(27)
CHZOH
+ N03-'
+
+ NO2- + OH+ H + --+ NO2 + OHCHzO
Nos-'
(20) (16)
An alternate mechanism based on reaction 4 would predict : (a) nitrite yields independent of the presence or absence of 0 2 , and (b) a yield of forinaldPhyde in the deaerated CH30H-SaS03 system of G(CH20) 0.6, both of which are contrary to the experimental observations. It is apparent from the above inechanisni that the nitrite yields should be dependent upon dose rate and hydrogen ion concentration. The dose rate effect in neutral Ji NaN03 solutions was found to be negligible (Table 11). However, this is not surprising in view of the fact that reaction 20 accounts for the ions. The hydrogen ion major proportion of the S03c2 concecrration dependence is clearly demonstrated by the results compiled in Table IV; thus lowering the pH favored reaction 16 and resulted in lower nitrite yields. The presence of lo-' 111 n'aN03 in these solutions ensured that no scavenging of eaq- by H30 + ions occurred at the lorn pH. The increase in G(SO2-j between 2 X and 2 x M ;C'aS03results from the scavenging of reaction 18 by nitrate ions. Thus G(A-Oz-) increased by -0.4 and the-molecular hydrogen yield decreased by G(H2) 0.2. This effect was also observed in the 2propanol-r\'*O system over the same solute concentration range.2 The further reduction in G(Hz) with increasing nitrate concentration (Table 111) suggests that nitrate ions can compete with methanol for the hydrogen atoms. The ratio of rate constants is calculated to be k H + C H 8 0 H , kH+NOs10 ( c j . ref. 3). 144 The nitrite ion and formaldehyde yields iii CH30H-10-3 M NaN03 solutions may be accounted for on the basis of the initial production of G(ek,-) = 2.80, G(H) = 0.45, GM(Hz) = 0.45, GM(Hz0z) = 0.6, and G(0H) = 2.93. The C€IzOH and N03-2 radicals are then formed in yields of G(CH20H) = 3.40 and G(N03-2j = 2.80 and react according to
-
-
-
2CHzOH CHzOH
+
G ?j03-'
G
= 0.6
G(CHzO)
=
0.1
= 22
___f
G(CHzO) = 2.2 and G(n'Oz-) = 2.2 (14) H. A. Alahlman, J . Phgs. Chem., 64, 1598 (1960); 67, 1466 (1963). (15) G. E. Challenger and B. J. Masters, J . Am. Chem. Soc., 77, 1063 (1955). (16) K . Kustin, Ph.D. Thesis, University of Minnesota, 1959.
RADIOLYSIS OF AQUEOUS METHANOL-SODIUM NITRATESOLUTIONS
2703
Conclusion The dose-rate dependence of GM(H202) shown in Table I11 indicates that there is also a contribution, G 0.1, to the formaldehyde yield from the chain process
-
CHzOH
+ HzO2
-
CH2O
+ HtO + OH
(28)
The combined yields, G(CHz0) = 2.40 and G(NO2-) = 2.50, are in good agreement with the experimental data. For 2 X low2A4 Nan'03 solutions where G(e,;-j = 3.20 and GM(H2)= 0.25 the above stoichiometrical interpretation predicts values of G =: 2.7 f 0.1 and G = 2.85 f 0.1 for the yields of formaldehyde and nitrite ion, respectively (cf. Table 11).
The reduction of nitrate to nitrite by radiationproduced electrons involves the intermediate forrnation of the species N03-2. In the presence of hydrogen ions N03-2reacts to give NO2,either directly by reaction 16 or possibly through the formation of a second intermediate NOSH-. However, with organic free radicals such as CH20Hand (CH&COH, N03-2yield~NOz-. The value for the yield of hydrated electrons in thie bulk of a dilute aqueous solution (pH 5-7) was found to be (?(e,,-) = 2.80 f 0.15 for the O2-H2O2l7and &propanol-N20 systeim2 This result is confirmed by the present studies of aqueous alcohol solutions containing 0 2 and/or NaN03 in concentrations of A4 or greater. (17) G. Czapski and A. 0.Allen, J. Phvs. Chem., 66, 262 (1962).
Volume 68, Nzimber 9 September, 1964
