JOURNAL O F T H E A M E R I C A N C H E M I C A L SOCIETY Regislered i n U.S. Paten1 Ofice.
@ Copyright,
i970,by the American Chemical Socicfy
VOLUME92, NUMBER 20
OCTOBER 7, 1970
Physical and Inorganic Chemistry Absorption Spectra and Reaction Kinetics of NO,, N20,, and N20, in Aqueous Solution A. Treinin’ and E. Hayon Contribution from the Pioneering Research Laboratory, U.S. Army Natick Laboratories, Natick, Massachusetts 01760. Received January 19, 1970 Abstract: The flash photolysis of NOz- ions in aqueous solution gives rise to three transient absorptions assigned to NO, (A, -400 nm), N2O4(A, -340 nm), and NzOa (A, 12 gave rise to a transient absorption with a maximum at 430 nm, identical with that of 03-radicals; see Figure 1. Up to 3 pmol of 0,- could be produced by one flash. In the presence of Br- or C0,2- ions, the photolysis of NO,- ions showed the presence of the transient spectra of Brz- and COS- radicals (Figure 1). These results suggest that the primary photolytic mechanism of nitrite ions produces 0- radicals, according to
R O N 0 +RO
NO
(2)
The radicals Bra- and COS- are formed by the reactions Br- + O H +
was suggested’o to be the primary process. It is analogous to the primary photochemical process in organic nitrites. l 1 hv
hu ---t
+ 00- + HzO $O H + OHNOz-
+ NO
Br
+
+ Br-
Br.
*
+ OHBrz-
cos2-+ O H +COa-
+ OH-
The decay rate of ozonide radicals obeyed the law
However, no evidence has been presented for the occurrence of a similar reaction in NO,-. Recent flash photolysis studies on S2032--,12HO.2 , l 3 NO 3-, l 4 and the oxyhalogen ions’j have shown that the 0- type of dissociation is a common process for excited oxyanions which can readily undergo bond rupture. In the case of H N 0 3 , photolytic dissociation to OH
where
+
(7) J. H . Baxendale, Radiaf. Res., 17, 312 (1962). (8) L. Dogliotti and E. Hayon, J . P h y s . Chem., 71, 2511, (1967); M. Daniels, ibid., 73, 3710 (1969). (9) M. Holmes, J . Chem. SOC.,1898 (1926). (10) R . J. Knight and H . C. Sutton, Trans, Faraday SOC.,63, 2623 (1967). ( 1 1 ) J . G . Calvert and J. N. Pitts, “Photochemistry,” Wiley, New York, N. Y . , 1967, p 480. (12) L. Dogliotti and E. Hayon, I . P h y s . Chem., 72, 1800 (1968). (13) D . Behar and G . Czapski, Israel J . Chem., 6 , 4 3 (1968). (14) U . Shuali, M. Ottolenghi, J. Rabani, andZ. Yelin,J.Phys. Chem., 73. 3445 (1969): M . Daniels, R . V. Meyers, and E. V. Belardo, ibid., 72, 389 (1968). (15) 0. Amichai, G . Czapski, and A. Treinin, Israel J. Chem., 7, 351 (1969); 0 . Amichai and A . Treinin, J . P h y s . Chem., 74, 830 (1970).
Journal of the American Chemical Society
p is a constant and q is a function of pH. The dependence of k o on [NOr-] and pH is shown in Figure 2. The same law was found to be applicable to the decay of 0 3 - produced from HOr- l 3 and the oxybromine and oxyiodine systems. l j This suggests a similar mechanism for the generation and decay of 03-,reactions 1-6 (16) E. Hayon and E. Saito, J. Chem. PhjJS.,43, 4314 (1965). (17) (a) L. Dogliotti and E. Hayon, J . Phys. Chem., 71, 2511 (1967); M . Langmuir and E. Hayon, ibid., 71, 3808 (1967); (b) M . Sirnic, P. Neta, and E. Hayon, ibid., 73, 3794 (1969); E. Hayon, J . Chem. Phys., 51, 4881 (1969).
/ 92:20 1 October 7, 1970
5823 I
I
3b0
I 350
I
1
IO
v
I
I
00
20
I 60
40
V@H-~, ~
-
O'
I
I
I
400
450
X , nm
80
1
3x10-4Y N O i , 2 5 * C
5
X,nm
I/[NOi]
+
,IO'M-'
Figure 2. Dependence of the first-order decay of 03-' upon OHand NOz- concentrations (see eq 7).
(4)
(7)
with
where K, = [H+][OH-] and KOH= [H+][O-]/[OH]. From the parameters of the lines in Figure 2 we could calculate ke = (5.5 f 0.5) X l o 3 sec-' (in good agreement with previous datai3I15),k3/k5 = 4.0 0.4, and k4/k3 40. Taking k5 = 2.5 X lo9 M-' sec-*,18 the values k3 = (1.0 f 0.1) X 10'0 M-' sec-' and k4 2.5 X lo8 M-' sec-' were derived, which are close to those recently reported.lg Equation 7 is based on the assumption that equilibrium 2 is established. In all our experiments, k3[N02-] and k4e [NO,-] were lower than 5 X lo6 and lo5 sec-l, respectively, whereas kz[H20] and k-,[OH-] were at least one order of magnitude higher.IQ This assumption is thus justified. B. Identification of NOz. According to the above mechanism, equimolar amounts of NO and NOz should be produced by the photolysis and, as in the gas phase,
*
-
Figure 3. Absorption spectra of transients produced on flash photolysis of NOz-, pH 6-8, at different temperatures and [NOt-], O D read 150 psec after start of flash: dashed curves, spectra corrected for depletion of NO2- (see text); insert, spectrum produced 1.7 X M K N O Z ,25",OD onflash photolysis ofO.1 M K N 0 3 read at 200 psec after start of flash.
-
( 1 8 ) M. Anbar and P.Neta, Int. .I. Appl, Radiat. Isotop., 18,493 (1967). (19) G. V. Buxton, Trans. Faraday Soc., 65, 2150 (1969).
Treinen, Hayon
they are expected to be in equilibria with N z 0 4 and Nz03. The absorption of N O in water is significant only below -220 nm, where NO,- absorbs strongly, but the other three nitrogen oxides absorb at longer wavelengths.6 For their identification, the formation of 0,- was avoided by using air-free solutions, or air-saturated solutions at pH below 10. Figure 3 shows some transient spectra produced under these conditions. On flash photolysis of 3 X M NOz- at 25", the spectrum displays three distinct bands: two weak bands appearing as "shoulders" at -400 nm (band A) and at -340 nm (band B) and the ascending branch of a relatively intense band (band C) at shorter wavelengths. The three bands decay within the millisecond range (see section HI), and at the end of the decay no depletion of NOscould be detected even at 230 nm, where cN02- = 2400 M-' cm-'. The latter result (which was obtained with 2 X 10-5 M NOz-) confirms that there is no net photolysis of NOn-, despite the high efficiency of its primary decomposition. Neither NzO nor O2 had any distinct effect on the transient spectrum, which indicates (a) that solvated electrons do not play a significant role in the photolysis (the effect of N 2 0 was tried with [Nz0]/[NO2-] -100); (b) that the reaction of NO with 0, in solution is slow relative to other processes in this system which involve NO. In general, a decrease in the amount of transients (either by changing their initial yield or their decay processes) affects bands B and C more than band A. Thus, when the intensity of band A is doubled, that of C increases by a factor of -4 (Figure 3, curves a and b). It was difficult to examine the effect on band B, a more detailed study of which was conducted by pulse radiolysis (see section 11). This
/ Absorption Spectra and Reaction Kinetics of NOs, N 2 0 r ,and N204
5824
explains our observation that the absorbance values measured within bands B and C were considerably scattered. This and the low intensity of band A made it necessary to average the results of several measurements, usually more than three. The spectrum of NO, in aqueous solution displays a broad band peaking at -400 nm, with E -200 M-l cm-l . 3-5 The assignment of band A to NO, is thus suggested and further supported by the following results. (a) The intensities of bands B and C were found to decrease on raising the temperature of the solution, and at 76” only band A was detected after the flash (Figure 3, curve c). The dashed curve shows the spectrum corrected for the depletion of NOs-; the correction was based on the assumption that 2 mol of NOz- is consumed for each mole of NO, produced (eq 1-3) and on the known spectrum of NO,-. The spectrum of NO,- measured by us was identical with that reported in the literature. This correction is significant near 360 nm, where NO2- has an absorption maximum. (Figure 3 includes such corrections for curves a and b, but here the conversion of NO, to N 2 0 , was also taken into account.) Band A appears to undergo temperature broadening. It also intensifies on warming, but this is not shown in Figure 3, where spectrum a represents the average of eight measurements. In the gas phase the degree of dissociation of N z 0 4 and N z 0 3 markedly increases with temperature (the heats of dissociation are 14.6 and 10.3 kcal, respectivelyz0); therefore at 76” the spectra of N,O, and N;03 should be practically quenched and that of NO, intensified. (b) N O a was also generated by flash photolysis of the NO3NO,- mixture. The concentrations of NO3- and NOz- were so adjusted that in effect all the light was absorbed by NO3-, and all the OH radicals produced l 4 by the reaction
+
hY
NOS- +NO2
+ 0-
(8)
were scavenged by NOz-, reaction 3. The insert in Figure 3 shows the spectrum produced by flashing a solution of 0.1 M NO31.7 x M NO?-. It was corrected for NO2- depletion: 1 mol of NO,is consumed for 2 mol of NO, produced. (This is a small correction and need be considered only near 360 nm.) In addition to the transient absorption, a permanent absorption peaking near 360 nm was observed after the flash; it is due to NOz- generated from NO3- by other14 processes. The transient absorption includes a contribution from N 2 0 4(1.7 pmol of NOr was produced by the flash; Le., [N204]/[N02] -0.1; see section 11). Still the spectra produced by methods a and b are rather similar. In particular, both differ from the spectrum of NO2 previously reported4 (and resemble more closely the gas-phase spectrum2I) by showing a smaller absorption below 300 nni. We believe that in the pulse work other transients contribute to the short-wavelength absorption (see section 11). (c) The assignment of band A to NOs leads to a 1 : 1 ratio for the yields of NO2 and 0- (or OH), in agree-
+
(20) M. C. Sneed and R . C. Brasted, “Comprehensive Inorganic Chemistry,” Vol. 5, Van Nostrand, Princeton, N. J . , 1956. (21) T.C. Hall and F. E. Blacket, J . Chem. Phj~s.,20, 1745 (1952).
Journal of the American Chemical Society
1 92:20
ment with the proposed mechanism (eq 1-4). The initial yield of NO, was calculated by extrapolating the maximum absorbance of A to zero time and cor= recting for the conversion of NO2 to Ns04,using 200 M-l cm-l and Kdis,(Nz04) = 1.3 X 10-5 M (section 11). No correction for N,O, was introduced. Such correction may increase the yields of NOz by -10% (section 1.C). Under the same conditions, the initial yield of 0- was determined from the yields of 03-,Brz-, or C03- produced when all the 0- (or OH) radicals are scavenged by 02,Br-, or CO,”, respectively. All the absorbance values were extrapolated to zero time. The values of emax used (in units of M-’ cm-l) were 1.9 X lo3, 7.8 X lo3, and 1.86 X l o 3for 0,-, Brs-, and C03-, respectively.22 The three scavengers led to the following values for [NOz]/ [0-1: 1.1 (from 0,-), 0.9 (from Br,-), and 1.5 (from cos-). (d) In neutral solution the decay kinetics of transient A closely resembles that of NOz (see section 111). In the presence of ethanol band A was not produced. With Br- the, suppression of C could be shown, but the intense Br,- absorption prevented reaching any conclusions about the other two bands (the decay of Br,- was accompanied by the buildup of a weak absorption below 300 nm, which may be due to NOBrz3). With C 0 3 , - no transient absorption was detected below 450 nm, but this could also be interpreted as a pH effect (see section 1II.C) since the pH of the solution was 11.1. Altogether, these results suggest that OH radicals are the precursors of the transients which are responsible for bands A, B, and C (Figure 3). The generation of NOz (band A) was also achieved by confining the absorption of light from the flash to the 360-nm band of NOs-. To show this, 0.1 M NO,- solution was flashed with 0.1 M phthalic acid as a filter (cutoff at -300 nm). This filter led to a considerable reduction in the yield of NOz. This and the fact that NOn (and the other two transients) was produced from very dilute NO,- solutions (-2 X 10-5 M ) suggest the absence of a wavelength effect on the mechanism of the photolysis of NOz- at X 2 2 2 0 nm. C. The Identification of Nz03. The effect of NO on the photolysis of NO,- was studied. The spectrum M Nos1.9 X obtained on flashing 9.5 X M NO is shown in Figure 4. Band A has apparently vanished and the spectrum now consists of a single band which resembles band C (Figure 3). Thus NO appears to convert transient A to transient C. Keeping the NO concentration constant, the intensity of this single band was found to increase with [Nos-], tending to some limiting value. The set of reactions which occurs in this system most likely includes reactions 1 and 2, with reaction 3 competing with the reaction
+
OH
+ NO +HONO
(9)
(22) (a) W. D. Felix, B. L. Gall, and L. M. Dorfman, J . Phys. Chem., 71, 384 (1967); (b) M. S . Matheson, W. A , Mulac, J. L. Weeks, and J. Rabani, ibid., 70, 2092 (1966); (c) J. L. Weeks and J. Rabani, ibid., 70, 2100 (1966). (23) N . Basco and R. G. W. Norrish, Proc. Roy. SOC.,Ser. A , 268, 297 (1962); C. Eden, H . Feilchenfeld, and S . Manor, Anal. Chem., 41, 1150 (1969).
October 7 , 1970
5825
'I "t! \\.\ \ I
0 n
004O
o
6
IIl.7~
~
l
;
~
,
ow
o
t
~
-1 ]
I
$5
002-
I
-
i 3 00
205 0
X , nm
0
3 50
/A-
5
10
I
-I
O
I
I
IO
15
I
20
Figure 4. Decay kinetics and transient spectrum produced on flash M ) at photolysis of NOz- solutions saturated with NO (1.9 X 25": (A) competition kinetics between NOZ- and NO for OH radicals; (B) transient spectrum of NzOaobtained with 9.5 X M NOz- (OD read at 100 psec), dashed curve is the normalized spectrum of Nz03 in the gas phase (ref 24); ( C ) first-order decay constant of NzOa as a function of pH.
and NO2 reacting with NO.
+ NO e NzOa
(10)
Under the conditions employed, HNOz rapidly dissociates to NOz- and almost all the NOz was converted to N203, as was indicated by the disappearance of band A (and also by the value of the equilibrium constant; see later). Thus this simple competition scheme leads to the following relations ODN0z400 1 = AODN~O," ere1
1 + T1 kg[NO] AT--[NO,-] re1
where ODN02400 and ODNzO: are the O D values for NO, at 400 nni (in the absence of NO) and for Nz03 at wavelength h (in the presence of 1.9 X M NO), respectively, obtained by flashing the same NO,solution. (The absorbance was extrapolated to zero time and ODNoz was corrected for the conversion of NO, to N204.) ere? is ENz0$/eN0z400. Figure 4A shows the validity of this relation at three wavelengths; from the parameters of the lines erel and k9/k3 could be calculated. Owing to the small value of the intercepts, the values derived from the 260- and 280-nm lines are only approximate. Thus, we found ere1300 = 10, kg/k3 = 1.6 f 0.4; ~~~1~~~ 25, k9/k3 2; ~~~1~~~ 40, k9/k3 2 . The proposed scheme is supported by (a) the apparent constancy of kg, and (b) the ratio e300:ez80:e260 = 10:25:40 being close to that exhibited by the 'pectrurn shown in Figure 4B' Using Our k3 = (1.0 rt 0.1) X 1Olo M-I sec-', we obtain ks =
-
-
430
450
500
Figure 5 . Pulse radiolysis of aqueous solutions of 9.5 X lo-' M KNOZ,pH 5.0, saturated with NzO (1 atm). Transient spectrum (a) read at 0.1 psec and (b) read at 160 psec after a 30-nsec electron pulse. -0represents OD corrected for depletion of N O z (assuming that 1 mol of NOz- is depleted for each mole of NO2 produced).
I/[NO;],102M~'
NOz
350
300
X,nm
1
/
250
-
-
Treinen, Hayon
(1.6 f 0.5) X 1010 M-I sec-'. This is considerably higher than the value previously reported,I8 but seems quite reasonable. The spectrum of Nz03 in the gas phase has been reported recently.24 Its shape is close to that of the spectrum recorded here (see dashed curve, Figure 4B) but its intensity is much lower (by a factor of -5). Before reaching any conclusion this spectrum should be checked. In conclusion we believe that band C is due mainly to N203. Only little of the absorption below 300 nm can be ascribed to NOz and N z 0 4 (section 11). From Figure 3 we could estimate that [N203] 0.12[NO2]. Assuming [NO] = [NO,] (section III.C), we obtain KN,os = [NO][N0,]/[N,03] = 2 X 10-6 M . In the gas phase KNzos is much higher.6b The same type of solvent effect is also exhibited by N,04 (section 11). 11. Pulse Radiolysis of NOz- Solutions. Band B in Figure 3 should be related to a third species; its assignment to N 2 0 4 will now be considered. For this purpose the pulse radiolysis of aqueous solutions of NOz- was studied, under conditions where high concentrations of NO, could be produced. The radiolysis of water produces the reactive species OH, eaq-, and H.
-
HzO -m+
OH, eaq-, H, Hz, HzOz
In the presence of NzO, eaq- can be converted to OH radicals (>98 with kea,-+ N z O = 6.5 X 109M-' sec-I. Is
z),
eaq-
+ N20+OH + Nz + OH-
Figure 5 shows the transient spectra produced on pulse radiolysis of 9.5 X M K N 0 2 saturated with NzO (-2.5 X lo-, M ) , pH 5.0, with the O D measured at 0.1 and 160 psec after the pulse. The former spectrum resembles that obtained in earlier s t ~ d i e s ,and ~ , ~ our value emax = 190 f 20 M-' cm-' for the 400-nm band is also in agreement. However, the short-wavelength part of this spectrum is not due to NO2 (section 1.B). The absorption below -300 nm could be accounted in part to the NO,,- radical4 (24) C . J. Hochanadel and J. A . Ghormley, J . Chem. PhJJs.,50,3075 (1969).
Absorption Spectra and Reaction Kinetics of NOZ,NzO3, and Nzo4
5826 Table I. First-Order Rate Constants for Decay of the Transients Absorbing at 400 and 280 nma kz80(av)
System
k400,sec-1
10-4 M NOZ-, air satd 1.2 x M NOz-, N 2 0 satd 2.6 X M NOZ-, Nz satd, borate buffer (pH 8.0) 2 . 6 x 10-4 M NOZ-, air satd, borate buffer (pH 7.4) 3X M NOz-, Nz satd
235, 275 265 268, 345, 318, 356 (ka,. = 322) 392
kzE0,sec-'
240, 319, 220, 309 (ksv = 272)
...
3 X lo-* M NO?-, air satd 6 . 9 x 10-4 M NOZ-, air satd 4 X 10-2 M NOZ-, air satd 0.1 M NOz-, air satd 0.1 M NO,-, NZsatd 0.1 M N O ~ - 1 . 7 x 10-4 M NOz-, Nz satd
390,' 440 230, 350, 340 370 190, 250 (kav = 220)
+
k400 (av)
...
...
5 20
508
1.96 1.58
...
...
499, 564, 540, 520 (kav = 531) 380 579
1.95
...
...
... ... ...
... ...
248
1.1
.
I
.
aTemperature = 25 i 2".
(or its protonated forms), to N203, and/or to H N 0 2 j (with maxima at -250 and 380 nm) produced from reactions with eaq- or H atoms. e&
H20 + NOz- +NOZ2-+ NO + 20HH + NO*- * N0z2- + H +
H+NO+HNO eaq-
+ NO *H N O + OH-
The formation of NOz,- was confirmed (by pulse radiolysis of 8 X M KNO, 0.5 M t-butyl ~ 2 6 nm, 5 emax 800 alcohol, pH 8.9, air free), A,, 150 M-' cm-I; the decay rate of NOS2- was found to be k = 7.7 X l o 4 sec-I. The NO, generated by pulse radiolysis was found to undergo two distinct stages of decay: in the microsecond and millisecond ranges, respectively, the first stage has already been studied4J and was ascribed to the equilibration process 2 N 0 2 e N 2 0 4 . Here we focused our attention on the second stage, which seemed more relevant to the flash photolysis work, where the equilibration process was completed within the duration of the flash. After reaching equilibrium the system clearly displays a new absorption with, , ,A 335 nm (Figure 5 , curve b). Its location and shape suggest its assignment 343 and 333 for N 2 0 , in hexane and to N 2 0 4,,A(, MeCN, respectivelyz6). Since the reactions with water are slow compared with the rate of equilibration, eNIo4and Kxzor = [NOs]2/[Nz0i] could be calculated by using the following expressions
+
0.4) X 10-5 M . The latter value is in agreement with previous r e ~ u l t s . ~ ~The 5 lowering of KN-204in solution compared to the gas phase is mainly an entropy effect; in solvents which function as Lewis bases, like water, K N ~ ois, further lowered because N,O, is a better Lewis acid than NO,.*' There is no previous report on the spectrum of N201 in water, but compared with other results (emax 179 and 233 in the gas phase and in hexane, respectivelyz6)our result is reasonable. Thus spectrum b of Figure 5 appears to represent the spectrum of Nz04 in the 300-380-nm region. Its absorption below 300 nm may be somewhat lower than that shown in the figure since some Nz03 should also be present in the system (NO is produced by the H atoms). Our values of KN~o,and E N ~ Omay ~ also need some correction owing to the presence of Nz03. 111. The Hydrolysis of the Nitrogen Oxides. A. The 2 N 0 2 a N 2 0 4System. The reaction NzOa
+ HzO +N03- + NOz- + 2H+
(11)
has been hitherto the accepted mechanism for the hydrolysis of NO,. For such a mechanism to be valid, the decay of NO, in equilibrium with N,04 should be second order when [Nn04]
