PHOTOCHEMISTRY OF AQUEOUS XITRATESOLUTIONS
Table 111. Position of Charge-Transfer Bands, mp (eV), for 7r
Acceptors with Chrysene and 1,2-Benzofluorene Donor-Acceptor'
Chrysene 'La 319 mr (3.38 eV)
1,Z-Benzofluorene 1L. 316 (3.92 eV)
DDQ TCNE CA TiNF TNF DTF
602' ( 2 . 0 5 ) 628 (1.97) 541 (2.29) 525 SC 5" (2.36) 480 rt 10 (2.58) 576 (2.15)
738 (1.68), 510 (2.43) 643 (1.92) 555 (2.23) 545 (2.27) 492 jI 5 (2.52) 588 (2.10)
a For abbreviations, see Table I. *Reference 28. insoluble.
e
Very
Fluorene (Ia) shows the largest deviation (AeV, Table 11). There is no possibility of n-n transition in the fluorene complexes. On the basis of its high ionization potential and the presence of the insulating -CH2group, fluorene is expected to be the weakest T donor. However, the positions of the C-T bands show that it is a better donor than its conjugated analog, phenanthrene (Table I). The stability constant of the fluorenechloranil complex (K,,, 0.61 1. mol-'; E,,, 1236 1. mol-'
3445 cm-l) is found to be quite close to that of phenanthrene (K,,, 1.27 1. mol-'; E,,, 1365 1. mol-' cm-l). Srivastava and Prasad2*found that the fluorene-DDQ complex has a slightly higher stability constant than the phenanthrene-DDQ complex. As an extension of this study, the charge-transfer transitions of lJ2-benzofluorene were compared with those of its fully aromatic analog, chrysene (Table 111). Again, the fluorene derivative turns out to be a consistently better donor than chrysene. It is obvious that the polarizing effect of the hybridized carbon orbitals (sp3) of the C-H bond plays a profound role in the donor ability of fluorene and its bene analog. Such polarization causes the weakening of the C-H bond, as is evidenced from the facile formation of fluorenide anion in the presence of proton acceptors. (26) N. Mataga, Y. Torihashi, and K. Esumi, Theor. Chim. Acta, 2, 158 (1964). (27) Grayish precipitate mixed with white solid separated from DDQcarbazole and TINF-carbazole solutions, which are assumed t o be salts and were not further investigated. Carbazole-TCNE and carbazole-TCNQ solutions showed epr absorption indicative of electrontransfer reactions. (28) R. D. Srivastava and G. Prasad, Spectrochim. Acta, 22, 1869 (1966).
On the Photochemistry of Aqueous Nitrate Solutions Excited in the 195-nm Band by Uri Shuali, Michael Ottolenghi, Joseph Rabani, and Ziva Yelin Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel
(Received March 11, 1969)
The photochemistry of aqueous nitrate solutions excited in the high-energy (195-nm) mr* band of the ion is investigated using steady-state and flash-irradiation techniques. T h e formation of hydroxyl radicals is demonstrated b y observing the absorption of characteristic transients formed by the reaction of O H (or 0-) with C082-, CNS-, and 02. T h e formation of pernitrite is also investigated and found t o be unaffected by [NO2-], [NOI-], [02], and [CH,CH20H]. T h e flash technique enables the determination of the dissociation constant of pernitrous acid, yielding pK, = 6.0 =t0.3. A third photochemical path, the evolution of 02 and the stoichiometric formation of NOz-, is also investigated. T h e effect of various solutes on the yields of molecular oxygen is analyzed, T h e results appear to be inconsistent with a mechanism involving intermediate oxygen atoms.
Introduction The photochemistry of the nitrate ion in aqueous solutions has been a subject of various The evolution of molecular oxygen and the formation of nitrite have been previously interpreted in terms of a primary act involving a dissociation, yielding oxygen atoms according to (NOa-)* 4 NO,0 (1)
+
The recent investigation of Daniels, et aL16in which the low-energy (300-nm) transition was excited, pro-
vided evidence for the occurrence of a competing dissociation (1) 0. Baudisch and E. Mayer, Ber,, 45, 1771 (1912); 0. Baudisoh and F. Bedford, i~'aturwissenscha~~en, 24,361 (1936), (2) E. Warburg, Sitzber. Preuss. Akad. Wiss. Phys. Math. Kl., 1228 (1918); 2.Elektrochem., 25,334 (1919). (3) W. T. Anderson, Jr., J . Amer. Chem. SOC.,46, 797 (1924). (4) D. S. Villars, ibid., 49,326 (1927). (5) H. hl. Papee and G. L. Petriconi, Nature, 204, 142 (1964). (6) M. Daniels, R. V. Meyers, and E. V. Belardo, J . Phys. Chem., 72, 389 (1968).
Volume 73, Number 10 October 196'9
3446
U. SHUALI, M. OTTOLENGHI, J. RABANI, AND Z. YELIN
It is also reported, in the same work, that small 0003 amounts of nitrite are formed independently of [NOz-]. An additional photochemical process, the formation of pernitrite, has been reported by Papee and P e t r i c ~ n i . ~ I n the present work me have carried out a photochem50002 ical study in which KOa- ions have been excited in their V W high-energy, high-intensity K K * band.' There were three main purposes of the research: (a) to carry out a detailed study of the chemical reactivity of the in5: 9 0001 4 termediate leading to the formation of molecular oxygen and to check its assignment to an 0 atom (this particular point has gained importance in view of the postulated roles of oxygen atoms in the radiation chemistry I 1 , , of waters and by the fact that 3P oxygen atoms have 380 460 540 620 700 not yet been unambiguously identified in liquid sohX . nm tions) ; (b) to obtain direct evidence for the formation Figure 1. Transient spectra recorded 1 rnsec after flashing of 0- in process 2 ; (c) to investigate the mechanism a -5 X M KNOI solution in the presence of added leading to the formation of pernitrite and to apply solutes: (a) M Na&Os, p H 10.9 (argon saturated); flash techniques in order to establish the dissociation ~ unbuffered (argon saturated); (c) (b) ~ 1 0 M- KCNS, 02-saturated solution a t pH 13. Solutions in the filter jacket constant of pernitrous acid.
[: II
1
Experimental Section (a) Steady-State Irradiation. Steady-state irradiation experiments were carried out using a Philips 93107 Cd lamp. Under standard experimental conditions ([Koa-] = M ) the only lines absorbed were those a t 229 nm and (low intensity) 216 nm. I n some experiments, when higher KOa- concentrations were employed, additional Cd lines (at 328, 341, 348, and 363 nm) were partially absorbed. Irradiations were carried out in a 1 X 1 cm square, 5 cm long quartz tube mounted on the inlet line of a gas chromatograph. The solution in the tube was first flushed with helium gas and then exposed to the exciting source. Stirring during the exposure was achieved by means of a Tefloncoated magnetic bar. After irradiation, the gas was again passed through the cell and then, after drying over CaS04, to the gas chromatograph, where (using molecular sieves on activated carbon columns) the 0 2 photoproduct was recorded. Applying analytical methods which have been previously developed, solution samples were also subjected to analysis of nitriteB and H202.'0 Absorption spectra prior to and after irradiation were recorded on a Gary 14 spectrophotometer. ( b ) Flaslz Excitation. The flash photolysis apparatus and irradiation procedure have been described previous1y.l' The flash duration (half-width) was -6 psec ; however, the actual time resolution under our experimental conditions was in the range of 0.1-1.5 msec. Although observing very small OD changes (see Figures 1, 3,4) our light signal to noise ratio ( 4 0 0 with a time constant of 150 psec) was sufficient to record the spectra of the observed (relatively long-lived) absorptions for which an accurate decay kinetics analysis was not required. A special Spectrosil-quartz The Journal of Physical Chemistry
-
were as described in the text. (Spectra a, b, c have been recorded under different excitation conditions and do not reflect relative extinction coefficients.)
jacket, mounted between the sample and the lamps, permitted the use of liquid solutions for the filtration of the exciting light. The monitoring beam was from a 120-W tungsten filament lamp for X >320 nm, or from a 60-W deuterium arc for wavelengths below 320 nm. (c) Materials. All chemicals used were analytical grade. KN03, KBr, CH3COONa, and KC1 were BDH products. C2H50H, HC104, NaAsOz, and NaOH were Baker products. KOH and NazCOa were supplied by Riedel-DeHaen Co., and iSaN02 by Mallinckrodt Co. NaHzPOz and NaH2P03 were products of City Chemical Corp. Water was always triply distilled.
Results ( I ) The Identification of Hydroxyl Radicals. Daniels, et al.,6 observed an increase in the rates of the photochemical nitrite formation upon the addition of characteristic OH radical scavengers to nitrate solutions illuminated at X >300 nm. They interpreted their results as due to OH-radical scavenging by the added solutes. In an attempt to obtain direct experimental evidence for the formation of hydroxyl radicals as intermediates, we have carried out flash experiments in -5 X M NOa- aqueous solutions to which the (7) D. Meyerstein and A. Treinin, Trans. Faraday SOC.,57, 2105 (1961). (8) (a) A. 0. Allen, Radiat. Res. Suppl., 4,54 (1964); (b) M. Daniels and E. Wigg, Science, 153,1534 (1966). (9) M. B.Shinn, Ind. Eng. Chem., Anal. Ed., 13,33 (1941). (10) H.A.Schwarz and A. J. Salzman, Radiat. Res., 9,502 (1958). (11) M.Ottolenghi and J. Rabani, J . Phys. Chem., 72,593 (1968).
U. SHUALI,11. OTTOLENGHI, J. RABANI,AND 2. YELIN
3448
0
0.007
1
I
I Oo0'
1-
'\
ooo*c
i
1
240
260
280
300
320
340
X , nrn
1
Figure 3. Absorption spectra of pernitrite ion and pernitrous acid: (a) . . . . . . . spectrum of OKOZ- according to ref 15; spectrum of photoproduct in alkaline (pH i Z 11.5) (b) KNOJ solution, recorded after 15-min irradiation with the Cd source. Initial absorbance changes recorded after flashing M KN03 solution: ., a t pH 11.5; 0 , a t pH 2. a5 X Note that below 245 nm the contribution of the 195-nm nitrate-band bleaching t o the absorbance change cannot be neglected. Thus the drop in the absorption below 245 nm cannot be attributed to a decrease in e(HON02). The dotted line is more likely to represent the true spectrum of HONOg.
An upper limit of pK,(ONO2-) E 6 has been estimated by Anbar and Yagil.17& A very recent publicationl8 reports a value of pK, = 6.5 obtained from decomposition kinetics data. In order to obtain an exact "photochemical" value for the dissociation constant of pernitrous acid we carried out flash experiments over the pH range between 0.5 and 12.7. Below pH 5 the stable peak at 300 nm is replaced by a transient (L12 i~ 2 sec) characterized by a tailing absorption (Figure 3b) with a maximum located below the limiting wavelength of our detection (X 240 nm). I n the range 5 < pH < 8 both transients are observed, exhibiting a common lifetime as well as an isosbestic point around 265 nm (result of several measurements at 265 nm, at different pH values in the range 5 < pH < 9). We thus identify the tailing absorption as due to pernitrous acid. I n Figure 4 the absorbance changes after flashing, observed at 290 and 245 nm, are plotted as function of pH. The symmetric relation between the two curves confirms the assignment of the two absorptions to the pernitrite ion (OXOz-) and pernitrous acid (HONO?), respectively. Interpreting the sharp inflection between pH 5 and 7 as due to the HONOz ONOz- equilibrium, a best fit between experimental
*
The Journal of Physical Chemistry
I
I
I
I
I
2
4
6
8
10
1
12
I
14
PH
Figure 4. pH dependence of absorbance changes measured 1 msec after flashing a 5 x 10-3 M KNOI (argon-saturated) solution: (a) 0 , A, absorbance change a t 290 nm; (b) W, absorbance change at 245 nm. The pH was regulated with NaOH or HClOd ( 0 ,m) or phosphate buffer (A).
and calculated curves (a) is obtained assuming pK, = f. 0.3. The value is in fair agreement with that reported in the kinetic work of Keith and Powell.ls The steady-state experiments showed that the yields of pernitrite in the alkaline range are not linear with irradiation time, a behavior which we attribute to photobleaching of ONOZ- by the low-energy lines (229 nm < X < 360 nm) of our Cd lamp. This is in keeping with preliminary photochemical experiments in which we have been able to eradicate the pernitrite absorption by exciting with a medium-pressure Hg source with a Pyrex glass filter. The reaction most probably leads to the regeneration of NOS-. From the initial slope of the D(300 nm) us. t curves at pH 11 (using e(OXOz-) = 1670 a t 302 nm17b), the value +NOzO-/+N02s 0.75 has been calculated for the quantum yield of pernitrite relative to that of NO?- formed in the process discussed below;. (111) The Evolution of Molecular Oxygelz. ( a ) E f e c t s of p H , [ N O z - ] , and [ N O , - ] . The 0 2 evolution induced by excitation a t 229 and 216 nm has been investigated under steady-state irradiation using the gas-chromatographic detection technique. Both oxygen and nitrite yields have been measured as functions of pH. The results, presented in Figure 5 , indicate that NO?,- and O2 are formed in equivalent amounts in the whole 1.5 < pH < 12.8 range, Up to pH 11,the general shape of the curve agrees with that of Daniels, et CTZ.,~ who ex-
6.0
PHOTOCHEMISTRY OF AQUEOUS NITRATESOLUTIONS
3449
8H 20
q yI
N 16
b
/
I
i
i
J
4 1
01
,
05 0
I
2
4
6
8
I
10
I
I
15
20
1
25
[Sl/[NOiI
I
12
I
10
14
PH
Figure 5. Relative yields of nitrite and oxygen as function of M):0, nitrite; A, oxygen. pH ([NOS-] =
Figure 7. Relative oxygen yields in the photochemistry of NOa- in the presence of added solutes (pH 10.5): (1) C Z H ~ O H , [CzHbOH] = 2 X loT3M; ( 2 ) NaAsOz, [KNOS] = M; (3) NaHZP03: A, [NaIlzPOa] = 5 X M ; A, [KNOS] = M; (4) NaHzPOz: 0 , [RTaHzP02] = 5 X M ; 0, M; (5) CHaCOONa, [C&COONa] = [KNOS] =
M.
-. I
I
d
I, min
Figure 6. Nitrite and oxygen concentrations as functions of irradiation time in solutions containing varying amounts of nitrate and nitrite (pH 10.5). All values in the absence of initial nitrite are [NOZ-]. In the experiment containing initial NOz- the values are 2[02].
cited within the low-intensity (300-nm) nitrate band. I n agreement with their data we found that the inflection around pH 9 is due to an efficient back-reaction, involving the NOz- photoproduct, which takes place in the low pH range. Thus, the yield values in the acid range, such as reported in Figure 5, do not reflect true primary photochemical yields. Above pH 10 (see Figure 6) the rates of 0 2 and NOZ- production show very little dependence (if a t all) on irradiation time and on both [NOz-] and [NO,-]. I n this range a zero intercept has always been observed for the latter plots. Below pH N 6 we observed small ("residual")
yields of YOz- and O2which are independent of irradiation time, The values are around 1% of the yields observed on the plateau in the alkaline range. This observation agrees with that of Daniels and coworkers who reported similar residual nitrite yields upon exciting in the 300-nm band. ( b ) The Effects of Added Solutes. The yields of 0 2 evolution in the alkaline range (10 < pH < 12.5) have been measured in nitrate solutions containing added solutes. I n all cases only a negligible fraction of the exciting light was absorbed by the additives. I n the cases of CH3COONa, NaH2P02, NaH2POa,NaAsOz, and CzH50H, the addition of a sufficient amount of solute is accompanied by marked drops in the slopes of the Oz vs. time plots. However, the linear character of such plots, as well as their zero intercepts, is maintained. The plots of So/S against the ratio [solute]/ [NOa-] are presented in Figure 7 (So and S represent the slopes in the absence and presence of the added solutes), The following solutes showed no detectable M nitrate effects on the rates of 0 2 evolution in M), solutions: C1- (up to 1 M ) , Br- (up to COS2- (up to loF2M ) , NO%- (up to M ) , and OH(up to 10-1 M ) . Due to experimental difficulties the effects of 0 2 and NzO on the oxygen yields have not been measured. However, no 02 or NzO effects on the rates of nitrite formation in 0 2 - and NzO-saturated alkaM-1 M ) solutions have been observed. line nitrate
Discussion Primary Processes in the Photochemistry of NO3-. Both steady-state and flash-photochemical experiments described above involve excitation of NO3- to its Volume 78, h'umber IO
October 1969
3450
U. SHUALI, M. OTTOLENGHI, J. RABANI, AND Z. YELIX
high-intensity mr* state. Four primary independent processes appear to take place in this state. (a) First is formation of pernitrite. The reaction mechanism responsible for the first-order growth observed by Barat and coworkers14is still obscure. A cage recombination involving hydroxyl radicals [NO,-* -t (NO2 O-)oago ---t ONO2-I3 can be excluded in view of the relatively slow (-15 psec) growing-in of OXO,-. A bulk recombination between the same radicals is inconsistent with the lack of sensitivity to 0- scavengers. Similar arguments apply when considering the analog process: NOI-* YO20 ONOZ(cage or bulk), in view of the expected reactivity of 0 atoms (see below). Alternative routes, involving the equilibria
the alkaline range (pH >lo) clearly indicate that a competition takes place between NO3- and the added solutes on an intermediate X. Thus, assuming that Nos-, leading to 02 evolution, is the process X suppressed by the process X solute (which leads to nongaseous products), the expression
+
-
(N02+.0H-)
-
+
+ Hf ONO2- + H+
_r ONO2-
(NO+.HOz-)
(4) (5)
may offer possible explanations. Both ion pairs involved in reactions 4 and 5 have been previously suggested18in the pernitrite system. (b) The generation of hydroxyl radicals formed via
hTOa-*
--.)
NO2
+ 0-
(6)
is established by the spectra of the CNS, COS-, and 03-radicals, known to be products of hydroxyl-radical scavenging by CYS-, COS2-,and 02. Additional evidence for process 6 is provided by the relative values of the corresponding scavenging rate constants, including the carbonate results which are sensitive to pH. It is interesting to note that an exact analog of process 6, which takes place from both 300- and 195-nm nitrate states, has been recently detectedls in the photolysis of BrOs-, Br02-, and BrO-. (c) The “residual” NO2- yields in the NOa- photolysis observed by Daniels, et a1.,6 at X >300 nm and by us for both O2 and NO3- at 229 nm may be accounted for by an “internal recombination” leading directly to O2 evolution. Such a process, which is probably taking place in the photolysis of BrOa-,20extends the above-mentioned analogy between the primary photochemical paths in the nitrate and oxybromide aqueous systems. (d) The formation of the intermediate leading to 02 evolution, via a reaction with NOa-, has been the first process to be detected in the nitrate photochemistry and is probably the least understood. I n the absence of added solutes, we have observed the formation of 02 and N02- in equivalent concentrations ([NOz-] = [ 0 2 ] / 2 ) . All previous suggestions explaining this process involve an oxygen atom intermediate. Daniels, et aLj6proposed the sequence Koa-* ---t yoz0
+ 0 + NOS- +NO2- +
0 2
The effects of solutes on the rates of 02 evolution in The Journal of Phusical Chemistry
+
+
(where So and S represent the slopes of the [ 0 2 ] us. time plots in the absence and presence of the added solute) is expected to be satisfied. The applicability of such a relation is shown in Figure 7. The slopes of the corresponding plots thus measure the relative rates between X and the various solutes. Such values are presented in Table I. To the same table we have also added an upper limit estimate for the lc(x+o,)/ k(x+NO^-) and k (x+xzo)/ k (X + NO^-) ratios derived from our observations concerning the lack of 0 2 and K20 effects on the XO2- yields.
Table I : Relative Rate Constants between Intermediate X and Various Solutes‘ k(solute+x)
Solute
NO,(CH&OO)(HzP0z)(HzP03)(AsOz)CHsCIIzOH OH-
k (ti03- +X)
1.0 0.4 2.2 3.5
6.7 14.5
