J . Phys. Chem. 1988, 92, 1156-1 162
1156
Kinetics and Spectroscopy of the NO3 Radlcal in Aqueous Ceric Nitrate-Nitric Acid Solutions P. H. Wine,* R. L. Mauldin, III,+ and R. P. Thorn$ Molecular Sciences Branch, Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received: April 28, 1987; In Final Form: August 17, 1987)
A pulsed laser photolysis-long-path absorption apparatus has been employed to investigate the kinetics and spectroscopy of the NO3 radical in aqueous nitric acid solution. NO3 was prepared by photolysis of cerium ammonium nitrate, Ce(NH4)2(N03)6.Much lower NO3 concentrations were employed than in all previous work on this well-studied photochemical system. Important new findings are (1) the N03(aq) extinction coefficient is considerably larger than previously thought, (2) the appearance of NO3 following absorption of a laser photon occurs on a time scale that is fast compared to our 50-ns time resolution, and (3) low concentrations of NO3 and other reactive species result in background NO3 decay rates which are about an order of magnitude slower than any reported previously. New rate data are reported for the reaction of NO3 with Ce(II1) and for the reaction SO4- NO3- NO3 + S042-.
+
-
Introduction Stimulated by the development of the flash photolysis and pulsed radiolysis techniques, a period of intense investigation of the kinetics and spectroscopy of small inorganic radicals in aqueous solution occurred during the 1960s and early 1970s. Interest in this area of research lessened over the past decade as the emphasis for studies of aqueous-phase transients shifted to systems of biological interest. In recent years, modeling studies have shown that free-radical chemistry in cloud water may play an important role in the atmospheric oxidation of SO2 to sulfuric acid.'" Radicals that may be important intermediates in aqueous-phase SO4-,NO3, C1 atmospheric chemistry include OH, H 0 2 02-, CI2-, and C03- H C 0 3 . The potential role of inorganic aqueous-phase radicals in acid rain chemistry has renewed interest in experimental investigation of the physical properties and chemical reactivities of these species.' We recently initiated a research program to investigate the kinetics and spectroscopy of small inorganic radicals in aqueous solution. We have two major goals. First, we hope to improve experimental methodology for real-time studies of aqueous-phase free-radical chemistry. Improvements in selectivity of radical production and both sensitivity and selectivity of radical detection are desirable. Secondly, we want to use the improved methodology to study aqueous-phase free-radical reactions, which are thought to be important in acid rain chemistry. With the above goals in mind, we recently constructed a laser flash photolysis-long-path absorption (LFP-LPA) apparatus. Compared to the traditional techniques of pulsed radiolysis and broad-band flash photolysis, monochromatic pulsed laser photolysis offers improved selectivity for radical production. A multipass detection scheme is employed to improve the sensitivity of UV-vis absorption as a radical detection technique. Our first experiments with the LFP-LPA apparatus, the results of which are reported in this paper, involved kinetic and spectroscopic studies of the NO3radical. There are several reasons for choosing NO3 for our initial studies. First, it is well documented in the literature that UV photolysis of ceric nitrate in aqueous nitric acid solution produces NO3,8-I3although there is some disagreement concerning the primary photochemical process.8*10~12~13 While the wavelength dependence of the N03(aq) absorption spectrum is w e l l - k n ~ w n , ~the ~ *only ~ ~ ~published ~~~*~~ value for the N03(aq) extinction coefficient'* is somewhat susp e ~ t . 'Also, ~ ~ it has been suggested that diffusion of NO3 into cloud droplets from the gas phase may initiate chemistry that can contribute significantly to the nighttime oxidation of atmospheric although it is questionable whether or not the solubility of
-
-
-
* Author to whom correspondence should be addressed.
Present address: NOAA, R/E/AL-2, 325 Broadway, Boulder, CO 80303. 'Also affiliated with the Georgia Tech School of Geophysical Sciences.
0022-365418812092-1156%01SO10
NO3 in water is large enough for such a process to be important.I6 In this paper, we report a new higher value for the N03(aq) extinction coefficient, demonstrate that NO3 appearance following light absorption by ceric nitrate is faster than previously thought, and show that background NO3 decay rates in acid solution are very slow as long as the concentrations of photolytically or radiolytically generated reactants are minimized. New kinetic data for the NO3 Ce"' and SO; NO< reactions are also reported.
+
+
Experimental Section A schematic of the laser flash photolysis-long-path absorption apparatus is shown in Figure 1. It consists of an all-Teflon reactor and liquid circulation system, a pulsed photolysis laser for creating NO3 radicals, a segmented aperture optical integrator for making the photolysis laser beam spatially uniform in intensity, a highpressure xenon arc lamp probe light source, White cell optics for multipassing the probe light beam through the reactor, a monochromator for isolating the monitoring wavelength of interest, a photomultiplier for detecting the probe radiation, and a signal averager for recording the time dependence of the photomultiplier output. The individual components and relevant operating conditions are discussed below. The Teflon reaction cell had an internal volume of 50 cm3, a maximum aperture for the photolysis beam of 3 cm X 3 cm, and a 3.5-cm-diameter round aperture for the probe beam. The path length traversed by the photolysis laser beam through the reactor was 4 cm in length. Windows were sealed to the reactor with '/,,-in.-thick Viton O-rings; Teflon O-rings were found to be too rigid to compress effectively against the reactor walls. The windows traversed by the probe beam were broad-band antireflection coated on the outside surface. Reaction mixtures were (1) Graedel, T. E.; Weschler, C. J. Reu. Geophys. Space Phys. 1981, 19, 505. (2) Chameides, W. L.; Davis, D. D. J. Geophys. Res., D: Atmos. 1982, 87, 4863. (3) Graedel, T. E.; Goldberg, K. I. J. Geophys. Res., D,Atmos. 1983,88, 10865. (4) Chameides, W. L. J. Geophys. Res., D Aimos. 1984, 89, 4739. (5) Chameides, W. L. J. Geophys. Res., D: Atmos. 1986, 91, 5331. (6) Jacob, D. J. J. Geophys. Res., D: Aimos. 1986, 91, 9807. (7) Neta, P.; Huie, R. E. J. Phys. Chem. 1986, 90, 4644. ( 8 ) Martin, T. W.; Henshall, A.; Gross, R. C. J. Am. Chem. SOC.1963, 85, 113. (9) Martin, T. W.; Rummel, R. E.; Gross, R. C. J. Am. Chem. SOC.1964, 86, 2595. (10) Dogliotti, L.; Hayon, E. J. Phys. Chem. 1967, 71, 3803. (11) Martin, T. W.; Glass, R. W. J. Am. Chem. SOC.1970, 92, 5075. (12) Glass, R. W.; Martin, T. W. J . Am. Chem. SOC.1970, 92, 5084. (13) (a) Stevens, M. V. dissertation, Vanderbilt University, 1971. (b) Martin, T. W.; Stevens, M.V. Presented at the XIIth Informal Conference on Photochemistry, Gaithersburg, MD, 1976. (14) Daniels, M. J. Phys. Chem. 1966, 70, 3022. (15) Broszkiewicz, R. K. Inr. J . Appl. Radiai. Isoi. 1967, 18, 25. (16) Mozurkewich, M. J. Geophys. Res., D Aimos. 1986, 91, 14569.
$3 1988 American Chemical Societv
NO3 Radical in Ceric Nitrate-Nitric Acid Solutions
Figure 1. Schematic of the laser flash photolysis-long-path absorption apparatus: B, baffle; CF, cutoff filter; DDG, digital delay generator; EM, energy monitor; HV, high voltage; IRF, infrared filter; L, lens; LR, liquid reservoir; M, monochromator; P, peristaltic pump; PG,pulse generator; PL, photolysis laser; PM, photomultiplier;RC, reaction cell; SA, signal averager; SAOI, segmented aperture optical integrator; T, telescope; WCM, White cell mirror; XAL, xenon arc lamp.
prepared in 1-L Teflon bottles and circulated through the reactor with a peristaltic pump; the flow rate was typically 50 cm3/min. All surfaces in contact with the reaction solution were Teflon (reactor, reservoir, and 1/8-in.-diameter tubing connecting the components), quartz (reactor windows), or Viton (O-rings and a short piece of tubing in the pump). The photolysis light source in most experiments was second harmonic radiation (532 nm) from a Quantel Model 481A Nd:YAG laser. The laser pulse width was 8 ns and up to 125 mJ/pulse was attainable. A few experiments employed 355- or 266-nm radiation; the light source for these UV wavelengths was third (355-nm) or fourth (266-nm) harmonic radiation from a Quanta Ray Model DCR-2A Nd:YAG laser. The laser pulse width was 6 ns and up to 150 mJ/pulse was attainable at 355 nm. The photolysis beam was shaped into a 2.2-cm square and made spatially uniform in intensity as it traversed the reactor through use of a segmented aperture optical integrator (SAOI). The operation of the S A 0 1 and the extent of uniformity of the integrated beam (typically 5% peak-to-peak) has been described in a previous pubication from our 1aboratory.I' The depth of focus of the integrated beam was such that the laser fluence was constant over a distance considerably longer than the 4-cm length of the reactor. The laser beam fluence was measured as the beam exited the reactor by using an EG&G photodiode-based radiometer capable of measuring individual pulsesl The calibration of the radiometer was checked frequently with a Scientech disk calorimeter. The analytical light source was a 150-W Osram Model XBOl50W/S xenon arc lamp contained in a PRA Model ALH 21 5 housing and powered by a PRA Model M304 supply. About 20 W of broad-band output is obtainable from this lamp. Light from the arc lamp passed through a liquid water filter to remove infrared radiation, then through two collimators and an appropriate cutoff filter before entering the reactor. The light beam was passed back and forth through the reactor at right angles to the photolysis beam by using external, 50-cm radius of curvature, White cell optics,I8 which were coated for high-reflectivity throughout the visible. Typically, 26 passes were employed, giving a path length for radical detection of 57 cm. The multipass path length was M Rhodamine calibrated by measuring the absorbance of B in H 2 0 (A = 590 nm, t = 2700 M-'cm-I) with the White cell aligned for 18 and 42 passes. In both cases, the measured path length was within 3% of the geometrical path length. The output beam from the White cell was directed through a 0.22-m (17) Ravishankara, A. R.; Eisele, F. L.; Kreutter, N. M.; Wine, P. H. J . Chem. Phys. 1981, 74, 2267. (18) White, J. U. J . Opt. SOC.A m . 1942, 32, 285.
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 1157 monochromator (SPEX Model 1681A) and onto the photocathode of a Hamamatsu Model R446 photomultiplier tube. Typical spectral resolution was 1.5 nm. Empirically, it was determined that the photomultiplier response was linear as long as the anode current was kept below 3 X lo4 A; most experiments were run with the anode current in the range (1-2) X lo4 A. To minimize noise in the long-path absorption monitoring system, all components were mounted on a vibrationally isolated optical table. In most experiments the output signal from the photomultiplier was fed into a Tracor Northern Model N S 570 multichannel analyzer (MCA) operating in the peak-height analysis mode. The MCA could digitize data in time intervals as short as 10 ps, its actual time resolution was -30 ps, and its voltage resolution was 12 bits. A few experiments required better time resolution than was attainable from the NS-570 MCA. Improved time resolution (at the expense of voltage resolution) was obtained by replacing the MCA with a Data Precision Model DATA 6000 transient digitizer/signal averager equipped with a Model D-1000 preamplifier. The DATA 6000 could digitize data in time intervals as short as 10 ns and had a voltage resolution of 8 bits. When the DATA 6000 was employed, the attainable time resolution was limited by the transit time of the arc lamp beam through the multipass system (typically 50 ns). The DATA 6000, which had pretrigger memory capability, was triggered by a photodiode, which monitored a small fraction of the photolysis laser pulse. When the NS-570 MCA was employed, a pulse generator-digital delay generator combination was used to allow measurement of a value of Io, the unattenuated monitoring beam intensity, immediately prior to the photolysis laser flash. Signal-averaged, digitized, time versus monitoring beam intensity data were transferred to a microcomputer for kinetic analysis. The salts used in this study were obtained from Aesar and had the following stated minimum purities: ceric ammonium nitrate, Ce(NH4)2(N03)6,99.5%; cerium nitrate, Ce(N03)3, 99.99%; potassium peroxydisulfate, K2S208,99%; sodium nitrate, NaN03, 99.997%. Nitric acid and perchloric acid were ACS reagent grade, 70% in H20. Water was purified by a Millipore Milli-Q system equipped with filters for removing particulates, ions, and organics. All solutions were prepared the same day as the experiment was carried out. Ceric ammonium nitrate is a primary standard. After several hours of drying at 100 OC, a stock solution of known concentration could be readily prepared by simply adding a known weight of Ce(NH,)(NO,), to nitric acid solution and diluting to a known volume. Because cerium nitrate is hydrated, the exact concentration of the Ce(II1) stock solution was determined gravimetrically with a method described by Martin et aL9
Results and Discussion The NO3 radical has been observed following flash photolysis of ceric nitrate solutions by Martin and and by Dogliotti and Hayon.lo These investigators disagree on the exact nature of the primary photochemical process. However, Glass and Martin12 present strong evidence that production of NO3 involves an intramolecular, excited-state charge transfer, which can be written schematically as follows. Ce"N0,-
+ hv
-
CeN03*
-
Ce1"
+ NO3
(1)
The ceric nitrate extinction coefficient is much lower in pure water than in nitric acid solution and, in the presence of nitric acid, the ceric nitrate spectrum is somewhat red shifted from that observed in water. This phenomenon is thought to result from the fact that the inner coordination sphere of Ce(1V) is occupied by OH groups in water or dilute H N 0 3 but by NO3-groups in more concentrated HNO, solution. In 6 M nitric acid the dominant ceric species is believed to be Ce(H20),,(N03)62-.19920 Martin and co-workers have shown that the quantum yield for NO3 formation is constant for 5 M I [HNO,] 5 14 M but tends to zero as [HN03] 0.11,13a Previous studies of the ceric nitrate photolytic system have employed broad-band flash or doubled ruby laser
-
(19) Wylie, A. W. J . Chem. SOC.1951, 1474. (20) Henshall, A. Dissertation, Vanderbilt University, 1963
Wine et al.
1158 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 phot~lysis.'~ In all cases the vast majority of photons absorbed by ceric nitrate were in the wavelength range 300-400 nm. In our initial experiments, we photolyzed ceric nitrate-nitric acid mixtures a t 532 nm. A transient absorption in the red region of the spectrum was readily observed. The absorption spectrum of the transient was mapped out point by point through repeated (Axop base 10 absorbance a short determinations of Ax0/A0635nm time after the laser fired, Le. when transient production was complete but no transient decay had occurred) with each absorbance measurement normalized to account for fluctuations in the photolysis laser power. The observed spectrum had three absorption maxima a t 600, 635, and 675 nm with the largest absorbance at 635 nm. The spectrum agreed quantitatively with other published spectra of N03(aq);7~8~'2,14*15 hence, the observed transient could unequivocably be identified as the NO3 radical. Daniels14 has shown that the NO,(aq) spectrum is independent of pH, Le. the same spectrum is observed following pulsed radiolysis of 6 M H N 0 3 and 5 M NaNO,. In our experiments, the observed spectrum was independent of ceric nitrate concentration M and independent of HNO, conover the range (1-7) X centration over the range 1-6 M. We also observed the identical spectrum following 266-nm photolysis of SzOS2-/NO3-mixtures in 0.1 M perchloric acid. Once it was established that NO3 was being detected with good sensitivity, we carried out three sets of experiments, each of which addressed a different aspect of the ceric nitrate/nitric acid/N03 photochemical system. First, we measured tp, the product of the NO3 extinction coefficient at the wavelength of maximum absorbance (635 nm), and the quantum yield for NO3 formation. Second, we examined the assertion of Stevens and Martin', that NO3 does not appear promptly following ceric nitrate photolysis, but with a rise time of about 13 p s . Finally, we examined the NO3 decay kinetics. Each of the three sets of experiments led to conclusions that differ substantially from those of previous investigators. The three sets of experiments are discussed below. I. Determination of tq. The square shape and spatial uniformity of the optically integrated laser beam make possible accurate determination of tcp. If we let A,, = absorbance at 635 nm at a time shortly after the photolysis pulse when NO3 production is complete but no NO, decay has occurred, then the following relationship is applicable: Amax
= tltN03lmax = c&f/N
(1)
where I is the absorption path length, F is the photolysis laser photon fluence, f is the fraction of photolysis photons absorbed per centimeter, and N is the number of solute molecules in 1 cm3 of 1 M solution, i.e. N = 6.023 X 10". Even though the photolysis beam was square, a 2.2 cm X 2.2 cm mask was placed over the reactor entrance window. The mask clipped the outer edges of the photolysis beam but allowed the absorption path length per pass to be known very accurately. As mentioned above, the laser power was measured as the photolysis beam exited the reactor. A 0.81-cm2 round aperture was placed over the radiometer for these measurements. The measured laser power was corrected upward for reflection at the outer surface of the exit window (5%) and for absorption by ceric nitrate in the rear half of the reactor (2-20%). The multipass analytical beam probed the middle 1.5 cm of the 4-cm path traversed by the photolysis beam. Sincefwas less than 0.1 under all experimental conditions, the laser fluence could be taken to be independent of position in the reactor and equal to its value at the center of the reactor without introducing significant error into the t9 determination. As mentioned above, the extinction coefficient for ceric nitrate increases with increasing nitric acid concentration. Over the range of Ce(IV) concentrations employed in the tcp measurements (( 1-7) X IO-, M), solute-solute interactions resulted in the "apparent" extinction coefficient decreasing with increasing Ce(1V). For this reason, the fraction of photolytic photons absorbed per centimeter, had to be determined from photometric measurements that employed solutions identical with those employed in the photolysis experiments. The photometric measurements were
made on a Cary 17 spectrophotometer using 10.0-cm absorption cells. The experimental procedure for measuring tcp involved circulating 1 L of freshly prepared cerium ammonium nitrate-nitric acid through the reactor, photolyzing at a repetition rate of 1 Hz, and recording the absorbance temporal profile on a time scale where the l / e time fsr NO3 decay was more than 50 channels. Typically, 64 flashes were averaged to obtain good signal to noise. The entire temporal profile was used to obtain the best value for Imax/Io (Io 2 analytical beam intensity before the photolysis laser fires; I,,, 2 analytical beam intensity after photolytic production of NO3 is complete but before any NO, decay has occurred; Amax log (Io/Imax). The radiometer monitored each individual laser pulse, allowing accurate determination of the average pulse energy. Each liter of solution was used to carry out between 3 and 10 experiments. We found that 1 L of solution could be exposed to room lights for several hours and subjected to over 1000 shots without significant bleaching of the ceric nitrate photolyte. This is because much of the photolytically destroyed ceric nitrate is regenerated: Ce"'
+ NO3
-
Ce'"N0,-
The results of the tcp determinations are summarized in Table
I. A total of 40 determinations was made. Parameters that were varied include the nitric acid concentration, the ceric nitrate concentration, the photolysis laser power, and the concentration of dissolved oxygen. The absorption path length was 57.2 cm in all experiments (26 passes). However, in preliminary experiments not aimed at accurately measuring tp, it was established that 635-nm absorbance scaled linearly with the number of passes. We obtain a lower value of ccp in 1 M H N 0 3 than at higher nitric acid concentrations. This is consistent with the observations of Stevens and Martin that €9decreases with decreasing nitric acid concentration.', Stevens and Martin employed 347-nm (doubled ruby laser) photolysis and found that the ratio of quantum yields was 0.46:0.92:1.00 for 1, 3, and 6 M nitric acid. Using 532-nm photolysis, we obtain a ratio of quantum yields of 0.68: 1.O: 1.O. Since the ceric nitrate absorption spectrum red shifts (and becomes much stronger) with increasing nitric acid concentration, it seems reasonable that the dependence of the NO3 quantum yield on [HN03] should show some dependence on photolysis wavelength. Our results for tcp in 3 and 6 M HNO, are, within experimental uncertainty, identical. The average of 36 determinations in 3 and 6 M HNO, is tp = 832 f 112 M-' cm-I; the uncertainty is 20 and represents precision only. We estimate that the absolute accuracy of the tcp determination is f20% (95% confidence). Because the primary photochemical process is a one-electron excited-state redox reaction, the quantum yield for NO3 production cannot be greater than one. Hence, our value for tp represents a lower limit for t . Both our results and those of Stevens and Martin', demonstrate that tp increases with increasing [HNO,], then levels off at higher HNO, concentrations. Stevens and MartinI3suggest that cp = 1 in the regime where tp is independent of [HNO,], but no proof for this assertion has been reported. Recently, we employed an analogous method to that described above for NO, to measure tcp for the sulfate radical (SO4-) at 443 nm, the absorption maximum for this species. SO[ was produced by 266-nm pulsed laser photolysis of the peroxydisulfate anion.
+
S2Oa2- hu(266 nm)
-
2S04-
(3)
Our result (details will be presented elsewhere2I) is tcp = 2770 f 550 M-' cm-I. Under the assumptions that the quantum yield for NO, production via reaction 1 is unity in 3-6 M HNO, and that the quantum yield for SO4- production via reaction 3 is 2, the ratio of the independently determined extinction coefficients is t(S04-, 443 nm)/t(N03, 635 nm) = 1.67 f 0.47. To check the consistency of our tp determinations for NO, and SO4- and obtain further information concerning the quantum (21) Tang, Y.; Thorn, R. P.; Mauldin 111, R. L.; Wine, P. H., to be submitted for publication.
The Journal of Physical Chemistry, Vol. 92, No. 5. 1988 1159
NO3 Radical in Ceric Nitrate-Nitric Acid Solutions
TABLE I: Summary of tq DeterminationsPd [HNO31, M
[Ce(Wl, 10-3 M
f‘
1
.oo
4.0
0.0093
3.00
4.0
0.0255
6.00
1.o
0.0155
6.00
4.0
0.0581
6.00
4.0
0.0581
6.00
7.0
0.0977
photons/cm2
Am..h
w3 M-‘ cm-’
2.85 3.21 2.33 2.75 2.50 0.78 2.11 2.44 0.75 2.38 1.16 2.51 2.33 2.86 0.63 2.70 2.76 1.34 2.06 2.62 1.81 0.54 1.80 1.77 0.50 1.54 1.82 1.79 0.50 1.02 2.79 2.84 2.8 1 2.31 0.53 1.19 2.21 2.34 0.5 1 1.46
0.0122 0.0165 0.0129 0.0137 0.0500 0.0146 0.0458 0.05 18 0.0155 0.0479 0.0247 0.05 18 0.0497 0.0382 0.0084 0.0342 0.0309 0.0151 0.0279 0.0310 0.0780 0.0230 0.0765 0.0791 0.0225 0.0670 0.0797 0.0740 0.0202 0.0410 0.134 0.134 0.145 0.186 0.0392 0.0970 0.174 0.183 0.0433 0.112
484 582 628 564 825 769 896 878 85 1 832 88 1 854 88 I 907 907 862 760 767 919 804 782 770 770 808 810 788 793 750 733 73 1 870 857 938 869 804 875 827 841 914 827
F,g
SGf
(V),‘
M-’ cm-‘
565 f 120
852 f 78
847 f 137
774 f 56 888 =k 87
851 f 74
“ t = NO3 extinction coefficient at 635 nm. b9 quantum yield for NO3 production from 532-nm photolysis of ceric nitrate. ‘The path length was 57.2 cm in all experiments. dExperimentsthat employed the same liter of solution are grouped together. ‘fl fraction of photolysis photons adsorbed per centimeter. f S G 2 saturating gas. gF 2 laser photon fluence. hAmaxE base I O absorbance at a time after the photolysis flash when NO3 production was complete but no NO3 decay had occurred. Errors are 2a, precision only.
yields for processes 1 and 3, we have examined the kinetics of the electron transfer reaction
-
SO4- + NO3+ NO3 (4) S20g2-/N03mixtures in 0.1 M perchloric acid (Le. pH 1) were photolyzed at 266 nm, a spectral region of minimum NO3- absorbance. Absorbance temporal profiles were monitored at 635 and at 443 nm. To eliminate contributions to the observed temporal profiles from the fast SO, self-reaction22and from reactions involving NO3- photofragments, a relatively long path length (84 cm) was employed in conjunction with relatively low photolysis laser powers. Typical data are shown in Figure 2. The data were analyzed by using the simple pseudo-first-order reaction scheme
SO4-
- ka
NO,
kd
X
(5)
At 635 nm, NO3 was the only absorbing species.23 Therefore, the absorbance temporal profile could be described by the following relationship:
kd was measured independently by monitoring the absorbance temporal profile after all .SO4- had reacted; kd was found to be (22) Ross, A. B.; Neta, P. “Rate Constants for Reactions of Inorganic Radicals in Aqueous Solution”; Natl. Stand. ReJ Data Ser. (CIS., Natl. Bur. Stand.); Washington, DC, 1979; NSRDS-NBS 65. (23) Hug, G. L. “Optical Spectra of Non-Metallic Inorganic Transient Species in Aqueous Solution”; Natl. Stand. ReJ Data Ser. (U.S.,Narl. Bur. Stand.); Washington, DC, 1981; NSRDS-NBS 69.
-50 s-I in all experiments. k, and C were determined from a nonlinear least-squares analysis. Values for ka, the pseudofirst-order NO, appearance rate, are plotted as a function of nitrate concentration in Figure 3 (the SO4-decay rate in the absence of nitrate was obtained from 443-nm absorption temporal profile measurements). The slope of the ka versus [NO3-] plot gives a value of (1.1 1 f 0.05) X lo5 M-’ s-l for k4, where the uncertainty is 2a and represents precision only; the absolute accuracy of our k4 determination is estimated to be f l 5 % . Two measurements of k4 have been reported p r e v i o ~ s l y . ~ ~ ~ ~ K r a l j i ~employed ~~ a competitive kinetics technique based on the fact that SO4- radicals react rapidly to bleach the R N O chromophore in p-nitrosodimethylaniline and obtained a value of 1.4 X lo6 M-’ s-l for k4 in weakly basic solution (pH 9). Kraljic pointed out potential interferences from secondary reactions involving NO3 and, therefore, considered his measurement of k4 to be only a tentative value. Neta and Huie’ recently employed the pulsed radiolysis-time-resolved absorption technique to obtain a s-l for k4 in 6 M H N 0 3 . The factor of value of 5.5 X lo5 M-’ five difference between our value of k4 and the value reported by Neta and Huie could be due to the existence of protonated H N 0 3 as a reactant under the very acidic conditions of their experiment. Also, the rate coefficient for reaction 4 would be expected to increase with increasing ionic strength, consistent with the higher value obtained by Neta and Huie at very high ionic strength. At 443 nm, SO4-absorbs strongly while NO3 absorbs weakly.23 The contributions of SO4- and NO3 to the measured 443-nm (24) Kraljic, I. Int. J . Radiat. Phys. Chem. 1970, 2, 59.
1160 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
Wine et al.
+
0.01
-
..%
8
635 nm
- d 3
1
0
TIME
2
(mr)
Figure 2. Temporal profiles of absorbance observed at 635 and 443 nm following 266-nm pulsed laser photolysis of 0.002 M S208*+ 0.02 M NO< 0.1 M HC104. The solid line drawn through the 635-nm data is the best fit to eq I1 by using the measured value of 48 s-I for kd;best fit parameters are k, = 2650 s-I and C = 0.01026. The dashed line drawn in the 635-nm temporal profile shows the absorbance that would have been reached at t m if kd = 0. The solid and dashed lines drawn with the 443-nm data show the contributions of SO; and NO3, respectively, to the 443-nm absorbance.
+
-
O
L
U
-
L-
2
0 [NO;]
1
I
4
(16' M )
Figure 3. Plot of k,, the NO, appearance rate, versus nitrate concentration. The solid line is obtained from a linear least-squares analysis and gives the bimolecular rate coefficient shown in the Figure.
absorbance temporal profile are shown in Figure 2. The pseudo-first-order SO,,- decay rate is equal to the pseudo-first-order NO3 appearance rate, as would be expected if reaction 4 was leading to conversion of SO, to NO,. The parameter C in eq I1 is related to the maximum absorbance that would have been observed at 1 if the NO3 decay rate were zero; we designate this absorbance A,(635 nm). Measured values for A,(635 nm) and A0(443 nm) provide a useful comparison with our independent measurements of ecp for NO3 and SO4-. For the data shown in Figure 2, we obtain A0(443 nm) = 0.0150 and A,(635 nm) = 0.01026. Under the conditions used to obtain the data shown in Figure 2, 86% of the photolytically produced SO, reacted with NO3- to produce NO,. Hence, the data in Figure 2 suggest that €(SO4-,443 nm)/c(N03, 635 nm) = 0.86 and A0(443 nm)/A,(635 nm) = 1.26. Similar analyses were carried out on the other data used to obtain a value for k4. Two data sets were rejected because the laser power drifted significantly during the runs. The average value of t(SO4-, 443 nm)/e(NO,, 635 nm) obtained from the other four determinations was 1.29 f 0.1 5. As mentioned above, our independent determinations of E ( P for SO4- and NO, give the result t(S04-,443 nm)/e(NO,, 635 nm) = 1.67 f 0.47 if it is assumed that the quantum yields for processes 1 and 3 are 1 and 2, respectively. The value for t(S04-, 443 nm)/t(N03, 635 nm) obtained from
-
the SO4- NO3- experiments is lower than the value obtained from the independent measurements but not by more than the combined experimental uncertainties. We consider the SO4- + NO,- experiments to be suggestive that the quantum yield for NO3 production from photolysis of ceric nitrate in 3-6 M HNO, is a little less than unity. The first measurement of cp for NO3 at 635 nm was reported by Glass and Martin.I2 They studied the kinetics of reaction 2 and used their measured bimolecular rate coefficient and the intercept of a plot of the pseudo-first-order NO3 decay rate versus the concentration of added Ce(II1) to calculate the concentration of Ce(II1) produced by the photoflash, [Ce"'lF. It was assumed that the NO, decay in the absence of added Ce(II1) was entirely due to reaction with flash-generated Ce(III), an assumption that probably was not valid under Glass and Martin's experimental conditions.13* Setting [CeI1'lF = [NO,] and estimating their absorption path length to be slightly less than the distance between electrodes in the concentric flash photolysis cell, Glass and Martin obtained the result tp = 250 f 90 M-' cm-' at 635 nm. Stevens employed a method similar to ours to obtain a value for tp.13aHe monitored the NO, produced following 347-nm doubled ruby laser photolysis of ceric nitrate and obtained the result tp = 480 f 48 M-' cm-l. The laser power was determined by potassium ferrioxalate actinometry, a potential problem since it is not clear that ferrioxalate actinometry can be applied to measure photon fluxes typical of high-energy pulsed lasers. Other potential sources of error in both previous determinations of t p are spatial nonuniformities in the photolysis beam and the use of optically thick ceric nitrate solutions, effects that lead to large gradients in [NO,] through the monitoring zone. The data base for NO, optical spectra has been critically reviewed by Hug.*, Since Stevens' results were never published in the open literature, the recommended 635-nm extinction coefficient is that reported by Glass and Martin. Our value is a factor of 3.3 higher. We believe that our determination of tcp is much less susceptible to systematic errors than either previous measurement, and should be considered the best available value. II. The NO3 Appearance Rate. Stevens and Martin', have reported that NO, does not appear instantaneously following 347-nm pulsed laser photolysis of ceric nitrate. In 6 M HNO,, they observed a rise time for NO, absorption of 12.9 ps. Martin and Glass have provided evidence that there is no radical precursor to NO, in the ceric nitrate photolysis system.I' Thus, Stevens and Martin concluded that a metastable excited-state ceric nitrate species must exist, i.e. Ce"N0,CeN03*
+ hv
-
-
Cel"
CeN03*
(la)
+ NO,
(1b)
with klb = 7.8 X lo4 s-l in 6 M H N 0 3 . Since the existence of a long-lived excited-state ceric nitrate species is an interesting photochemical phenomenon, we decided to reinvestigate the kinetics of NO3 appearance. A 6 M HNO, solution was used as the solvent in all experiments. As discussed in the Experimental Section, the apparatus was configured so the overall time resolution was limited predominantly by the transit time of the monitoring beam through the multipass optical system (about 2 ns/pass). Initially, we employed 532 nm as the photolysis wavelength. The appearance of NO, was found to be instantaneous on the time scale of our experiments, i.e. klb > I x 10~s-I. It is possible that absorption at 532 nm produces a different excited state of ceric nitrate than that produced at 347 nm, the excitation wavelength employed by Stevens and Martin.', To investigate this possibility, we repeated the experiment using 355-nm (third harmonic, Nd:YAG laser) photolysis. The 355-nm photolysis experiments were difficult in our apparatus because ceric nitrate has such as huge extinction coefficient at 355 nm (-2 X lo4 M-' cm-' in 6 M HNO,) that, in order to avoid total absorption in the front part of the reactor, we had to work with very low ceric M). High laser fluences and nitrate concentrations (- 1 X a long absorption path length (42 passes) were required to obtain measurable levels of NO,. Nevertheless, it was possible to obtain
The Journal of Physical Chemistry, Vol. 92, No. 5, 1988 1161 0.85
-
TABLE 11: Summary of NO3 Kinetic Data Obtained in the Absence of Added Reactantsec
v * ~ ~ * w s
-P -e
0.80
~~~~-,..:~:---~.-L-L.-.--.. 0.70-
I
I
I
1.o
0.0040
3.0
0.0040
I
1.o 1.o 0.25 1.o 1.o 1.o 0.25 1.o 1.o 0.25 0.5 1 .o
ii
6.0
0.0010
6.0
0.0040
'6.0
0.0040
6.0
0.0070
110-8
0
r
c m
P
d
e
5 n
a
time(ms)
Figure 5. Typical plots of log (absorbance) versus time for data obtained in the NO3decay kinetics studies. Experimental conditions: 6 M HNO,; 0.004 M Ce(1V); N2saturated; 1 Hz repetition rate; 64 flashes averaged; absorption path length = 57 cm; laser fluence = (a) 5.6 and (b) 1.6 mJ cm-2, Solid lines are obtained from linear least-squareanalyses and give the first-order decay rates shown in the figure.
good quality data; a typical absorbance temporal profile is shown in Figure 4. As was the case with 532-nm excitation, the 355-nm data show that k l b> 1 X lo7 s-'. It seems likely that the slow NO3 appearance rate reported by Stevens and Martin was the result of an experimental artifact such as photomultiplier saturation or an unidentified slow-response component in their detection electronics. 111. NO3 Decay Kinetics. The kinetics of NO3 removal in aqueous nitric acid solution have been studied by a number of investigators. In pulsed radiolysis studies the background NO3 decay rate has been found to be lo4 s-1.7915*25,26 Neta and Huie have suggested that reaction with radiolytically generated NOz In ceric nitrate flash photolysis is the dominant decay mechani~m.~ s t ~ d i e s ~background * l ~ . ~ ~ NO, decay rates around lo3 8' have been reported. Further NO3decay rates are reviewed in ref 22. Martin and c o - w ~ r k e r s generated ~~'~ very large concentrations of NO, and Ce(II1) (M). Under their experimental conditions, NO, removal was found to be nearly second order and was attributed to reactions 2 and 6. Dogliotti and Hayonlo generated somewhat lower concentrations of NO, and Ce(II1) (- lo4 M). They reported that background NO3 decays were first order, although their conclusion is open to question since they followed the NO3 decay for less than 1 half-life. (25) Daniels, M. J . Phys. Chem. 1969, 73, 3710. (26) Kozlowska-Milner, E.: Broszkiewicz, R. K. Radial. Phys. Chem. 1978, 1 1 , 253.
1.o 1 .o 1.o 1.o 1.o 1.o 0.25 1.o 1.o 1.o 1.o 1.o 1.o 0.25 1.o 1.o 1.o 1.o 1.o 1.o 1.o 1.o 1.o 0.25 1.o 1.o 1.o 1.o
0.0113 54 0.0163 46 0.0117 61 0.0132 53 0.0483 66 0.0140 67 0.0438 66 0.0494 68 0.0149 67 0.0465 68 0.0235 71 0.0499 74 0.0465 120 0.0356 86 0.0082 82 0.0297 88 0.0292 87 0.0140 81 0.0270 87 0.0273 98 0.0733 59 0.0217 57 0.0720 62 0.0725 70 0.0210 170 0.0624 182 0.0746 178 0.0683 176 0.0181 181 0.0371 179 0.1280 129 0.1295 137 0.1155 310 0.1674 214 0.0363 204 0.0896 211 0.1567 235 0.1650 238 0.0352 ,223 0.0913 223
'Experiments that were run consecutively on the same liter of solution are grouped together. bThe absorption path length was 57.2 cm in all experiments. CThephotolysis wavelength was 532 nm in all experiments. dSG 0 saturating gas. eLRR laser repetition rate. /A,,, 1 f
base 10 absorbance at a time shortly after the photolysis laser fired when NO3production was complete but no NO, decay had occurred. gk' 3 first order NO, decay rate. hSame as preceding run except circlating solution flashed for 15 min at 1 Hz,full laser power, between runs. 'Static solution (Le. not circulated) with reactor isolated from reservoir. 'Same as preceding run except solution flashed for 1 h at 1 Hz,full laser power, between runs. kSame as preceding run except 0, bubbled through solution for an additional 10 min between runs.
In our experiments, NO, concentrations were 1-3 orders of magnitude lower than those employed in all previous work, while total concentrations of photolytically or radiolytically generated reactants were at least 2 orders of magnitude lower than in all previous studies. As a result, we observe background NO, removal rates that are much slower than those reported previously. Typical plots of log (absorbance) versus time observed in our experiments are shown in Figure 5 . The data in Figure 5 show that NO, decays via a strictly first-order process, the rate of which is independent of photolysis laser power. All kinetic data obtained in the absence of added reactants is summarized in Table 11. Our results demonstrate that the NO3 decay rate increases with increasing [ H N 0 3 ] , [Ce'"], and total photolysis dose, but is independent of laser photon fluence, photolysis repetition rate, and oxygen concentration. The kinetic results described above suggest that a minor impurity, which builds up as the solutions are exposed to light, is reponsible for NO, removal (light absorption is enhanced when [HNO,] and/or [Ce'"] are larger). A likely candidate for the
Wine et al.
1162 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
TABLE 111: Comparison of Our Kinetic Results for the NO3 + Ce"' Reaction with Results of Other Investigators ' 6 M HNO,
P
4-
investigators
Martin, Rummel, and
technigue'
[HNO,], M
FP-A
6.0
17.0 f 0.4
FP-A FP-A
0.22'
3.7 f 0.1 IO 6.41 f 0.10 11 7.44 f 0.15 9.17 f 0.09 10.1 0.1 11.1 f 0.2 12.7 f 0.2 14.4 f 0.1 17.5 f 0.1 19.8 f 0.5 21.7 f 0.5 24.1 f 0.5 25.5 f 0.7 36.1 f 0.7 40.7 f 1.2 41.3 f 1.3 39.8 f 0.6 35.9 f 0.5 15 f 3 27 15 f 3
k2,6 l o 5 ref
M-l s-1
9
Gross
Dogliotti and Hayon Martin and Glass
2
0
4
[Ce(111)l(10-3M)
Figure 6. Kinetic data for the NO3 + Ce"' reaction. The pseudo-
first-order NO3 decay rate is plotted versus the Ce(II1) concentration. Solid lines are obtained from linear least-squares analyses and give the following bimolecular rate coefficients (units are lo5 M-' s-' , emors are 2u and represent precision only): 1 M HN03,4.34 f 0.14; 3 M HN03, 7.73 f 0.13; 6 M HN03, 13.7 f 0.3. background reactant is Ce(II1). Some Ce(II1) is expected to be present even in freshly made CeIV/HN03solutions and, clearly, the concentration of Ce(II1) should increase as the solution is exposed to light.I2 For this reason, we decided to investigate the kinetics of the NO, + Ce"' reaction NO,
+ Cetl'
-
Ce"N0,-
(2) Our results are shown in Figure 6 and compared with previous measurements of k2 in Table 111. We observe that k2 increases from 4.34 X lo5 M-'S-' in 1.0 M HN03 to 1.37 X lo6 M-ls-l in 6.0 M HNO,. This trend agrees with the trend observed by Martin and Glass, who hypothesized that the reactive species is not bare Ce(II1) but rather a series of Cet''(N03)x complexes, each of which reacts with NO, at a different rate." Our rate constants are consistently about 25% lower than those reported by Martin and Glass. In 6 M H N 0 3 , Ce(II1) concentrations of 1 X IO4 M would be required to account for the NO3 decay rates M Ce(1V). It seems unlikely that Ce(II1) observed in 4.0 X impurity levels of several percent were present in the Ce(IV) solutions. Hence, we conclude that a significant fraction of NO3 removal must be occurring via a process other than reaction 2. Further experimentation will be required to identify this process. It should be pointed out that trace organics not removed by the water purification system or leached from the viton components in the flow system cannot be ruled out as background reactants, nor can trace metal ion impurities in the H N 0 3 and/or ceric nitrate samples. The experiments reported in this paper employed very low NO3 concentrations. Hence, our experimental conditions were not favorable for measuring the rate coefficient for the reaction NO, NO, N206 (6)
+
-
Nevertheless, our results do allow a meaningful upper limit for k6 to be established. The highest NO3concentrations employed M. We conservain our kinetic measurements were -3 X tively estimate that reaction 6 contributed less than 100 s-l to the NO3 decay at short times after the photolysis pulse. Hence, our results suggest that 2k6 < 6 X lo7 M-' s-l , i .e. 2k6/€ < 7 x los cm s-'. Three measurements of 2k6/e are reported in the literature; the reported values are 2.8 X 106,253.16 X 103,12and 1.3 X lo6 cm s-1.27 Our data are consistent only with the low value reported (27) Pikaev, A. K.; Sibirskaya, G . K.; Shirshov, E. M.; Glazunov, P.Ya.; Spitsyn, V. I . Dokl. Akad. Nauk SSSR 1974, 215, 645.
1.o 2.0 2.5 3.0 3.5 4.5 5.0 6.0 6.5 7.0 1.5 8.0 10.0
12.0
Pikaev, Sibirskaya Shirshov, Glazunov and Spitsyn Wine, Mauldin, and Thorn
PR-A LFPLPA
13.0 14.0 15.0 2 5 1 .o 3.0 6.0
*
4.34 f 0.14 this 7.73 f 0.13 work 13.7 f 0.3
FP, flash photolysis; A, time-resolved absorption; PR, pulsed radiolysis; LFP, laser flash photolysis; LPA, longpath time-resolved absorption. Errors are those reported by the investigators and typically represent precision only. c O . l M K2Ce(N0& in HzO; pH 0.65 due to hydrolysis. by Martin and Glass.I2 The higher values for 2k6/t were both determined in pulsed radiolysis s t u d i e ~ ? ~It~appears *~ that reaction of NO3 with another radiolytically generated radical was mistaken for reaction 6 in these studies.
Summary A laser flash photolysis-long-path absorption apparatus has been employed to investigate the kinetics and spectroscopy of NO,(aq) produced via photolysis of cerium ammonium nitratenitric acid solutions. Selective production of NO3and the improved sensitivity of the long-path detection scheme (compared to single-pass absorption detection) allow kinetics studies to be carried out with considerably lower concentrations of photolytically or radiolytically generated reactants than all previous studies of N03(aq). This results in the observation of much longer NO3 lifetimes than previously observed. A new higher value for the N03(aq) extinction coefficient has been obtained. Also, the rate of NO3 appearance following the photolysis pulse has been found to be at least 100 times more rapid than previously thought. Rate coefficients for the NO3 + Ce"' reaction have been measured in 1, 3, and 6 M HNO,; the results agree reasonably well with previous studies of NO3 + Ce"' kinetics. The rate coefficient for NO3- reaction has been measured in 0.1 M perchloric the SO, acid and found to be considerably slower than previous measurements indicated.
+
Acknowledgment. We thank Dr. Y. Tang for assisting with a few of the experiments. We also thank J. M. Nicovich for his help in designing the reaction cell and for other helpful discussions. This work was supported by the Electric Power Research Institute through Contract No. RP2023-6. Registry No. NO,, 12033-49-7; Ce(NH&(N03),, 10139-51-2; HN03, 7697-37-2; Ce4*, 16065-90-0; NO3-, 14797-55-8; SO,-, 1214345-2.
