J . Phys. Chem. 1992, 96, 7674-7679
7674
of the TICT states in the excited states in polar solvents shifted the fluorescence maxima to longer wavelengths, decreased the fluorescence yields, and increased the fluorescence decay time. The increase of the contribution of the TICT state in the excited state decreased the ring-closure quantum yield.
Acknowledgment. We express our gratitude to K. Uchida of Kyushu University, and Y.Horikawa, R. Sumiya, and M. Hanazawa of Kanebo Co.Ltd. for valuable discussion. We also thank T. Minami of Horiba Co.Ltd. for the measurement of fluorescence decay times. This was supported in part by a Grant-in-Aid for Scientific Research on New Program (03NW301) from the Ministry of Education, Science, and Culture in Japan. References and Notes (1) Heller, H. G. IEE Proc. 1983, 130-1, 209. (2) Wilson, A. E. J. Phys. Technol. 1985, 15,232. (3) Lenoble, C; Becker, R. S . J. Phys. Chem. 1986,90, 2651. (4) hie, M.; Mori, M. J. Org. Chem. 1988, 53, 803. (5) Tamura, N.; Asai. N.; Seto, J. Bull. Chem. SOC.Jpn. 1989,62,358. ( 6 ) Suzuki, H.; Tomoda, A.; Ishizuka, M.; Kaneko, A.; Furui, M.; Matsushima, R. Bull. Chem. s&. Jpn. 1989,62,3968. (7) Parthenopoulos, D. A.; Rentzepis, P. M. Science 1989,245, 843. (8) Irie, M.Jpn. J. Appl. Phys. 1989,28-3, 215.
(9) Yokoyama, Y.; Iwai, T.; Kera, N.; Hitomi, I.; Kurita, Y. Chem. Lett. 1990, 263.
(IO) Uchida, K.; Nakayama, Y . ;hie, M. Bull. Chem. Soc. Jpn. 1990,63, ,,,, IJII. ’(I 1) Kurita, S.; Kashiwagi, A.; Kurita, Y; Miyasaka, H.; Mataga, N. Chem. Phvs. Lett. 1990. 171. 553. (12) Ulrich, K.; Port; H.; Wolf, H. C.; Wonner, J.; Effenberger, F.; Ilge, H.-D. Chem. Phys. 1991, 154, 311. (13) Yokoyama, Y.; Yamane, T.; Kurita, Y. J. Chem. Soc.. Chem. Commum 1991, ii22. (14) Hanazawa, M.; Sumiya, R.; Horikawa, Y.; hie, M. J. Chem. Soc., Chem. Commun. 1992,206. (15) Lippert, E. Z . Naturforsch. 1955,AlO, 541. (16) Mataga, N.;Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956,29, 465. (17) The radius was estimated from the molecular model. (18) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986,25, 971. (19) Hicks, J. M.; Vandersall, M.; Babarogic, Z.; Eisenthal, K. 8. Chem. Phys. Lett. 1985, 116, 18. (20)Van Damme, M.;Hofkens, J.; De Schryver, F. C.; Ryan, T. G.; Rettig, W.; Klock, A. Tetrahedron 1989, 45, 4693. (21) Majumdar, D.;Sen, R.; Bhattacharyya, K.; Bahattachayya, S . P. J. Phvs. Chem. 1991. 95.4324. 122) Tominaga,’K.;’Walker, G. C.; Jarzeba, W.; Barbara, P. F. J. Phys. Chem. 1991,95, 10475. (23)Nakamura, S.; Irie, M. J. Org. Chem. 1988, 53, 613. (24) Frederick. J. H.: Fuiiwara. Y.: Penn. J. H.: Yoshihara. K.: Petek. H. J. Phys. Chem. 1991, 95, 2845.
Heterogeneous Interactions of CIONOp and HCI on Nitric Acid Trihydrate at 202 K J. P. D. Abbatt*It and M. J. Molina Department of Earth, Atmospheric and Planetary Sciences and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received: March 31. 1992; In Final Form: June 12, 19921
-
-
Using a low-pressure flow tube coupled to a mass spectrometer, reaction probabilities (7’s)for C10N02 + H 2 0 HOCl + HNO, (1) and C10N02 + HCI C12+ HNO, (2) have been measured on nitric acid trihydrate (NAT) films at 202 K for reactant partial pressures in the lod Torr range. When the water vapor pressure over the NAT film approaches that of ice (PH = 1.5 X lo-’ Torr, H20-richNAT), y I = 0.002 f 0.001 and y2 > 0.2. For lower water partial pressures (PH20 = 2 X lo4 Torr), characteristic of HN03-rich NAT, the y’s decrease by 2 orders of magnitude. For HCl partial pressures of 5 X lod Torr, the experiments indicate that H20-rich NAT films take up HCl in amounts similar to those taken up by water-ice surfaces (-1 X 1015molecules/cm*); HN03-rich NAT films take up 2 orders of magnitude less HCl (1 X lo4 Torr), very much greater uptake by both H20-richand HN03-rich films is observed, indicating that the NAT films melt under these conditions.
Introduction Reactions between reservoir chlorine species on the surfaces of polar stratospheric clouds (PSCs) are now widely believed to be instrumental in the springtime loss of ozone which occurs at polar latitudes. Reactions such as (1) and (2) convert chlorine
CIONOt
+ H20
--
C10N02 + HCl
HOCl
+ HN03
CI2 + HNO,
(1)
(2)
from the reservoir species C1ONO2 and HCl over to C12 and HOCl, which photolyze readily and form chlorine-containing free radicals capable of destroying 0z0ne.l-~ The polar stratospheric clouds which act as catalysts for these reactions are believed to be composed of either nitric acid trihydrate (type I PSC) or water-ice (type I1 PSC). The type I PSCs are more prevalent because the frost point for nitric acid trihydrate (NAT) is higher than that of water-ice. The kinetics of these reactions on solid surfaces of composition similar to the PSCs have been studied by several groups. Molina ‘Resent address: Department of the Geophysical Scienca, The University
of Chicago, 5734 South Ellis Ave., Chicago, IL 60637.
et a1.,4 Tolbert et al.,5 and Leu6 reported that reactions 1 and 2 proceed readily on cold water-ice surfaces. Studies on frozen mixtures of HNO, and H 2 0by Tolbert et al.,5, Moore et al.,’ and Leu et a1.* showed that the observed reactivity can be a strong function of the sptcifc compsition of the HN03-d0pedice. More recently, Hanson and Ravishankara9 have shown that these processes occur readily at stratospheric partial pressures of C10NO2 and HCl on both water ice films and on thin NAT films in coexistence with water-ice. Our understanding of the physical chemistry of these solid substrates has also improved since the discovery of the ozone hole. The thermodynamically stable form of NAT in the stratosphere is determined by the ambient conditions: at a given temperature, fixing the H 2 0 vapor pressure determines the HNO, vapor pressure and hence the composition of the surface layers of the NAT solid.I0J1 When the H 2 0 pressure approaches that of ice, we refer to the NAT solid as “H,O-rich” and when it approaches values characteristic of the coexistence between NAT and nitric acid monohydrate, we refer to it as “HN03-rich”. Abbatt and Molina12have shown that there is a large variation in the reaction probability (y) between these two types of NAT for the reaction of HOCl with HCl.
0022-3654/92/2096-7674$03.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, NO. 19, 1992 1615
Interactions of ClON0, and HCl on NAT H~
I
COOLING OUTLET
-
COOLING JACKET
VACUUM JACKET
PRESSURE
THERMOCOUPLE " SPECTROMETER
Figure 1. Schematic diagram of the experimental apparatus.
Studies of the interactions of gas-phase HC1 with water-ice now indicate that under stratospheric conditions monolayer quantities or less of HCl reside in the surface layers of type I1 PSCS.'~-'~At much higher partial pressures, substantial uptake of HCl from the gas phase occulg as a result of substrate melting.I4 Although bulk solubility measurements of HCl in H20-rich NAT the HCl uptake as a function of the water have been perf~nned,~~J~ vapor pressure of NAT has not been studied so far. We report in this work the first measurements of reaction probabilities of (1) and (2) as a function of the water vapor pressure of the NAT film. By fixing the water vapor pressure, the thermodynamic state and the surface activity of the NAT surface are thus determined. We find that the reaction probabilities show strong dependencies on the type of NAT film with the H20-rich forms being more reactive than HN03-rich forms. Low partial pressures of HCI were used so that melting did not occur. We also report here measurements showing that, at these low HC1 partial pressures, H20-rich NAT surfaces take up substantially more HCl than do HN0,-rich surfaces. Experimental Section The experimental apparatus (Figure 1) consists of a 2.4 cm i.d. flow tube which is connected to a quadrupole mass spectrometer (Extrel EXM-280) used to detect the concentration of gas-phase species and to a roughing pump (Edwards E2M80) used to attain high velocities of the He buffer gas (up to 1750 cm/s at 1 Torr). The flow tube is surrounded by an inner jacket through which flows cold ethanol and by an outer jacket which is evacuated for thermal insulation. The flow tube has a jacketed injector (1 .O cm 0.d.) kept warm by flowing a room temperature solution of ethylene glycol in water, a second, sealed injector (0.3 cm 0.d.) which houses a thermocouple, and a third, open injector (0.3 cm 0.d.). The c l c d source ionizer of the mass spectrometer is directly coupled to the flow tube via a 8 cm long, 0.16 cm i.d. glass-lined stainless steel tube. Typically, in addition to pumping through valve 1, a small part of the flow is pumped via valve 2 (seeFigure 1). This arrangement allows for more rapid conditioning of surfaces along the sampling line leading to the narrow sampling tube. To prepare a NAT surface, an ice film was initially deposited on the cold walls of the flow tube by flowing H 2 0 in 12 sccm of He through the warmed injector and by pulling the injector to increasingly upstream flow tube positions over a period of 10-15 min. Ice films approximately 10 Fm thick and with 50-100 cm2 geometric area were formed by operating at 0.5 Torr total pressure of He, with flow velocities of 1000 cm/s and temperatures of 202 K. The film was then exposed to HNO, in the gas phase at partial pressures of (5-1 0) X 10-5 Torr in order to form a NAT film a few tenths of a micrometer thick. This is an easy method of NAT film generation which does not require the precise coaddition of HzO and HNO, in amounts corresponding to the stoichiometry of NAT. Initially, the HNO, (as monitored by the mass spectrometer at mass 46)is taken up by the ice film, but, after a period of 30-60 min, the film becomes saturated with H N 0 3 and the water vapor pressure simultaneouslydrops to values a factor of 5-10 lower than that of pure water-ice film. By addition of small amounts of water vapor to the flow to establish the HzO partial pressure the composition of the surface layers of the NAT film could be altered: measurements of both the H 2 0and HNO,. vapor pressures over the film, along a 202 K isotherm, are similar to
those reported by Hanson and Mauersberger'O of NAT crystals. The slope of a plot of log (HN03) versus log (HzO) is close to -3 which, with the Duhem-Margules equation, confirms the identity of the NAT film. Reaction probabilities were obtained from first-order rate constants for reactant loss or product growth which had been corrected for diffusion effects according to the approach detailed by Brown18 and utilized by other workers (see, for example, refs 4, 6-9, and 13). The ClONO, diffusion coefficient employed in the calculation of this correction was 179 cmz/s at 1 Torr and 202 K, as calculated using the procedure outlined in Hirahfelder et al.I9 Flow velocities ranged from 150 to 1700 cm/s, He buffer gas pressures ranged from 0.5 to 1.0 Torr, and temperatures were 202 f 1 K. For 7's (defined relative to the number of collisions with the flow tube wall) with values of 0.02 this correction for loss of plug flow conditions is small (=lo%). For 7's of 0.2 where the loss rate is strongly diffusion limited, the correction can be as large as a factor of 4. The uncertainties in the calculation of the 7's increase significantly for large 7's (>0.1) which are very sensitive to the diffusion coefficient and the observed first-order rate constant. On the other hand, it is possible that the effects of porosity and surface roughness, which are not evaluated in this work, may significantly affect observed decay rates for small Y ' S . ~ ClONO, was synthesized by the reaction of FCl with a slight excess of HNO, at temperatures between 195 and 253 Ka2' The sample was distilled at 195 K to separate the excess HNO, and was then purified by pumping at 195 K until the vapor pressure was 0.8 Torr, close to the literature value.21 The C10N02samples contained at most a few percent of C12which was observed initially in the mass spectrum at mass 70 with a vapor pressure ~ 3 0 % of the C10N02 pressure. To deliver C10N02 to the flow tube, 1-5 sccm of He flowed through a trap containing ClONO, at 195 K. Diluted with 10 sccm of He, this flow passed through a 10.4 cm long UV absorption cell prior to passing through a needle valve and entering the flow tube through the unjacketed injector. Typical detection limits in the UV absorption system at 220 nm (uC10N02= 3.44 X cm2)22were 3 X lo-, Torr which, upon dilution, corresponded to a pressure of 1 X Torr in the flow tube. The mass spectrometer detection limit of C10N02 (by detection of the NO2+fragment ion at mass 46) was determined by the magnitude of NO2+background signal arising both from Torr) and from the the flow tube and sampling line (=1 X H N 0 3 vapor pressure of the NAT film. The mass spectrometer Torr. HC1 detection limit was 3 X H 2 0 signals were calibrated with the vapor pressure of an ice film at 202 K. HC1 and HNO, were delivered to the flow system from mixtures in He (0.1-1%). Their pressures were determined by observing the pressure rise in the flow system upon their addition ( i l X l P 3Torr). HCl was obtained from Matheson (99%) and FC1 from Ozark-Mahoning Co. HNO, was collected from a 3:l solution of H$04 (96%) and HNO, (70%).
Results HCI Uptake Experiments. HCl uptake measurements were performed by first establishing a steady-state flow of HCl in He (10 sccm) through the unjacketed injector pushed in just past the NAT film. The injector was then quickly withdrawn to an upstream location while the mass spectrometer HC1 signal was monitored. Uptake from the gas phase is calculated from the decline and recovery in the HCl signal and from the known flow rates (see, for example, refs 12 and 13). Depending on the HCl partial pressure, the uptakes took one of two general forms. When PHclwas kept below a threshold value, the uptakes were relatively small, corresponding to a monolayer coverage or less (as calculated by assuming the cross sectional area of the HC1 molecule to be 10 A2 and using the geometrical surface area of the film); when PHclwas above the threshold, very large uptakes were observed. Results of a typical experiment in the low P H Cregime ~ are shown as the upper trace in Figure 2: the injector is pulled upstream at 0.45 min, and the HCl signal falls and then later returns to its initial value, indicating that the surface has reached steady state with the HCl in the gas
7676 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 I
I
Abbatt and Molina
I
I
1
(a)
0 L
$
I
0
I
I
I
I
In
‘2
I 0
(b)
1.5-
ob
0
-
I
1
I
4
I
0.5
I
I .5
2
2.5
Time (minutes) Figure 2. Typical HCI uptake profiles on a NAT film at 202 K (see text). Trace a: PHlo= 1.1 X IO-’ Torr, PHCl = 6 X 10” Torr; trace b: PHlo = 1.5 X 10-3 Torr, PHcI = 1.2 X Torr.
0 . 0
E
1Ol2I o-6 i
I O+.
I o l-
~
IO-’J
PHCl ( t o r r 1 Figure 4. HCI surface coverages on a HN0,-rich NAT film at 202 K as a function of PHCl (for P H ~=O5 X lo4 Torr).
0
0
.
0 0 0
0
0.
0
0.
%
0
o 0
0 0 0
0
104O
I
I
I
I
I
0.2
0.4
0.6
0.8
I
I
1.2
I
1.4
I
1.6
T
Ice
1
1.8
Figure 3. HCI surface coverages on a NAT film at 202 K as a function of PHlo (for PHcl = 5 X 10” Torr).
phase. When the injector is returned to its original position at 1.35 min, the HCl desorbes from the surface and is observed as a peak which has approximately the same area as the uptake peak. For a fixed PHClof 5 X 10” Torr, the amount of HCl taken up was dependent on the state of the NAT surface as determined by the partial pressure of H 2 0 over the film: HN03-rich NAT took up only very small amounts of HCl ( ~ Pc”s (for P ~ l o N q= (1-9) X lod Torr, PHcl (4-10) X 10“ Torr, PHc = 0.7-0.95 Tom, velocity = 1400-1750 cm/s). Reaction probabilities determined by (i) C 1 0 N 0 2 decay (open circles) and (ii) CI2 growth (closed circles).
For pure ice films exposed to C10N02 and HCl at partial pressures of approximately 5 X 1 P Torr (with the HCl slightly in excess), 7;s larger than 0.2 were observed for C10N02 loss (as monitored at mass 46). As described above, we believe this represents a measurement of the reaction probability for reaction 2 on a NAT surface in coexistence with ice.
Discussion HCI Uptake Experiments. Previous work on the interaction of HCl vapor with water-ice surfaces has demonstrated that the amount of HCl taken up corresponds to a fraction of a monolayer surface coverage for HC1 pressures below those which characterize the coexistence of liquid HC1 solutions and When the HCl partial pressures are above this level, substantial uptake is observed as a result of melting of the ice film.I4 Qualitatively similar behavior is observed for HC1 uptake on NAT: when pressures are above the level which leads to 10i4-10’s molecules/cm2 surface coverage, very large uptake is observed. By analogy with the HC1/H20 system, we believe that melting occurs at this point to form a liquid HN03/HCl/H20 solution. The observation that the H 2 0 signal rises to levels characteristic of ice at 202 K when large uptake occurs mast likely indicates that the melting has reached the underlying ice film. Interestingly, for a fixed P,,, the threshold for substantial HCl uptake corresponds to the PHclvalue which leads to a partial monolayer coverage of the surface (lOI4-l OiS molecules/cm2); presumably, melting can occur when a large fraction of the surface is covered with HCl. For low, fixed values of PHCI,we believe that HN03-rich surfaces take up less HC1 than HzO-rich NAT because of the reduced chemical potential (Le,, activity) of the surface water molecules. That is, there are fewer Yfreenwater molecules in the surface layers available to interact with the HCl vapor. HN03-rich NAT can, however, take up somewhat larger amounts of HCl if the PHclis raised and PHtois kept constant (see Figure 4). This is compatible with adsorption isotherm models for submonolayer coverages. Our data are similar to those of Hanson and Ravishankara” who reported HCl surface uptakes of (2-3) X lOI4 molecules/cm2 on films believed to consist of coexisting phases of NAT and ice, a value slightly smaller than our measurements for high PH are also in qualitative agreement with the recent work of hu and We LeuI5 who report solubilities of HCl in NAT of 10-5-104 mole fraction for the PHclrange of 10-7-10” Torr. As in our experiments, for much higher PHCl(7 X Torr) these investigators see significantly larger uptakes. Lastly, the observations of Leu et a1.* that the sticking coefficients of HCl on frozen HN03/H20 films decrease from 0.01 to 1 X 10“ as the HNO, content of the fdm changes from 45 to 57 wt % can be compared with our results.
d“
7678 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 Presumably, the films in the Leu et al. study with HNO, compositions less than that of pure NAT (54 wt% HNO,) were composed of H,O-rich NAT/ice mixtures; at the HCI pressures used (>2 X lo4 Torr), HCl will be taken up by ice in large amounts to form a liquid layer. Results from Hanson and Ravishankaral, and from our laboratory indicate large sticking coefficients (>0.2) on such surfaces. For HNO, compositions greater than 54%, presumably corresponding to mixtures of HN0,-rich NAT and nitric acid monohydrate, the sticking coefficients measured by Leu et al. were very small, which is in accord with our results which show that very little HCl is taken up by HN03-rich NAT. However, it is unclear whether surface melting occurred at the milliTorr pressures of HCl used by Leu et al. in which case the sticking coefficients might have been expected to be somewhat larger. Because our experiments were conducted on short time scales (minutes or less) and because solid-phase diffusisn is very slow, the results are only relevant to the interactions of HCl with NAT surface layers and not to interactions with bulk NAT. Thus, they cannot be directly compared to the 0.3 mol % solubility of HCI in NAT reported by Hanson and Mauersberger16 and by Marti et a1.I’ in longer time scale experiments involving both surface and bulk uptake. CIONOz HzO HOCl+ HNOJ. We believe the reaction probability for reaction 1 decreases as PHzOis lowered because the water molecules on the surface layers of HN0,-rich NAT are more tightly bound than in H20-rich NAT and hence less available for reaction with CIONO,. The same tendency for HNO&h films to have reduced reactivities with respect to reaction 1 has been observed previously by both Tolbert at al.5 and Leu et aL8 Tolbert et al. noted that the HOCl product was not observed from frozen mixtures of HNO, and H 2 0 with H 2 0 H N 0 3molar ratios of 1 :2 and 2:1, whereas mixtures of 4.5: 1 did show evidence for reaction. This observation was confirmed and reaction probabilities were measured over a wide range of compositions by Leu et a1.8 For NAT films in coexistence with ice we obtain reaction probabilities on the order of 0.002 both from the results in Figure 5 and from the experiments conducted on pure ice with C10N02 partial pressures of 6 X lod Torr. This value is similar to the results of Leu et aL8 which have been corrected for porosity effects (yi = 8 X lo4 to 1.7 X lo-, for H N 0 3 compositions less than 50 wt % HNOJ and slightly smaller than those of Hanson and Ravishankara (yl = 0.006).9 Because of the manner by which we form the NAT films, we were unable to vary the thickness of the films to investigate the effects of film thickness and porosity on the reaction probabilities. Although previous results from our laboratoryi2and from Hanson and Ravi~hankaral~ indicate at most a weak dependence of measured y’s on the thickness of ice films, Keyser et a1.20have shown a strong dependence for HN03-H20 films somewhat thicker and probably more porous than those used in this work. This dependence is most pronounced for small 7’s where internal diffusion in a porous solid leads to a high number of collisions with the surface; hence our measured y’s may be upper limits to the true reaction probabilities. This effect warrants further study since it may be significant to the small y’s measured for reaction
+
1.
+
-
-
+
ClONOZ HCl C12 HN03. The reaction probabilities for reaction 2 (see Figure 7) show a strong dependence on the H 2 0 vapor pressure of the NAT surface which is similar to that observed earlier in our studies of the reaction between HC1 and HOCl on NAT.I2 In light of the results which show that little HCl is taken up by HN0,-rich forms of NAT, it is reasonable to suggest that reduced surface concentrations of HCl lead to the small y2(s at low PHo. The observations that raising the PHcl on a HN03-rich film feads to larger reaction probabilities supports this model since higher HCl pressures result in higher HCl surface concentrations (see Figure 4). It is also possible, however, that part of the reduction of the reaction probabilities at low PHzois a result of the low sticking coefficient of CIONO, on a HN0,-nch NAT surface.
Abbatt and Molina The results for the reaction probabilities on H20-rich films are in good agreement with those of previous workers. Specifically, the study of Hanson and Ravishankara9 reported ys; larger than 0.3 for reaction 2 on NAT films in coexistence with ice. This is similar to the large 7;s we observe on true NAT films as PHzo approaches the vapor pressure of ice, and on water-ice films which have been contaminated with a thin layer of HN03. Similarly, Leu et a1.8 have measured large 7;s on HN03-H20 films with compositions ranging from 42 to 54 wt % HNO,. We believe that these investigators do not observe a decrease in their reaction probabilities for compositions greater than 54%, which correspond most likely to mixtures of HNO,-rich NAT and nitric acid monohydrate, because at the HCl pressures used in their study ((2-8) X lo4 Torr), substantial uptake of HCl by the NAT surface will have occurred. In fact, our results for experiments on HN03-rich NAT with PH,-I)ssimilar to those used in the Leu et al. study ((1-2) X 10-4 Torr) gave rise to both substantial HCl uptake and reaction probabilities similar to those of Leu et al. (y2 > 0.5). On a water-ice surface, it is likely that the mechanism for reaction 2 involves a two-step process which consists of reaction 1 followed by the reaction HOCl + HCl Clz H20.12J3Large reaction probabilities have been measured for both of these reactions? and they are consistent with the large y measured for the overall process. As has been concluded previously,13 for NAT surfaces such a mechanism probably does not occur: the small y’s for reaction 1 measured in this work and elsewhere are inconsistent with the large y’s for the overall process. Rather, on NAT surfaces the reaction is more likely of a direct nature, probably occurring via an ionic mechanism. Because it has been shown in this work and by Chu and LeuI5 that the HCl surface coverage on NAT is a function of PHcl,to better simulate stratospheric conditions it becomes important to measure yz at HCl partial pressures 1-2 ordm of magnitude lower than those used in this study, in particular as the NAT becomes HN03-rich. Although Hanson and Ravishankara9J3have camed out experiments for NAT in coexistence with water-ice, such experiments have not yet been performed as a function of the H 2 0 vapor presure of NAT. This might be important since NAT exists in the polar stratospherewith compositions intermediatebetween those corresponding to the coexistence of NAT with either ice or nitric acid monohydrate (see, for example, refs 10 and 23).
-
+
Summary
The principal conclusion from this work is that the water vapor pressure of the NAT surface, and so the thermodynamic state of NAT, has a strong influence on the reaction probabilities of reactions 1 and 2 and on the tendency for NAT surfaces to take up HCI. As observed previously for the reaction between HOCl and HCl on NAT,l2 when PHo, the water vapor pressure of the NAT film, approaches that o# ice the reaction probabilities are much higher than when PHzois low. Since NAT can exist over a range of conditions in the wintertime polar stratosphere, these results imply that atmospherically-relevant reaction probabilities should be determined in the laboratory not only by operating with reactant partial pressures typical of the stratosphere but also by characterizing the vapor pressures of the NAT surface. In particular, pure NAT is expected to exhibit reactivities intermediate between those of solid substrates which are coexistence mixtures of either NAT and ice or of NAT and nitric acid monohydrate.
Acknowledgment. This work was supported by the National Science Foundation under grant ATM-9017150 and J. A. acknowledges the receipt of a NSERC (Canada) postdoctoral fellowship. We thank Steve Lloyd for synthesizing the C1ONO2, and L. T. Chu and M.-T. Leu for sending a preprint of their work. References and Notes ( 1 ) Anderson, J. G.; Toohey. D. W.; Brune, W. H. Science 1991,251, 39. ( 2 ) Molina, M. J. Oceonus 1988, 31, 47. (3) Solomon, S. Reo. Geophys. 1988, 26, 131. (4) Molina, M. J.; Tso,T.; Molina, L. T.; Wang, F. C. Y. Science 19fJ7,
238. 1253. .-, . -- - .
( 5 ) Tolbert, M. A.; Rossi, M. J.; Malhotra, R.; Golden, D. M. Science 1987, 238, 1258.
J. Phys. Chem. 1992, 96,1679-1682 (6) Leu, M.-T. Geophys. Res. Lerr. 1988, 15, 17. (7) Moore, S. B., Keyser, L. F.; Leu, M. T.; Turco, R. P.; Smith, R. H. N a m e 1990, 345, 333. (8) Leu, M.-T.; Moore, S. B.; Keyser, L.F. J. Phys. Chem. 1991,95,7763. (9) Hanson, D. R.; Ravishankara, A. R. J. Geophys. Res. 1991,96,5081. (IO) Hanson, D.; Mauersberger, K. Geophys. Res. Leu. 1988, 15, 855. (1 1) Molina, M. J. In CHEMRAWN VII: Chemistry of rhe atmosphere: The impact of global change; Calvert, J. G., Ed.; Blackwell Science: Oxford,
U.K., in press. (12) Abbatt, J. P. D.; Molina, M. J. Geophys. Res. Lerr. 1992, 19, 461. (13) Hanson, D. R.; Ravishankara, A. R. J . Phys. Chem. 1992,96,2682. (14) Abbatt, J. P. D.; Beyer, K. D.; Fucaloro, A. F.; McMahon, J. R.; Wooldridge, P. J.; Zhang, R.; Molina, M. J. J . Geophys. Res., in press. (15) Chu, L. T.; Leu, M.-T. Preprint, 1992. (16) Hanson, D.; Mauersberger, K. Geophys. Res. Lerr. 1988, 15, 1507.
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(17) Marti, J.; Mauersberger. K.; Hanson, D. Geophys. Res. Lerr. 1991, 18, 1861. (18) Brown, R. L. J. Res. Natl. Bur. Stand. (US.)1978, 83. 1. (19) Hirshfelder, J. 0.; Curtiss, C. F.; Bird, R. B. Moleculur Theory of Gases and Liquids; Wiley and Sons: New York, 1954. (20) Keyser, L. F.; Moore, S. B.;Leu, M.-T. J . Phys. Chem. 1991, 95, 5496. (21) Schack, C. J. Inorg. Chem. 1%7,6, 1938. (22) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Molina, M. J.;
Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. “Chemical kinetics and photochemical data for use in stratospheric modeling”; JPL Publ. 90-1, NASA, 1990. (23) Fahey, D. W.; Kelly, K. K.; Ferry, G. V.;Poole, L. R.; Wilson, J. C.; Murphy, D. M.; Loewenstein, M.; Chan, K. R. J. Geophys. Res. 1989, 91, 11,299.
An EPR and EMF Study of Belousov-Zhabotinsky Oscillators: Veratric Acid and Veratraldehyde in a Water-Acetonitrile Medium P. V. Lalitha, R. Ramaswamy, Department of Chemistry, Indian Institute of Technology, Madras 600 036, Tamilnadu, India
Geetha Ramakrishnan, and P. Sambasiva Rao* Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, Madras 600 036, Tamilnadu, India (Received: May 8, 1992) The Belousov-Zhabotinsky oscillatory reaction involving veratric acid and veratraldehyde as substrates in (water + 20% acetonitrile) mixed medium has been investigated using electron paramagnetic resonance (EPR)and potentiometric techniques. The oscillatory characteristics (induction time, total time, and number of oscillations) obtained from these two techniques show a good correlation. EPR is a very reliable and powerful technique to study the chemical oscillations in these substrates.
1. Introduction The Belousov-Zhabotinsky (B-Z) reaction has been studied with various combinations of metal ions (Ce3+,MnZ+,and Fe3+ in the form of ferroin) and In the earlier studies, the come of the B-Z reaction, for several organic substrates, was followed, with Mn(I1) as the metal ion, potentiometrically, using a platinum indicator electrode/ion selective electrode (the EMF method), and spectrophotometrically. In the EMF technique, one measures the ratio of the concentration of the metal ion that oscillates between the two oxidation states. Further, in a detailed study with malic acid substrate in aqueous medium, we obtained a good agreement between electron paramagnetic resonance (EPR) and potentiometric methods.’ The reaction mixture contains manganese (Mn) ions in addition to substrate, iodate, hydrogen peroxide, and sulfuric acid. The oscillations are due to the Mn ions that cycle between Mn(I1) and Mn(II1) oxidation states. The former is a paramagnetic ion (ds) and is EPR active. Hence, one can study the changes in the concentration of Mn(I1) ions using EPR spectroscopy. Experiments using a complexing agent such as tetrasodium pyrophosphate in the malic acid-bromate system with Mn(I1) ions showed evidence for the formation of Mn(II1) ions, when studied by EPR and EMF te~hniques.’~ An EMF study on the veratraldehyde (3,4-dimethoxybenzaldehyde) substrate has been reported in an uncatalyzed system? Later on, Ramaswamy et al? have done a systematic study on the veratraldehydebromate system (both uncatalyzed and ferroin catalyzed) in aqueous medium. Substrates with sufficient solubility in water alone have been investigated so far in the B-Z and B-R (Briggs-Rauscher) reactions. In order to understand the behavior of many substrates that are sparingly soluble in water, a new methodology has been reported, in which mixed media, i.e., water and an organic solvent, have been substituted for water. This technique has widened the
scope of investigation of chemical oscillators, where a number of new substrates have been employed.I1J2 Since veratric acid (3,4-dimethoxybenzoic acid) and veratraldehyde are sparingly soluble in water, we have studied these systems in a mixed medium. No report has been published with a catalyzed system in a mixed medium for these two substrates. Hence, the current work has been undertaken for these two systems by EPR and EMF techniques, using a mixed medium of water and 20% acetonitrile. The two techniques yield a good correlation for the various oscillatory parameters such as induction time, total time, and number of oscillations.
2. Experimental Metbods The potentiometric method was followed as discussed in ref 8. The EPR measurements were carried out in a Varian E- 1 12 EPR spectrometer operating at X-band frequency (v is around 9.5 GHz) having a 1OO-kHzfield modulation and phase sensitive detection to obtain a first derivative EPR signal. Veratric acid (3.4-dimethoxybenzoic acid), veratraldehyde (3,4-dimethoxybenzaldehyde), KBrO,, MnS04, HzS04, and acetonitrile were of AnalaR purity and were used without further purification. Triply distilled water was used for preparing all the solutions. Oscillations were triggered by the addition of potassium bromate to a solution containing all other constituents (substrate, sulfuric acid, and MnS04). Oscillations were followed potentiometrically with a platinum indicator electrode coupled to a saturated calomel electrode. The following procedure was adopted to obtain oscillations from the EPR spectrometer. The mixture containing all other constituents except KBr03 was taken in a Varian E-248 quartz aqueous cell and the EPR spectrum was recorded. The six-line pattern thus obtained was due to Mn(I1) ions with a g value of 2.004 (3) and a hyperfine coupling constant of 94 (3) G. These are typical values for a Mn(I1) ion.I3 The EPR os-
0022-3654f 92f 2096-7619%Q3.QQ f 0 0 1992 American Chemical Society
