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Eiceman, G. A. Crit. Rev. Anal. Chem. 1991,22,471. Hill, H. H., Jr.; Siems, W. F.; St. Louis, R. H.; McMinn, D. G. Anal. Chem. 1990,62,12OlA. Eiceman, G. A,; Snyder, A. P.; Blyth, D. A. Int. J. Environ. Anal. Chem. 1990, 38,415. Hagen, D. H.; Markell, C. G.; Schmitt, G. A. Anal. Chim. Acta 1990,236, 157. Markell, C.; Hagen, D. F.; Bunnelle, V. A. LC-GC 1991,9, 332. Markell, C. G.; Hagen, D. F. Extraction of Phenolic Compounds from Water Samples Using Styrene-Divinylbenzene SPE Disks. Proceedings of the US.E P A Seventh Annual Waste Testing and Quality Assurance Symposium; Washington, DC, Jul 8-12, 1991; pp (11)27-(11)37. O’Donnell, A. D.; Anderson, D. R.; Bychowski, J. T.; Markell, C. G.; Hagen, D. F. Evaluation of Liquid/Solid Extraction for the Analysis of Organochlorine Pesticides and PCBs in Typical Ground and Surface Water Matrices. Proceedings of the US.E P A Seventh Annual Waste Testing and Quality Assurance Symposium; Washington, DC, Jul 8-12, 1991; pp (11)182-(11)194. Moye, H. A,; Moore, W. B. A Remote Water Sampler Using Solid-Phase Extraction Disks. Proceedings of the U.S. E P A Seventh Annual Waste Testing and Quality Assurance
(10) (11) (12) (13) (14) (15) (16)
Symposium; Washington, DC, Jul 8-12, 1991; pp (I)245-( I)26 1. Browers, E. R.; Lingemann, J.; Brinkman, U. A. Th. Chromatographia 1990,29, 415. Poziomek, E. J.; Eastwood, D.; Lidberg, R. L.; Gibson, G. Anal. Lett. 1991, 24 (lo), 1913. Spangler, G. E.; Collins, C. I. Anal. Chem. 1975, 47, 393. Eiceman, G. A.; Martinez, P. M.; Fleischer, M. E.; Shoff, D. B.; Harden, C. S.; Snyder, A. P.; Watkins, M. L. Anal. Chem. 1989,61, 1093. Karasek, F. W.; Kim, S. H. Anal. Chem. 1975, 47, 1166. Kim, S. H.; Karasek, F. W. J . Chromatogr. 1982,234, 13. Eiceman, G. A.; Garcia-Gonzalez, L.; Wang, Y.-f.; Pittman, B. Talanta, in press.
Received for review August 29,1991. Revised manuscript received January 22, 1992. Accepted February 7, 1992. Although the information in this paper has been funded in part by the U.S. Environmental Protection Agency under Cooperative Agreements CR814701 and CR818353 with the University of Nevada-Las Vegas, it does not necessarily reflect the views of the Agency and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Atmospheric Chemistry of Hydrofluorocarbon 134a: Fate of the Alkoxy Radical CF,CFHO T. J. Walllngton,” M. D. Hurley, J. C. Ball, and E. W. Kalser
Research Staff, SRL-3083, Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053
rn The atmospheric fate of the alkoxy radical CF3CFH0 produced as a result of the atmospheric photooxidation of hydrofluorocarbon 134a (HFC-134a) in the presence of NO, has been determined using Fourier transform infrared spectroscopy. Decomposition and reaction with O2 are important loss mechanisms for CF,CFHO radicals. Decomposition yields HC(0)F and CF3radicals; reaction with O2yields CF,COF and HOP: CF,CFHO + O2 CF3COF + HOz (reaction 4); CF3CFH0 CF, + HC(0)F (reaction 5). While reaction 5 is near the high-pressure limit at ambient temperature and pressures above 1000 Torr, it shows significant fall off for pressures below 700 Torr. At 2 atm total pressure, reaction 5 is at the high-pressure limit, and the ratio of the rate constants for reactions 4 and 5 is given by k,/k5 (&20%) = 1.58 X lo-%exp(3600/T) cm3molecule-l. Use of this expression enables the relative importance of reactions 4 and 5 in the atmospheric oxidation of HFC-134a to be estimated for a given temperature and oxygen concentration, and hence altitude. At 6-km altitude both loss processes are equally important; decomposition dominates below 6 km.
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Introduction Recognition of the adverse effect of chlorofluorocarbon (CFC) release into the atmosphere has led to an international effort to replace CFCs with environmentally acceptable alternatives (1-3). Hydrofluorocarbon 134a (1,1,1,2-tetrafluoroethane)(HFC-134a) is a potential substitute for CFC-12 used in automotive air conditioning systems. Prior to large-scale industrial use of HFC-l34a, the environmental consequences of its release into the atmosphere need to be considered. To define the environmental impact of the release of HFC-134a four questions need to be addressed. First, “What effect will HFC-134a have on the ozone layer?”. Second, “What effect 1318
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will HFC-134a have on potential global climate change?”. Third, “What are the atmospheric photooxidation products of HFC-l34a?”. And fourth, ”What is the atmospheric chemistry of these photooxidation producta?”. The answer to the first question is straightforward HFC-134a contains no chlorine and therefore will have zero impact on the ozone layer. The second question has been addressed by atmospheric modeling studies ( 4 , 5 ) . The global warming potential of HFC-134a is approximately 1 order of magnitude less than the CFC it will replace, CFC-12 (based upon a 500-year time horizon). The present paper addresses the third and fourth questions. The atmospheric lifetime of HFC-134a is determined by reaction with the OH radical, reaction 1. Recent studies of the kinetics of this reaction (6) have shown that the atmospheric lifetime of HFC-134a is approximately 15 years. The alkyl radical formed in reaction 1reads rapidly (within 1M) under tropospheric conditions with molecular oxygen to give the peroxy radical CF3CFHO2. In the
+ -
CF3CFH2 + OH CFBCFH
+0 2
CFSCFH + H2O
M
CFSCFH02 + M
(1) (2)
atmosphere, this peroxy radical will react with one or more of the following: NO, NO2, HOP, or other alkyl peroxy radicals (RO,). We have recently shown that reaction with NO is rapid (k,= 1.3 x 10-l’ cm3molecule-l s-l) and that it is an important sink for CF3CFH02radicals (7). CFSCFHO2 + NO CFSCFHO + NO2 (3)
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Furthermore, we have shown that, as expected for a reaction of a peroxy radical with NO, the products of reaction 3 are NO2 and the alkoxy radical CF,CFHO (7). In the atmosphere, CF,CFHO radicals will either decompose or react with 02:
0013-936X/92/0926-1318$03.00/0
0 1992 American Chemical Society
- +
CF3CFHO + 02 CFSCFHO
CF3COF + HO2
CF3
(4)
!
Experimental Section Three different reaction vessels were used in the present work. All have been described before (10-12) and are only briefly discussed here. At ambient temperature (297 K) and total pressures of 1000 Torr and below, a 140-L, 2-mlong evacuable Pyrex chamber surrounded by 24 UV fluorescent lamps and interfaced to a Mattson Instruments Inc. Sirius 100 FT-IR spectrometer was used. At ambient temperature and pressures above 1000 Torr, a 0.06-L stainless steel reactor equipped with quartz windows was used. At temperatures other than ambient, a 0.2-L Pyrex reactor was used, which was equipped with a temperature-controlled jacket through which a 50-50 mixture of ethylene glycol and water was circulated. In all experiments, chemical analysis was performed using FT-IR spectroscopy. Mixtures from the stainless steel reactor and the temperature-controlled 0.2-L Pyrex reactor were expanded into the large Pyrex chamber for analysis. To generate the alkoxy radical CF3CFH0, we have chosen to use the self-reaction of the corresponding peroxy radicals (reaction 6a). CF3CFH02 + CF3CFH02 CF3CFH0 + CF3CFH0 + O2 (6a) CF3CFHO2 + CFSCFH02 CF3COF
CFSCFHOH + 02 (6b)
Experiments were performed by the irradiation of mixtures of HFC-134a and C12 in the presence of 15-5650 Torr partial pressure of 02. C12 + hv C1+ C1 (7) C1+ CF3CFH, CF3CFH + HC1 (8) CF3CFH + 0
--
+M
-
CF3CFHO2 + M
(2) In each set of experiments, at least two gas mixtures were prepared, irradiated, and analyzed to check the experimental reproducibility. In all cases, indistinguishable results were obtained from successive experiments. The loss of HFC-134a and the formation of products were monitored using Fourier transform infrared spectroscopy. The path length for the analyzing infrared beam was 26.6 m. The spectrometer wag operated at a resolution of 0.25 cm-l. Infrared spectra were derived from 32 coadded interferograms. Reference spectra were obtained by expanding known volumes of the reference material into the long pathlength cell. Systematic uncertainties associated with quantitative analyses using these reference spectra are estimated to be 99.9% purity) and 0.1-81 Torr chlorine (>99% purity). CF3COFand COF, were obtained from PRC Incorporated at a stated purity of 99%. HC( 0 ) F was prepared from the reaction of benzoyl chloride with dry formic acid and anhydrous potassium fluoride (13). Ultrapure synthetic air or mixtures of ultrapure N2 and O2 were used as diluents. Experiments were performed over the temperature range 261-353 K and at totaJ pressures from 15 to 5650 Torr.
Results Following the irradiation of mixtures of HFC-l34a, Clz, and 02,the observed products were CF3COF, HC(O)F, COF,, and HC1. In addition, an unidentified product was observed that showed infrared features at 1170,1253, and 1291 cm-l. The yield of this product increased as the oxygen pressure decreased, suggesting that the product is related to decomposition products of the CF3CFH0 radical. The yield of the unknown product increased linearly with the difference in yields of HC(0)F and COF2. By comparison of our unknown spectrum with literature spectra (14-16) we tentatively identify the unknown as bis(trifluoromethy1) trioxide, CF3000CF3, formed following the generation of CF302radicals: CF3CFHO CF3 HC(0)F (5) CF3 + O2 + M CF302+ M
--+ + + -
CF3O2 + CF3O2 CF,O CF30 + O2 (9) CF30 + CF302 M CF3000CF3+ M (10) We do not have a spectrum of CF3000CF3in our reference library and are thus not able to positively identify this species. Infrared spectra acquired immediately after 20-9 irradiations of HFC-134a/C12/02mixtures, and after the resulting product mixtures were allowed to stand in the dark for 10-60 min, revealed that a small amount of chemistry was still proceeding in the absence of UV irradiation. This “dark” chemistry will be discussed later. Yields of CF3COFand HC(0)F observed following the C1 atom initiated oxidation of 100 mTorr HFC-134a in the presence of 15 Torr oxygen at 700 Torr total pressure and 297 K are plotted as a function of the HFC-134a loss in Figure 1. Linear least-squares analysis of the data in Figure 1gives product yields (expressed as mole of product formed per mole of HFC-134a consumed) of 84 f 4% and 9 f 2% for HC(0)F and CF,COF. Errors are 2a. In the derivation of accurate product yields, care must be taken to avoid, or correct for, any losses of products. Environ. Scl. Technol., Vol. 26, No. 7, 1992
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1 0
t
I
1
I T
297K
O 00 0 0
0 5 2
. 0
5
1 5
21 0
Ln ( [ C ~ , l , o / [ C ~ , l . )
Flgure 2. Plot of In ([HC(O)F], /[HC(O)F],) versus In ([CD,],J[CD4],) following the irradlatlon of HC[O)F/CD,/CI, mixtures In 700 Torr air. The solid line is a linear least-squares fit.
Possible losses in the present system include photolysis and reaction with C1 atoms. To test for photolysis of HC(O)F, CF3COF,and COF,, mixtures of these species in air were irradiated for 10 min; no photolysis was observed. To test for reaction with C1 atoms, mixtures of HC(O)F, CF3COF,and COF2with molecular chlorine in air at 700 Torr and 297 K were irradiated. There was no observable loss of either CF3COFor CF20 showing that C1 atoms do not react with these species. In contrast, HC(0)F was observed to decay in the presence of C1 atoms. To ascertain the rate of reaction 11 C1+ HC(0)F HC1+ C(0)F (11)
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a relative rate technique was used in which the rate of decay of HC(0)F was measured relative to that of CD4 with both exposed to attack by chlorine atoms. The experimental techniques used have been given before (17) and are not discussed here. Figure 2 shows a plot of the observed decay of HC(0)F as a function of that of CD4 following the irradiation of HC(O)F/CD,/Cl,/air mixtures at 700 Torr and 297 K. Linear least-squares analysis of the data in Figure 2 yields a rate constant ratio, kll/k12 = 0.32 f 0.02 that can be combined with a value of k12 = 6.1 X cm3molecule-l s-l (18)to give kll = (2.0 f 0.2) X cm3 molecule-l s-l. Quoted errors are 2a. C1+ CD4 DC1+ CD3 (12) +
Variation of the initial concentration ratio [CD4]/[HC(O)F] by 1 order of magnitude had no observable effect on kll/k12, suggesting the absence of complicating secondary reactions sometimes encountered in C1 atom relative rate studies of lower reactivity organics (19). The rate constant kll measured in this work was used to correct our measured HC(0)F product yields for loss by reaction with C1 atoms. Corrections ranged from 2 to 30%. For the correction of measured HC(0)F yields at temperatures other than ambient, we have used the rate constant ratio kll/k12 measured at room temperature. In so doing we assume that the activation energies for reactions 11 and 12 are similar. This assumption seems justified in light of the similarity in rate constants for reactions 11and 12 at 297 K and the expected similarity in mechanism (hydrogen atom abstraction). Experiments to determine the yields of CF3COF and HC(0)F were conducted over a wide range of oxygen partial pressures. Measured product yields at 297 K are plotted as a function of the O2 partial pressure in Figure 3. For experiments using O2 partial pressures less than 700 Torr, nitrogen diluent was added to bring the total pressure up to at least 700 Torr. To check for the effect of total pressure on our measured product yields, experiments were performed in which 700 Torr O2was employed 1320
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I
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O
1000
2000
3000
4000
5000
6000
Partial Pressure of 0, (torr)
Flgure 3. Ylelds of HC(0)F and CF,COF as a function of the oxygen partial pressure at 297 K. For experiments using oxygen partial pressures
