J. Phys. Chem. 1992,96,9301-9304 2. CSTR. ( a ) Pumps. The pump system consisted of two identical Bishoff HPLC pumps. The flow rates could be adjusted over a range of 10 p L m i d to 5 mL m i d . (6)Reactor. The body of the reactor consisted of a quartz cylinder closed at both ends with Teflon blocks. The lower Teflon block contained the injectors, while the upper block was slightly conical to enable escape of bubbles. A micromagnetic stirrer was placed in the cylinder. The speed of stirring was controlled by a stepper motor. The setup was placed in a thennostated chamber inside a diodearray spectrophotometer (HP 8451). The internal volume of the reactor was 2.1 mL. The absorbance of the reacting solution was recorded through a 0.5 mm wide calibrated slit, giving an overall optical path length of 1.1 cm. Pure water was used as reference. 3. Kinefics in h t c h Reactor. Measurements were made with the same solutions in the same reactor used as batch or CSTR. One pump fed in a solution of permanganate [KMn04] = 1.09 M and [H2S04]= 0.54 M. The other pump fed in a x solution of oxalic acid (condition I, [H2C204] = 1.82X lo-’ M; condition 11, [H2C204]= 2.73 X M. The solutions were pumped in at the maximum flow rate (5 mL/min for both pumps). The pumps were switched off and the UV/visible spectra d e d . The initial concentrations in the reactor were as follows: [KMnO4Io = 5.45 X lo4 M; [H2S04]o= 0.27 M. I, [H2C204]0 M. = 9.10 X lo-’ M; 11, [H2CZ04]0= 1.37 X 4Det”m ’ tionofSteady§$atesintbecsTRThetwopumps operated at the same flow rate D (mL/min). The residence time 7 (s) is expressed by the relationship 7
= 60 X 2.1/2D
The absorbance at 560 nm was recorded until a steady state was reached. This took up to 207. To describe the mne of bistability, 7 was reduced from the thermodynamic to the flux branch; 7 was then increased from the flux to the thermodynamic branch. The two transitions from one branch to another were not observed at the same values of 7 .
9301
5. Data Treatment. The experimental data (Abs versus f ) w e n transferred from the HP 8451 spectrophotometer to a HP 9OOO Series 330 workstation running under UNIX. The software was written in C. Smoothing and derivation. Both operations were carried out using the Savitsky+olay’s alg~rithm?~ with a smoothing window width of 1 1 points and a third degree polynomial. Registry No. MnO,-, 14333-13-2; oxalic acid, 144-62-7.
References pad Notes (1) Lin, K. F. Can. J. Chem. Eng. 1979,57,476. (2)Lin, K. F. Chem. Eng. Sci. 1981, 36(9), 1447. (3) A typical example is the photolysisof thioxanthone in aerated toluene. Bistabdity was predicted from the accelerating nature of batch kinetics, but in fact monostable behavior was always okperved in a CSTR. (4)Noyes, R. M.; Epstein, I. R. J. Phys. Chem. 1983,87,2700. ( 5 ) Eptein, I. R. J. Phys. Chem. 1984,88, 187. (6)Matsuura, T.; Kato, M. Chem. Eng. Sci. 1967,22, 171. (7)Fazekas, T.;Mrakavova, M.; Nagy, A.; Olexova, A,;Treindl,L. React. Kinet. Catal. Lett. 1990,42, 181. (8) de Kepper, P.; Eptein, I. R.; Kustin, K. J. Am. Chem. Soc. 1981,103, 6121. (9)Brynestad, J.; Smith, G. P. J. Phys. Chem. 1968,72,296. (10)Polster, J.; Lachman, H. Spectrometric Titration; VCH Verlagegesellschaft: Weinheim, Germany, 1989;ISBN 3-527-26436-1. (11) Cohen, M. D.; Fischer, E. J . Chem. Soc. 1962,3044. (12)Coleman, J. S.;Varga, L. P.; Mastin, S.H. Inorg. Chem. 1970,9, 1015. (13) H a W , A. V.; EEson, W. Phifar. Trans.R. Soc. London 1866,156, 193. (14)Launer, H. F. J . Am. Chem. Soc. 1932,54,2597. (15) Malcolm, J. M.; Noyes, R. M. J. Am. Chem. Soc. 1952,74,2769. (16)(a) Scott, S.K. Acc. Chem. Res. 1987,20,186. (b) Gray, P.; Scott, S.K. Ber. Bunsenges. Phys. Chem. 1986,90,985. (17) Ganapathisubramanian, N.J. Phys. Chem. 1988, 92,414-7. (18) Adler, S.J.; Noyes, R. M. J. Am. Chem. Soc. 1955,77,2036-42. (19) Reckley, J. S.;Showalter, K. J. Am. Chem. Sa.1981,103,7012-3. (20)de Kepper, P.; Ouyang, Q.;Dulos, E. In Nonequifibrium Dynamics in Chemical Systems; Vidal, C., Pacault, A., E&; Springer: New York, 1984; PP 44-9. (21) Ainsworth, S.J . Phys. Chem. 1961,65, 1968. (22)Hugus, Jr., 2.2.;El-Awadi, A. A. J . Phys. Chem. 1971,75,2954. (23) Savitzky, A.; Golay, M. J. E. Anal. Chem. 1984,36, 1627.
Rate Constants for the Reactions of the Hydroxyl Radical with Several Partially Fluorinated Etherst Z. Zhang, R. D.Saini, M. J. Kurylo, and R. E. H i e * Chemical Kinetics and Thermodynamics Division, National Institute of Standards and Technology. Gaithersburg, Maryland 20899 (Received: April 8, 1992: In Final Form: August 17, 1992)
We have measured rate constants for the reactions of the hydroxyl radical, OH, with a number of fluorinated ethers. These ethers and their calculated atmospheric lifetimes for removal by OH (in years), estimated relative to CH3CC13,are as follows: CF3-0-CH3, 3.0; CF3-O-CHF2, 19; CHFz-O-CHF2, 2.6; CFpCH2+CHp, 0.1; CFoCHz-O-CHF2, 5.2; CyClOCFZCHFCF2-0, 26;~yclo-(CF2)3-0,>330.
Iatroduction Because of the concerns regarding the role of chlorofluorocarbons (CFCs) in ozone depletion and greenhouse warming, partially fluorinated ethers are now being considered as CFC replacements in industrial applications. Since these compounds contain neither chlorine nor bromine, they do not contribute to ozone depletion, but still they must be evaluated as possible greenhouse gases. The assessment of the greenhouse warming potential of any compound requires an accurate determination of its atmospheric lifetime, which, for many species, is dictated largely by the rate of reaction with the hydroxyl (OH) radical in the troposphere. Presently, there is very little experimental information on the OH reactivity of fluorinated ethers. We have Contribution of the National Institute of Standards and Technology.
therefore measwed the rate constantsfor the reactions of OH with a representative group of hydro fluoro ethers and have used the results to estimate their tropospheric lifetimes by means of a comparison with the reactivity of CCl3CH3. The rate constant data also provide preliminary information on the effects of fluorine substitution on OH/ether reactivities, which may help in predicting the rate constants for reactions of other hydro fluoro ethers with OH.
Experimental Section Rate measurements were conducted at 296 K by the flash photolysis resonance fluorescence technique.’-3 A pyrex reaction cell (100 cm3 volume) was used in the flow mode. Reaction mixtures containing the hydro fluoro ether, H20, and Ar were prepared manometrically in glass storage bulbs and slowly flowed
This article not subject to US.Copyright. Published 1992 by the American Chemical Society
Zhang et al.
9302 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 TABLE I: Rate Colpt.ntS for the Rmction~of OH rsith P8&8& ether CF,-OCHa CF;-OCHF2 CHF2-OCHF2 CF3CH2-OCH3 CF3CH2CHF2 cyclo-CF2CHFCF2-O cycb(CF2)p-O a Approximate tropospheric
h h t d Ethers 8t 2% K k, cm3 molecule-' S-I
P
3.3 k v
4.6 kV (2.14 f 0.15) X (3.38 0.4ij x 10-15 (2.53 f 0.24) X (6.24 i 0.67) X lo-" (1.25 f 0.09) X (2.51 f 0.35) X lo-''
