New technique for generating high concentrations of gaseous OH radicals in relative rate measurements
✍ Scribed by L. Chen; S. Kutsuna; K. Tokuhashi; A. Sekiya
- Publisher
- John Wiley and Sons
- Year
- 2003
- Tongue
- English
- Weight
- 127 KB
- Volume
- 35
- Category
- Article
- ISSN
- 0538-8066
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✦ Synopsis
We have developed a technique for generating high concentrations of gaseous OH radicals in a reaction chamber. The technique, which involves the UV photolysis of O 3 in the presence of water vapor, was used in combination with the relative rate method to obtain rate constants for reactions of OH radicals with selected species. A key improvement of the technique is that an O 3 /O 2 (3%) gas mixture is continuously introduced into the reaction chamber, during the UV irradiation period. An important feature is that a high concentration of OH radicals [(0.53-1.2) × 10 11 radicals cm -3 ] can be produced during the irradiation in continuous, steady-state experiment. Using the new technique in conjunction with the relative rate method, we obtained the rate constant for the reaction of CHF 3 (HFC-23) with OH radicals, k 1 . We obtained k 1 (298 K) = (3.32 ± 0.20) × 10 -16 and determined the temperature dependence of k 1 to be (0.48 ± 0.13) × 10 -12 exp[-(2180 ± 100)/T] cm 3 molecule -1 s -1 at 253-328 K using CHF 2 CF 3 (HFC-125) and CHF 2 Cl (HCFC-22) as reference compounds in CHF 3 -reference-H 2 O gas mixtures. The value of k 1 obtained in this study is in agreement with previous measurements of k 1 . This result confirms that our technique for generating OH radicals is suitable for obtaining OH radical reaction rate constants of ∼10 -16 cm 3 molecule -1 s -1 , provided the rate constants do not depend on pressure. In addition, it also needed to examine whether the reactions of sample and reference compound with O 3 interfere the measurement when selecting this technique.