New technique for generating high concentrations of gaseous OH radicals in relative rate measurements

Abstract 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. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 317–325, 2003