Kinetics of the Reaction of OH Radicals with Hydrogen Iodide Between 225 and 950 K

Reaction OH + HI → I + H2O (1) is an important atmospheric process transforming inactive HI into chemically active iodine atoms. In the present work, the reaction kinetics have been studied in a discharge fast-flow reactor coupled with an electron impact ionization mass spectrometer at nearly 2 Torr total pressure of helium and over a wide temperature range, T = 225–950 K. The reaction rate constant was determined both by a relative rate method (with the OH + Br2 reaction as a reference) and by absolute measurements carried out under pseudo-first order conditions by monitoring the OH consumption kinetics in excess of hydrogen iodide. U-shaped temperature dependence was observed for the reaction rate constant, negative at low temperatures and positive at high temperatures. Recommended expression over the 225–950 K temperature range: k1 = 1.13 × 10−11 exp(354/T) + 6.93 × 10−11 exp(−1010/T) cm3 molecule−1 s−1 or in the form of a modified Arrhenius expression, k1 = 4.2 × 10−12 × (T/298)1.36 exp(666/T) cm3 molecule−1 s−1, with a total estimated uncertainty of 15% at all temperatures. The rate constant data obtained in this study are compared with the results of previous experimental works.