Kinetic Studies of Hydroxyl Radicals in Shock Waves. IV. Recombination Rates in Rich Hydrogen—Oxygen Mixtures

Abstract

The rate of disappearance of OH following [OH]_max in shocked H₂–O₂–Ar mixtures has been measured by a refined ultraviolet line absorption technique. Thirty‐one experiments have been done with mixtures of 4% H₂, 1% O₂, 95% Ar; 2% H₂, 0.5% O₂, 97.5% Ar; and 8% H₂, 1% O₂, 91% Ar at initial pressures between 50 and 200 mm Hg. The approximate rate equation found to describe the OH disappearance is $\mathrm{-d[}\mathrm{OH}{\mathrm{]/dt=k}}_{\mathrm{apparent}}[\mathrm{OH}{]}^2\mathrm{[M]}$ , where [M] is total gas concentration. Assuming that the equilibrium relationship $\mathrm{\alpha =[}\mathrm{OH}\mathrm{]/[}\mathrm{H}\mathrm{]=K[}{\mathrm{H}}_2\mathrm{O}\mathrm{]/[}{\mathrm{H}}_2]$ is maintained, and that the recombination reactions are: $$\mathrm{H}+\mathrm{H}\mathrm{+M\to }{\displaystyle {lim}_{k_1}}{\mathrm{H}}_2\mathrm{+M,}$$ and $$\mathrm{H}+\mathrm{OH}\mathrm{+M\to }{\displaystyle {lim}_{k_2}}{\mathrm{H}}_2\mathrm{O}\mathrm{+M,}$$ k_apparent may be interpreted as (A/α) (k₁+αk₂) where A is a factor near unity which incorporates the definitions of the k's as volumetric rate coefficients, the small variation of α and density during the reaction, and the participation of O and O₂. (k₁+αk₂) increased linearly with α between α=0.01 and α=0.1, yielding k₁=6±1×10⁸ liter² mole^—2·sec^—1 and k₂/k₁≈10±5. No dependence on temperature was evident. Estimation of the contributions of H₂, H, and H₂O as third bodies indicates that k₁^Ar=4±2×10⁸ liter² mole^—2·sec^—1 near 1700°K.