A shock tube study of the enthalpy of formation of OH

Abstract

The standard enthalpy, of formation of the hydroxyl radical (OH) at 298 K, ΔfH0298(OH), has been determined from shock tube measurements spanning the tmeperature range 1964–2718 K and at pressures of 1–2.4 atm. Low-concentration, lean and stoichiometric mixtures of H2 and O2 in Ar produce well-controlled leels of OH in a “partial equilibrium” state, with little or no sensitivity to the reaction kinetics. the partial equilibrium OH concentrations are dependent only on the thermochemical parameters of the reacting species, with the heat of formation of OH being the most significant and uncertain parameter. Narrow line width UV laser absorption at 306.7 nm is used to measure OH concentrations with sufficient accuracy (2%–4%) to clearly determine the value of the enthalpy of formation. Over the whole range of experimental conditions, the average determination is ΔfH0298(OH)=8.92±0.16 kcal/mol (37.3±0.67 kJ/mol) with a standard deviation of σ=0.04 kcal/mol (0.17 kJ/mol). This value is 0.40–0.48 kcal/mol (1.7–2.0 kJ/mol) below the previously accepted values, and it agrees with recent theoretical calculations, experimental studies using the positive-ion cycle, and calculations using thermochemical cycles.