The standard enthalpy of formation of the hydroxyl radical (OH) at 298 K, Δ_fH_(298)~0(OH), has been determined from shock tube measurements spanning the temperature range 1964-2718 K and at pressures of 1-2.4 atm. Low-concentration, lean and stoichiometric mixtures of H_2 and O_2 in Ar produce well-controlled levels 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 Δ_fH_(298)~0(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.
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