UV spectroscopic determination of the chlorine monoxide (ClO) / chlorine peroxide (ClOOCl) thermal equilibrium constant

Klobas, J. E. and Wilmouth, D. M.: UV spectroscopic determination of the chlorine monoxide (ClO) ∕ chlorine peroxide (ClOOCl) thermal equilibrium constant, Atmos. Chem. Phys., 19, 6205–6215, https://doi.org/10.5194/acp-19-6205-2019, 2019.

The thermal equilibrium constant between the chlorine monoxide radical (ClO) and its dimer, chlorine peroxide (ClOOCl), was determined as a function of temperature between 228–301K in a discharge flow apparatus using broadband UV absorption spectroscopy. A third law fit of the equilibrium values determined from the experimental data provides the expression: Keq = 2.16×10−27 e(8533±25K/T)cm3molecule−1. A second law analysis of the data deviates minimally: Keq = (2.06±1.27)×10−27 e(8546±123K/T)cm3molecule−1. From the slope of the van't Hoff plot in the third law analysis, the enthalpy of formation for ClOOCl is calculated, ∆Hf (298K) = 129.9±0.6kJmol−1. The equilibrium constant results from this study suggest that the uncertainties in Keq recommended in the most recent (year 2015) NASA JPL Data Evaluation can be significantly reduced.