Kinetics of the oxidation of ammonia and amines with hydroxyl radicals in the aqueous phase

Abstract

While many studies have reported on the oxidation kinetics of ammonia and amines with the hydroxyl radical (OH) in the gas phase, the analogous reactions in the aqueous phase have not been adequately studied. In this work, the reaction rate constants of ammonia, dimethylamine (DMA) and diethylamine (DEA) with hydroxyl radicals in the aqueous phase were investigated using ion chromatography. The neutral and protonated forms of each base were shown to have differing rate constants with OH by performing the measurements over a range of pH from 7.0 to 11.0. Excess hydrogen peroxide was used as the precursor for hydroxyl radicals, while monochloroacetic acid and benzoic acid were chosen as the reference compounds for the relative rate method. The rate constants of both protonated forms and neutral forms were calculated for DMA ((9.5 ± 1.2) ×10⁶ M⁻¹ s⁻¹ and (3.3 ± 0.2) ×10⁹ M⁻¹ s⁻¹) and DEA ((1.5 ± 0.4) × 10⁸ M⁻¹ s⁻¹ and (4.9 ± 0.1) × 10⁹ M⁻¹ s⁻¹) using the relative rate method. The rate constant of ammonium ion and neutral ammonia were calculated to be (2.3 ± 0.5) × 10⁶ M⁻¹ s⁻¹ and (1.8 ± 0.4) × 10⁸ M⁻¹ s⁻¹, respectively. With a pK_a of 9.25, the rate constant of the protonated form is applicable to the overall rate constant of ammonia at pH <7, indicating that this oxidation pathway is not a significant sink for ammonia in acidic aqueous environments. Because of their partitioning characteristics, oxidation of DMA and DEA by OH in aerosol particles could be competitive with oxidation in the gas phase.