Solubility and Solution-phase Chemistry of Isocyanic Acid, Methyl Isocyanate, and Cyanogen Halides

Abstract

<p><strong>Abstract.</strong> Condensed phase uptake and reaction are import atmospheric removal processes for reduced nitrogen species, isocyanic acid (HNCO), methyl isocyanate (CH<sub>3</sub>NCO) and cyanogen halides (XCN, X =Cl, Br, I), yet many of the fundamental quantities that govern this chemistry have not been measured or are understudied. Solubilities and first-order reaction rate of these species were measured for a variety of solutions using a bubble flow reactor method with total reactive nitrogen (N<sub>r</sub>) detection. The aqueous solubility of HNCO was measured as a function of pH, and exhibited the classic behavior of a weak acid, with an intrinsic Henry's law solubility of 20 (±2) M/atm, and a Ka of 2.0 (±0.28) × 10<sup>−4</sup> M (which corresponds to pKa = 3.7 ± 0.06) at 298 K. The temperature dependence of HNCO solubility was very similar to other small nitrogen-containing compounds and the dependence on salt concentration exhibited the “salting out” phenomenon that was also similar to small polar molecules. The rate constant of reaction of HNCO with 0.45 M NH<sub>4</sub><sup>+</sup> was measured at pH = 3, and found to be 1.2 (±0.1) × 10<sup>−3</sup> M<sup>−1</sup> sec<sup>−1</sup>, which is much faster than the rate that would be estimated from rate measurements at much higher pHs, and the assumption that the mechanism is solely by reaction of the un-dissociated acid with NH<sub>3</sub>. The solubilities of HNCO in the non-polar solvents n-octanol (n-C<sub>8</sub>H<sub>17</sub>OH) and tridecane (C<sub>13</sub>H<sub>28</sub>) were found to be higher than aqueous solution for n-octanol (87 ± 3 M/atm at 298 K) and much lower than aqueous solution for tridecane (1.7 ± 0.17 M/atm at 298 K), but the first-order loss rate of HNCO in n-octanol was determined to be relatively slow 5.7 (±1.4) × 10<sup>−5</sup>sec<sup>−1</sup>. The aqueous solubility of CH<sub>3</sub>NCO was measured at several pHs and found to be 1.3 (±0.13) M/atm independent of pH, and CH3NCO solubility in n-octanol was also determined at several temperatures and ranged from 4.0 (±0.5) to 2.8 (±0.3) M/atm. The aqueous hydrolysis of CH3NCO was observed to be slightly acid-catalyzed, in agreement with literature values, and reactions with n-octanol ranged from 2.5 (±0.5) to 5.3 (±0.7) × 10<sup>−3</sup> sec<sup>−1</sup> from 298 to 310 K. The aqueous solubilities of XCN was determined at room temperature and neutral pH were found to increase with halogen atom polarizability from 1.4 (±0.2) M/atm for ClCN, 8.2 (±0.8) M/atm for BrCN, to 270 (±41) M/atm for ICN. Hydrolysis rates where measurable, were in agreement with literature values. The atmospheric loss rates of HNCO, CH<sub>3</sub>NCO, and XCN due to heterogeneous processes are estimated from solubilities and reaction rates. Lifetimes of HNCO range from about 1 day against deposition to neutral pH surfaces in the boundary layer, but otherwise can be as long as several months in the mid troposphere. The loss of CH<sub>3</sub>NCO due to aqueous phase processes is estimated to be slower than, or comparable to, the lifetime against OH reaction (3 months). The loss of XCNs due to aqueous uptake are estimated to range from quite slow, lifetime of 2–6 months or more for ClCN, 1 week to 6 months for BrCN, to 1 to 10 days for ICN. These characteristic times are shorter than photolysis lifetimes for ClCN, and BrCN, implying that heterogeneous chemistry will be the controlling factor in their atmospheric removal. In contrast, the photolysis of ICN is estimated to be faster than heterogeneous loss for average mid-latitude conditions.</p>

Surface Wetness as an Unexpected Control on Forest Exchange of Volatile Organic Acids

“…In contrast to the alkanoic acids, simple weak acid equilibria suggest that HNCO will remain dominantly in the gas phase. However, HNCO hydrolyzes through multiple pathways including reaction with aqueous ammonia (Borduas et al, 2016; Leslie et al, 2019; Roberts & Liu, 2019) that are not considered by Equation —thus, leading to an overestimate of the gas phase fraction. However, some of these reactions are permanent sinks for HNCO and could not lead to equilibrium partitioning back out of surface wetness.…”

“…Temperature influences gas‐water partitioning. The K wa of HNCO is particularly temperature sensitive (Roberts & Liu, 2019). For example, a temperature range of −10–30°C implies a range of 21–3.0% for the aqueous HNCO fraction (pH 8).…”

“…Figure S8 plots the natural logarithm of gas phase HNCO mixing ratio against inverse temperature, with the slope providing Δ H obs in kJ mol −1 . Wet and dry periods at MEFO yield distinct enthalpies, which we plot against literature values for Δ H solvation (Roberts & Liu, 2019; Sander, 2015) in Figure S9. Observed enthalpies agree with literature values during dry periods but become more enthalpically favorable during wet periods.…”

Sources of isocyanic acid (HNCO) Indoors: A focus on cigarette smoke

“…In addition to temperature, partitioning of HNCO on indoor surfaces may also be affected by a change in pH. 51,52 Similarly, a surface reservoir for nitrous acid (HONO) that undergoes gas-surface partitioning was required to explain gas-phase HONO mixing ratios in this residence. 31 Evidence for gas-surface partitioning of organic compounds, such as formaldehyde, 53 has also been identified indoors.…”