Solubility and solution-phase chemistry of isocyanic acid, methyl isocyanate, and cyanogen halides

Abstract

Abstract. Condensed-phase uptake and reaction are important atmospheric removal processes for reduced nitrogen species, isocyanic acid (HNCO), methyl isocyanate (CH3NCO), and cyanogen halides (XCN, X = Cl, Br, I); yet many of the fundamental quantities that govern this chemistry have not been measured or are not well studied. These nitrogen species are of emerging interest in the atmosphere as they have either biomass burning sources, i.e., HNCO and CH3NCO, or, like the XCN species, have the potential to be a significant condensed-phase source of NCO− and therefore HNCO. Solubilities and the first-order reaction rate of these species were measured for a variety of solutions using a bubble flow reactor method with total reactive nitrogen (Nr) detection. The aqueous solubility of HNCO was measured as a function of pH and had an intrinsic Henry's law solubility of 20 (±2) M atm−1 and a Ka of 2.0 (±0.3) × 10−4 M (pKa = 3.7±0.1) at 298 K. The temperature dependence of HNCO solubility was very similar to other small nitrogen-containing compounds, such as HCN, acetonitrile (CH3CN), and nitromethane, and the dependence on salt concentration exhibited the “salting out” phenomenon. The rate constant of reaction of HNCO with 0.45 M NH4+, as NH4Cl, was measured at pH = 3 and found to be 1.2 (±0.1) × 10−3 M−1 s−1, faster than the rate that would be estimated from rate measurements at much higher pHs. The solubilities of HNCO in the non-polar solvents n-octanol (n-C8H17OH) and tridecane (C13H28) were found to be higher than aqueous solution for n-octanol (87±9 M atm−1 at 298 K) and much lower than aqueous solution for tridecane (1.7±0.17 M atm−1 at 298 K), features that have implications for multi-phase and membrane transport of HNCO. The first-order loss rate of HNCO in n-octanol was determined to be relatively slow, 5.7 (±1.4) × 10−5 s−1. The aqueous solubility of CH3NCO was found to be 1.3 (±0.13) M atm−1 independent of pH, and CH3NCO solubility in n-octanol was also determined at several temperatures and ranged from 4.0 (±0.5) M atm−1 at 298 K to 2.8 (±0.3) M atm−1 at 310 K. 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) × 10−3 s−1 from 298 to 310 K. The aqueous solubilities of XCN, determined at room temperature and neutral pH, were found to increase with halogen atom polarizability from 1.4 (±0.2) M atm−1 for ClCN and 8.2 (±0.8) M atm−1 for BrCN to 270 (±54) M atm−1 for ICN. Hydrolysis rates, where measurable, were in agreement with literature values. The atmospheric loss rates of HNCO, CH3NCO, 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 middle troposphere. The loss of CH3NCO 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 is estimated to range from being quite slow, with a lifetime of 2–6 months or more for ClCN and 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 midlatitude conditions.

Isocyanic acid (HNCO) and its fate in the atmosphere: a review

“…2 We note that similar values were obtained by Roberts et al for atmospherically relevant temperatures and pH. 4 The K a value used for this calculation was 2.1 Â 10 À4 M. At this temperature, the k 1 term dominates the overall rate constant equation at pH values between 0 and 1; while at pH values above 9 the k 3 term dominates. The k 2 term dominates the overall rate constant in the pH range of 3-7.…”

“…4 The Setschenow constant for HNCO is also well predicted with its octanol-water constant, K ow , 96 which was recently measured to be 4.4. 4 Once HNCO is dissolved into an aqueous solution, it can undergo hydrolysis, through three pH dependent reactions, to irreversibly produce NH 3 and CO 2 . These two products may then form ammonium and bicarbonate depending on the pH (R8-R10).…”

“…The lifetime of HNCO with respect to heterogenous uptake and hydrolysis depends strongly on atmospheric parameters such as the liquid water content, temperature, and pH of cloud droplets. 2,4 The air-water phase partitioning of HNCO is thus an important parameter to investigate. Roberts et al reported an intrinsic Henry's law coefficient of 21 M atm À1 , inferred from measurements made at pH ¼ 3.…”

“…Additionally, HNCO exhibits a salting out effect-a decrease in the solubility of a solute with the increase in ionic strength of the solutionat increasing NaCl concentrations at pH 3. 4 Roberts and Liu reported the Setschenow constant, k s , a measure of a solute's solubility as a function of salt content, of HNCO to be 0.097 AE 0.0011 M À1 from a series of solubility experiments at varying ionic strengths. 4 The Setschenow constant for HNCO is also well predicted with its octanol-water constant, K ow , 96 which was recently measured to be 4.4.…”

“…4 Roberts and Liu reported the Setschenow constant, k s , a measure of a solute's solubility as a function of salt content, of HNCO to be 0.097 AE 0.0011 M À1 from a series of solubility experiments at varying ionic strengths. 4 The Setschenow constant for HNCO is also well predicted with its octanol-water constant, K ow , 96 which was recently measured to be 4.4. 4 Once HNCO is dissolved into an aqueous solution, it can undergo hydrolysis, through three pH dependent reactions, to irreversibly produce NH 3 and CO 2 .…”