Solubility and Solution-phase Chemistry of Isocyanic Acid, Methyl Isocyanate, and Cyanogen Halides
Authors: Roberts James M. , Liu Yong 
Published: November 2018

Abstract. Condensed phase uptake and reaction are import 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 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 (Nr) 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) × 10−4 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 NH4+ was measured at pH = 3, and found to be 1.2 (±0.1) × 10−3 M−1 sec−1, 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 NH3. 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 ± 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) × 10−5sec−1. The aqueous solubility of CH3NCO 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) × 10−3 sec−1 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, 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 mid 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 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.