Solubility and solution-phase chemistry of isocyanic acid, methyl isocyanate, and cyanogen halides
Authors: Roberts James M. , Liu Yong 
Published: April 2019

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.