Solubility and solution-phase chemistry of isocyanic acid, methyl isocyanate, and cyanogen halides
<p>Condensed-phase uptake and reaction are important atmospheric removal processes for reduced nitrogen species, isocyanic acid (HNCO), methyl isocyanate (<span class="inline-formula">CH<sub>3</sub>NCO</span>), and cyanogen halides (XCN, X <span...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-04-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/19/4419/2019/acp-19-4419-2019.pdf |
| Summary: | <p>Condensed-phase uptake and reaction are important atmospheric removal
processes for reduced nitrogen species, isocyanic acid (HNCO), methyl
isocyanate (<span class="inline-formula">CH<sub>3</sub>NCO</span>), and cyanogen halides (XCN, X <span class="inline-formula">=</span> 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 <span class="inline-formula">CH<sub>3</sub>NCO</span>, or, like the XCN species, have the potential to be
a significant condensed-phase source of <span class="inline-formula">NCO<sup>−</sup></span> 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 (<span class="inline-formula">N<sub>r</sub></span>) detection. The aqueous solubility of HNCO was
measured as a function of pH and had an intrinsic Henry's law solubility of
20 (<span class="inline-formula">±2</span>) M atm<span class="inline-formula"><sup>−1</sup></span> and a <span class="inline-formula"><i>K</i><sub>a</sub></span> of
2.0 (<span class="inline-formula">±0.3</span>) <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>−4</sup></span> M (<span class="inline-formula"><i>p</i><i>K</i><sub>a</sub></span> <span class="inline-formula">=</span> <span class="inline-formula">3.7±0.1</span>)
at 298 K. The temperature dependence of HNCO solubility was very similar to
other small nitrogen-containing compounds, such as HCN, acetonitrile
(<span class="inline-formula">CH<sub>3</sub>CN</span>), and nitromethane, and the dependence on salt concentration
exhibited the “salting out” phenomenon. The rate constant of reaction of
HNCO with 0.45 M <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="97c709e7ff43e05afce356dc3f53b497"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-4419-2019-ie00001.svg" width="24pt" height="15pt" src="acp-19-4419-2019-ie00001.png"/></svg:svg></span></span>, as <span class="inline-formula">NH<sub>4</sub>Cl</span>, was measured at
pH <span class="inline-formula">=</span> 3 and found to be 1.2 (<span class="inline-formula">±0.1</span>) <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>−3</sup></span> M<span class="inline-formula"><sup>−1</sup></span> s<span class="inline-formula"><sup>−1</sup></span>, faster than the rate that
would be estimated from rate measurements at much higher pHs. The
solubilities of HNCO in the non-polar solvents <span class="inline-formula"><i>n</i></span>-octanol
(<span class="inline-formula"><i>n</i></span>-<span class="inline-formula">C<sub>8</sub>H<sub>17</sub>OH</span>) and tridecane (<span class="inline-formula">C<sub>13</sub>H<sub>28</sub></span>) were found to be
higher than aqueous solution for <span class="inline-formula"><i>n</i></span>-octanol (<span class="inline-formula">87±9</span> M atm<span class="inline-formula"><sup>−1</sup></span> at
298 K) and much lower than aqueous solution for tridecane (<span class="inline-formula">1.7±0.17</span> M atm<span class="inline-formula"><sup>−1</sup></span> at 298 K), features that have implications for
multi-phase and membrane transport of HNCO. The first-order loss rate of HNCO
in <span class="inline-formula"><i>n</i></span>-octanol was determined to be relatively slow, 5.7 (<span class="inline-formula">±1.4</span>) <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>−5</sup></span> s<span class="inline-formula"><sup>−1</sup></span>. The aqueous solubility of
<span class="inline-formula">CH<sub>3</sub>NCO</span> was found to be 1.3 (<span class="inline-formula">±0.13</span>) M atm<span class="inline-formula"><sup>−1</sup></span> independent
of pH, and <span class="inline-formula">CH<sub>3</sub>NCO</span> solubility in <span class="inline-formula"><i>n</i></span>-octanol was also determined at
several temperatures and ranged from 4.0 (<span class="inline-formula">±0.5</span>) M atm<span class="inline-formula"><sup>−1</sup></span> at 298 K
to 2.8 (<span class="inline-formula">±0.3</span>) M atm<span class="inline-formula"><sup>−1</sup></span> at 310 K. The aqueous hydrolysis of
<span class="inline-formula">CH<sub>3</sub>NCO</span> was observed to be slightly acid-catalyzed, in agreement
with literature values, and reactions with <span class="inline-formula"><i>n</i></span>-octanol ranged from 2.5 (<span class="inline-formula">±0.5</span>) to 5.3 (<span class="inline-formula">±0.7</span>) <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>−3</sup></span> s<span class="inline-formula"><sup>−1</sup></span> 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 (<span class="inline-formula">±0.2</span>) M atm<span class="inline-formula"><sup>−1</sup></span> for ClCN and 8.2 (<span class="inline-formula">±0.8</span>) M atm<span class="inline-formula"><sup>−1</sup></span> for BrCN to
270 (<span class="inline-formula">±54</span>) M atm<span class="inline-formula"><sup>−1</sup></span> for ICN. Hydrolysis rates, where measurable,
were in agreement with literature values. The atmospheric loss rates of HNCO,
<span class="inline-formula">CH<sub>3</sub>NCO</span>, 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 <span class="inline-formula">CH<sub>3</sub>NCO</span> 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.</p> |
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| ISSN: | 1680-7316 1680-7324 |