Reaction of Nitric Oxide with Acetylene and Oxygen: Implications for Denitrification Assays
1997
McKenney, D. J. | Wang, S. W. | Drury, C. F.
Nitric oxide reacts with C₂H₂ at ambient temperature under aerobic conditions with relatively large quantities of O₂ present. We used a gas-flow system at 20°C to determine the extent, if any, of reaction of NO with C₂H₂ under nearly anaerobic conditions with addition of trace (<500 µL L⁻¹) quantities of O₂, and to determine whether the process could account for the apparent C₂H₂ inhibition of NO production in soil. Results showed that reaction occurred only when critical trace quantities of O₂ were present. For example, if [NO] was in the range 0.5 to 2.5 µL L⁻¹, which is typical of concentrations in our soil studies, approximately 200 to 300 µL O₂ L⁻¹ was required before measurable NO loss was observed. The reaction was first order with respect to NO and variable order, becoming zero order, with respect to O₂ when [O₂] exceeded ≈400 µL L⁻¹ and maximum NO depletion rates (≈0.01–0.06 µL L⁻¹ s⁻¹) were obtained. The effect of C₂H₂ on NO production with addition of O₂ (≈20–730 µL L⁻¹) in Brookston clay loam (fine-loamy, mixed, mesic Typic Argiaquoll) and Fox sandy loam (fine-loamy over sandy or sandy-skeletal, mixed, mesic Typic Hapludalf) under conditions similar to the gas-phase studies was also investigated. Depletion of NO in the presence of C₂H₂ vs. [O₂] followed the same pattern with similar rates as the gasphase process. A simplified C₂H₂-catalyzed scheme for the oxidation of NO to NO₂ provides a plausible, partial explanation for these results. Since NO depletion requires only trace amounts of O₂, the results suggest that in many previous studies involving assays of NO or N₂O production based on C₂H₂ inhibition, significant gas-phase reaction may have occurred leading to serious errors of interpretation.
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