A new method for measuring carbonate radical reactivity toward pesticides

Abstract

AbstractThe carbonate radical is produced from the scavenging of the hydroxyl radical by carbonate/bicarbonate and is strongly electrophilic toward electron‐rich compounds such as aromatic amine and sulfur‐containing compounds. Carbonate radical reactivity (CO3⋅−) was measured by setting up a competition between a chemical of known reactivity and a compound with unknown reactivity toward CO3⋅−. The carbonate radical was produced using a novel method whereby KOONO (potassium peroxonitrite) was dissolved into 0.01 NaHCO3 solution initially producing a HO⋅, which was rapidly scavenged by HCO3⋅−, yielding CO3⋅− CO3⋅− production was monitored using a stopped‐flow spectrometer at the absorption band for CO3⋅− of 600 nm. Results using this competition kinetic method indicated the second‐order rate constants for substituted anilines corresponded to reference values with a relative error less than 10%. Using σpara+ values, the rate constants of substituted anilines at pH 9.0 correlated well (R2 = 0.98) with the Hammett equation yielding ρ = −0.89. To measure the reactivity of pesticides toward the carbonate radical, p‐nitroaniline (PNA) was selected as the standard competitor. Of the nine pesticides tested, fenthion was the most reactive, followed by phorate, with rate constants above 107/M/s. Fluometuron and atrazine were of intermediate reactivity, and methyl parathion was one of the least reactive.