A new method for measuring carbonate radical reactivity toward pesticides

The 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 sigma(para)(+) values, the rate constants of substituted anilines at pH 9.0 correlated well (R-2 = 0.98) with the Hammett equation yielding rho = -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 10(7)/M/s. Fluometuron and atrazine were of intermediate reactivity, and methyl parathion was one of the least reactive.