Rate constants for aqueous‐phase reactions of SO₄⁻ with C₂F₅C(O)O⁻ and C₃F₇C(O)O⁻ at 298 K

Perfluorocarboxylic acids and their anions (PFCAs), such as perfluorooctanate (C 7 F 15 C(O)O − ), have been generally recognized to be global pollutants and are believed to persist in the environment. Kinetic data for reactions of sulfate anion radicals (SO 4 − ) with PFCAs are needed to evaluate the residence times of PFCAs in the environment, but no kinetic data have been reported, except for the rate constant for the reaction of SO 4 − with trifluoroacetate (CF 3 C(O)O − ) ( k 1 ). In this study, using the fact that PFCAs react with SO 4 − to form shorter chain PFCAs, we determined rates relative to k 1 of the reactions of photolytically generated SO 4 − with two short‐chain PFCAs, pentafluoropropionate (C 2 F 5 C(O)O − ; k 2 ) and heptafluorobutyrate (C 3 F 7 C(O)O − ; k 3 ), along with conversion ratios for conversion of C 2 F 5 C(O)O − into CF 3 C(O)O − ( α ) and conversion of C 3 F 7 C(O)O − into C 2 F 5 C(O)O − (β) and CF 3 C(O)O − ( γ ) at 298 K. Values of k 1 , k 2 , or k 3 might change over the course of reaction with increasing ionic strength. Nevertheless, if the values of k 1 / k 2 , k 2 / k 3 , α , β , and γ remain almost constant during the reaction, a simple equation involving relative rates, such as k 1 / k 2 , can be used to relate the concentrations of C 3 F 7 C(O)O − , C 2 F 5 C(O)O − , and CF 3 C(O)O − . We compared the relative rates, such as k 1 / k 2 , and the conversion ratios determined from various experimental runs with different initial conditions to check whether relative rates and conversion ratios remained almost constant during each experimental run. The values of k 1 / k 2 , k 2 / k 3 , α , β , and γ seemed to remain almost constant, which facilitated determination of k 2 / k 1 = 0.89 ± 0.07, k 3 / k 1 = 0.84 ± 0.08, α = 0.88 ± 0.05, β = 0.75 ± 0.05, and γ = 0.17 ± 0.02.