Determination of Acidity Constants of Perfluoroalkanoic Acids

Abstract

Acidity constants (Ka) of perfluoroalkanoic acids were determined by pH titration and by electric conductivity for shorter alkanoic acids (C1 to C5) with 1 to 5 carbon atoms in the alkyl chain. The acidity constants obtained by the two methods are in good agreement. They increase from C1 (ethanoic) to C3 (butanoic acid) and then decrease from C3 to C5 (hexanoic acid) with increasing alkyl chain length. An abrupt decrease in the Ka value for the C5 acid occurred; the attempt to explain this in terms of viscosity of the solutions did not succeeded. The decrease in the Ka values with increasing alkyl chain length was substantiated by the values for C9 (decanoic) to C11 (dodecanoic) acids, which were determined from the solubility change with the solution pH. For the intermediate alkanoic acids (C6 to C8), the Ka values could not be determined precisely by these methods due to the extreme difficulty in separating the colloidal acid particles from the aqueous phase, because the particles have an emulsifying action of their own.

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

“…Similarly, we estimate P 1 to be less than 1.4% (pH 2.34) or 4.7% (pH 1.78) for pK a = 0.47 [13]. We estimate P 3 to be less than 1.7% (pH 2.25) or 4.8% (pH 1.78) for pK a < 0.48 [14]. Using data for CH 3 C(O)O − and CH 3 C(O)OH, we estimate that the rate constant for the reaction of SO 4 − with C n F 2n+1 C(O)OH is much smaller than the rate constant of SO 4 − with C n F 2n+1 C(O)O − (the rate constant for reaction of SO 4 − with CH 3 C(O)OH (2.5 × 10 5 mol −1 dm −3 s −1 ) at 298 K is two orders of magnitude smaller than the rate constant for reaction of SO 4 − with CH 3 C(O)O − (2.8 × 10 7 mol −1 dm [6].…”

“…(17) and pK a ≤ 0.38 [14], we estimate Q 2 to be greater than 91 for pH 2.34 or greater than 25 for pH 1.78. From Eq.…”

“…The literature values of pK a for equilibria (14), (13), and (15) at 298 K are 0.47 [13], 0.29-0.38 [14], and 0.36-0.48 [14], respectively. Panel A in Fig.…”

“…That is, we evaluate the decay rates of CF 3 Table III, runs 15, 17, and 19). Pseudo-first-order decay rate constants (k pseudo ) were obtained in a way similar to that described for runs [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The values of k pseudo obtained are listed in Table III.…”

Perfluoroalkyl Compounds

“…It should be noted, however, that trifluoroacetic acid (TFA, 1 carbon atom) is also a common and wellstudied PFCA [2,3]. Similar to the PFSAs, the PFCAs also have small pKa values, typically 2-3 [4,5] and are dissociated at environmental pH values:…”