The dissociated and nondissociated species of any organic acid differ largely in their physicochemical behavior. The ratio of both species in aqueous systems is determined by the respective pKa value. For perfluorooctanoic acid recent fate-modeling studies have applied a pKa value of 2.8. This value likely is too high by 3 log units. Here, the correct value is estimated to be close to -0.5 based on analogy considerations and molecular modeling. Calculated pKa values for other highly fluorinated carboxylic acids are also presented.
