Simultaneous determination of acid dissociation constants and true partition coefficients by analysis of the apparent partition coefficients. III. The application of the Hammett acid function.

Abstract

Equations for the simultaneous determination of acid dissociation constants (pKa) and true partition coefficients (P) of a very weak base or acid have been derived by applying the Hammett acid functions Ho and H- and by analyzing the acid function dependence of the apparent partition coefficient (Pa). The equations show that Pa-1 is linearly related to ho (ho=10-Ho) and h^ (h-=10-H-) for a base and an acid, respectively. Some experimental systems for bases have been successfully measured using octanol as an organic solvent phase and suitably diluted H2SO4 solution as an aqueous phase, the Ho value being used. The results obtained were reasonably consistent with the literature values. The application of this technique makes it possible to determine log P and pKa simultaneously, the latter values being smaller than 0 for bases and larger than 14 for acids.