A new electrochemical method to study the distribution of weak acids at the aqueous|organic solution interface

The distribution process of a weak acid, HA, between an aqueous and an organic solution, W and O, was assumed to be the transfers of the dissociation products, H+ and A−, between W and O combined with the associations of H+ and A− in W and O. The transfers were considered to proceed depending on Gibbs energies for the transfer of H+ and A− from W to O, ΔGtr,H+o and ΔGtr,A−o, and maintaining electroneutrality in both W and O. Then, a theoretical equation was derived for quantitative expressions of distribution ratios, D, of species relative to HA between W and O at various pH of W by using ΔGtr,H+o and ΔGtr,A−o and acid dissociation constants of HA in W and O, Ka,W and Ka,O. The D calculated by substituting Ka,W,Ka,O,ΔGtr,H+o and ΔGtr,A−o determined with the aid of electrochemical methods for those in the derived equation agreed well with D determined by the distribution experiments with W of wide pH range. The HA investigated were chelating agents such as 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone, 1-phenyl-3-methyl-4-trifluoroacetyl-5-pyrazolone and thenoyltrifluoroacetone.