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 DeltaG(tr,H+)(O) and DeltaG(tr,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 DeltaG(tr,H+)(O) and DeltaG(tr,A-)(O) and acid dissociation constants of HA in W and O, K-a,K-W and K-a,K-O. The D calculated by substituting K-a,K-W, K-a,K-O, DeltaG(tr,H+)(O) and DeltaG(tr,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. (C) 2003 Elsevier B.V. All rights reserved.