The distribution ratio, D, of a metal ion, Mn+, between an aqueous solution, W, and an organic solution, O, in solvent extraction with a chelating agent, HA, was evaluated by using physiochemical constants determined electrochemically such as standard Gibbs energies for transfers of H+,A(-), and Mn+, form W to O, Delta(W)(O) G(tr,H+)degrees, Delta(W)(O) G(tr, Mn+)degrees, the overall complex formation constants of Mn+ with A(-) in W an O, beta(n,W) and beta(n,O), and the acid dissociation constants of HA in W and O, K-a,K-W and K-a,K-O. Electrochemical methods adopted were voltammetry for the transfer of H+ or A(-) at the W vertical bar O interface (for Delta(W)(O) G(tr,H+)degrees or Delta(W)(O)G(tr,A-)degrees), that for the transfer of Mn+ or H+ facilitated by A(-) in O (for beta(n,O) or K-a,K-O), anodic stripping voltammetry ata hanging mercury drop electrode (for beta(n,W)), and acid-base titration in W (for K-a,K-W). The Delta(W)(O)G(tr,Mn+)degrees was determined by using Delta(W)(O)G(tr,H+)degrees and distribution ratios of Mn+ and H+ obtained by shaking W and O in the presence of Mn+ and H+. The D thus evaluated agreed well with those determined by the distribution experiment. Cu2+ was adopted as an example of Mn+, and thenoyltrifluoroacetone, 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone or 1-phenyl-3-methyl-4-trifluoroacetyl-5-pyrazolone as HA. The O used was 1,2-dichloroethane. (c) 2007 Elsevier B.V. All rights reserved.