Electrochemical evaluation of the distribution of a metal ion at the aqueous∣organic solution interface in chelate extraction

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 physicochemical constants determined electrochemically such as standard Gibbs energies for transfers of H+, A− and Mn+ from W to O, ΔOWGtr,H+∘, ΔOWGtr,A-∘ and ΔOWGtr,Mn+∘, the overall complex formation constants of Mn+ with A− in W and O, βn,W and βn,O, and the acid dissociation constants of HA in W and O, Ka,W and Ka,O. Electrochemical methods adopted were voltammetry for the transfer of H+ or A− at the W∣O interface (for ΔOWGtr,H+∘ or ΔOWGtr,A-∘), that for the transfer of Mn+ or H+ facilitated by A− in O (for βn,O or Ka,O), anodic stripping voltammetry at a hanging mercury drop electrode (for βn,W), and acid–base titration in W (for Ka,W). The ΔOWGtr,Mn+∘ was determined by using ΔOWGtr,H+∘ 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.