Interfacial phenomena in ion-association extraction kinetics of iron(II) with 1,10-phenanthrolines.

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The role of the interface in ion-association extraction kinetics of iron(II) with 1,10-phenanthroline(phen), 4,7-dimethyl-1,10-phenanthroline(DMP) or 4,7-diphenyl-1,10-phenanthroline(DPP) from sodium salt solutions was investigated by means of a high-speed stirring technique and a stopped-flow method. The initial extraction rates of the ion pairs were proportional to the first order of both iron(II) and the ligand concentrations in all cases studied; it was therefore suggested that the rate-controlling step is the formation of mono-complexes. From analyses using the adsorption constants and the distribution constants of the ligands and the formation-rate constants of the mono-complexes in the aqueous phase it was concluded that the location where the rate-controlling reaction takes place depends on the ligands: a bulk aqueous phase in phen, an interface in DPP, and both of a bulk aqueous phase and an interface in the case of DMP. The extraction rates were apparently reduced by an interfacial adsorption of the ion pairs, which was governed by the hydrophobicity of the ligands and the hydrophilicity of the counter anions.
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