UNSPECIFIED. (2002) Investigation of cupric ion extraction kinetics in a two-phase organic/water system using microelectrochemical measurements at expanding droplets (MEMED). JOURNAL OF ELECTROANALYTICAL CHEMISTRY, 538 (Sp. Iss. SI). pp. 277-283. ISSN 0022-0728

Abstract

The extraction kinetics of metal ions from aqueous solution to an organic phase containing an extractant ligand are investigated using microelectrochemical measurements at expanding droplets (MEMED). Specifically, the extraction of cupric ion from an aqueous phase by the oxime ligand, Acorga P50 (HL) in 1,2-dichloroethane (DCE) droplets, is considered. The MEMED approach involves the periodic formation of DCE droplets containing HL, by flowing this organic phase at a slow rate through a fine capillary tip submerged in an aqueous receptor phase. The depletion of cupric ion in the aqueous phase, adjacent to the advancing droplet, is probed amperometrically at a fixed ultramicroelectrode. Time-dependent spatially resolved concentration measurements made in this way could be related to the kinetics of the extraction process, by solving a well-posed mass transport problem. The extraction process is immeasurably slow at pH I in the aqueous bulk phase, but becomes faster with increasing pH, ultimately attaining a transport-controlled rate on the MEMED timescale at pH 4. A first-order dependence of the extraction rate on the bulk concentration of Cu2+ (1.0-6.0 mM) was found, while there was an inverse first-order correlation with proton concentration (pH 2.0-3.0). The dependence of the extraction rate constant on [HL] in the DCE phase was studied in detail. The extraction rate was first-order in [HL] in the range 10-200 mM. A mechanistic model for Cu2+ extraction by HL with the formation of CuL+ at the water I DCE interface as the rate-limiting step, is proposed to explain these observations. (C) 2002 Elsevier Science B.V. All rights reserved.

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