Microelectrochemical measurements at expanding droplets (MEMED): investigation of cupric ion stripping kinetics in a two-phase oil/water system

Microelectrochemical measurements at expanding droplets (MEMED) are used as a new approach for investigating the stripping kinetics of metals ions between an organic phase containing an extractant ligand, which complexes the metal ion of interest, and an aqueous acid solution. The approach is illustrated with measurements of the stripping of cupric ion from 1,2-dichloroethane (DCE) droplets containing CuL2 (where L is the oxime ligand, Acorga P50) that are formed periodically through a capillary tip submerged in an aqueous receptor phase containing HCl. First-order dependences of the stripping rate constants on the bulk concentration of CuL2 in the DCE phase (5–20 mmol dm−3) and HCl in the aqueous phase (0.25–2.0 mol dm−3) were found, with an effective bimolecular rate constant of (6.2±1.0)×10−5 cm s−1 M−1. The results obtained demonstrate that the cupric ion stripping process is likely to be heterogeneous, occurring at the immiscible liquid/liquid interface, with the first attack of protons on CuL2 as the rate-limiting step. The prospects for using MEMED more generally to investigate metal extraction/stripping kinetics are discussed.