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Integrated Microanalytical System Coupling Permeation Liquid Membrane and Voltammetry for Trace Metal Speciation. Theory and Applications

We recently described a mini permeation liquid membrane (miniPLM) cell that allows one to record in real time the complete time evolution of the metal strip concentration during an accumulation experiment, where metals are transported uphill from the sample (source) solution to the strip solution through a thin flat sheet PLM. To correlate our results with a theoretical basis and to further improve this miniPLM cell, we present here a simplified model based on diffusion-limited transport under steady-state conditions. This model takes into account the cell geometry (depth of the strip channel), diffusional parameters of metal species in both source and strip solutions, and the equilibrium constants of metal complexes in the source and strip solutions. Also included in the model are the inert and labile complexes formed by the metal in the source solution with hydrophilic ligands. Analytical expressions have been derived by assuming steady-state conditions. Theoretical calculations show the influence of the source diffusion layer thickness, of the strip channel depth, and of the complexation strength of the strip ligand on both the equilibration time and fluxes. The model predicts that, under conditions where the equilibrium in the source solution is little perturbed, the concentration in the strip solution at the end of the accumulation is directly proportional to the concentration of the free metal ion in the source solution, irrespective of the nature of the hydrophilic complexes. If the transport through the source solution is the limiting step, the concentration of labile complexes can be calculated from the initial flux. The results were found to accurately describe the experimental results obtained with the miniPLM cell for Pb and Cd in noncomplexing media. The distribution coefficient K D for Cd was found to be 13, and under optimal conditions, the source diffusion layer thickness δ so was estimated to be 9.2 ± 0.7 μm.