Influence of the settling of the resin beads on diffusion gradients in thin films measurements

Abstract

Binding resin beads used in DGT (diffusion gradients in thin films) tend to settle to one side of the resin during casting. This phenomenon might be relevant for metal accumulation when partially labile complexes dominate the metal speciation, especially after recognizing the important role played by complex dissociation in the resin domain. The influence of the inhomogeneity of the binding agent distribution on metal accumulation is here assessed by numerical simulation of DGT devices with binding beads in only one half of the resin disc, as a reasonable model of the standard resin discs. Results indicate that a decrease in mass accumulation of less than 13% can arise in these inhomogeneous devices (as compared with an ideal disc with homogeneous dispersion of the resin beads) when complexes with stability constant K<10(2)m(3)mol(-1) (K<10(5)Lmol(-1)) dominate the metal speciation. The loss increases as K increases, but the percentage of mass loss always remains lower than the volume fraction of resin disc without beads. For very labile or inert complexes, the impact of the inhomogeneous distribution of binding resin beads is negligible. As kinetic dissociation constants of complexes can be estimated from the distribution of the metal accumulation in a DGT device with a stack of two resin discs, the influence of the inhomogeneity on the recovered kinetic constant is also assessed. For the cases studied, the recovered kinetic dissociation constant, kd,recovered, retains the correct order of magnitude, being related to the true kd by kd≈f(-1)kd,recovered, quite independently of K and kd values, being f the fraction of volume of the resin disc where resin beads are dispersed.

New methodology to measure low free indium (III) concentrations based on the determination of the lability degree of indium complexes. Assessment of In(OH)3 solubility product

“…). As it can be shown that the lability degree is -among others-a function of the free ligand concentration [46], the obtained lability degrees are plotted against the free oxalate concentration in Fig 3 . Given the dispersion of results, the lability degree of indium-oxalate complexes can be considered practically constant (despite some decrease could be predicted for increasing ligand concentration [47]). In principle, ξoxalates is expected to be a function of all conditions, pH included, because -for instance-the relative composition of the mixture of sequential complexes changes with pH (see eqn.…”

Accumulation of Mg to Diffusive Gradients in Thin Films (DGT) Devices: Kinetic and Thermodynamic Effects of the Ionic Strength

“…Additionally, since the thickness of the region where the potential changes is much smaller than the thickness of the resin domain (where the electrostatic potential is mostly constant), a model that summarizes the electrostatic effects in a partitioning factor at the resin-gel interface is also used. The numerical solution of this model has been carried out in Fortran as explained in the SI of 30 . The checking of the accuracy of such model is of interest because it allows finding approximate analytical expressions in some cases 21 .…”

Dynamics of trace metal sorption by an ion-exchange chelating resin described by a mixed intraparticle/film diffusion transport model. The Cd/Chelex case

“…The study of the physicochemical mechanisms that regulate the metal accumulation is essential for the interpretation of the DGT performance in varying environmental conditions (pH, ionic strength, etc.) [17][18][19][20]. However, the mechanisms that govern the kinetics of metal uptake by the free resin beads need further study.…”