A comparison between the determination of free Pb(II) by two techniques: Absence of gradients and Nernstian equilibrium stripping and resin titration

Abstract

Absence of gradients and Nernstian equilibrium stripping (AGNES) is an emerging electroanalytical technique designed to measure free metal ion concentration. The practical implementation of AGNES requires a critical selection of the deposition time, which can be drastically reduced if the contribution of the complexes is properly taken into account. The resin titration (RT) is a competition method based on the sorption of metal ions on a complexing resin. The competitor here considered is the resin Chelex 100 whose sorbing properties towards Pb(II) are well known. The RT is a consolidated technique especially suitable to perform an intercomparison with AGNES, due to its independent physicochemical nature. Two different ligands for Pb(II) complexation have been analyzed here: nitrilotriacetic acid (NTA) and pyridinedicarboxylic acid (PDCA). The complex PbNTA is practically inert in the diffusion layer, so, for ordinary deposition potentials, its contribution is almost negligible; however, at potentials more negative than -0.8 V vs. Ag/AgCl the complex dissociates on the electrodic surface giving rise to a second wave in techniques such as normal pulse polarography. The complex Pb-PDCA is partially labile, so that its contribution can be estimated from an expression of the lability degree of the complex. These new strategies allow us to reduce the deposition time. The free Pb(II) concentrations obtained by AGNES and by RT are in full agreement for both systems here considered. The main advantage of the use of AGNES in these systems lies in the reduction of the time of the experiment, while RT can be applied to non-amalgamating elements and offers the possibility of simultaneous determinations.

Determination of Free Metal Ion Concentrations Using Screen‐Printed Electrodes and AGNES with the Charge as Response Function

“…The gains used during this study are low for ex- 3 for HMDE). The deposition times t 1 could be reduced, taking advantage of the presence of labile oxalate complexes [20] which contribute to the flux, lasting 400 s for SPE and 200 s for HMDE, irrespective of the aimed Y. Moreover, the experimental results were compared with the theoretical ones of MINTEQ [3].…”

The use of microelectrodes with AGNES

“…We have performed a titration by adding different concentrations of PDCA into a solution that contains a total Pb(II) concentration of 1.82 · 10 À6 M and a MES concentration of 10 À2 M to fix the value of the pH to 6.1. We used the two-pulse (2P) strategy for the experiments with HMDE, where the deposition time is split into a diffusion limited substage during t 1,a seconds and an The gain Y was selected so that the blank was practically negligible in front of the measured current, while the characteristic times t 1,a and t 1,b were found by a bisection method [15], consisting in seeking that there is no significant variation in the measured current when the equilibration time (t 1,b ) is enlarged.…”

Comparison of AGNES (absence of gradients and Nernstian equilibrium stripping) and SSCP (scanned stripping chronopotentiometry) for trace metal speciation analysis

“…Recent applications of AGNES include the measurement of free zinc in seawater [21], the use of microelectrodes [22], the study of Purified Aldrich Humic Acid complexation with cadmium and zinc [23], and the comparison of AGNES with the Resin Titration technique [24]. The concentration gain associated with E 1,a is denoted by Y 1,a and that associated with E 1,b by Y 1,b .…”