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The complexation of nitriloacetic acid (NTA) with copper and lead was studied by square wave stripping voltammetry (SWSV) using a mercury iridium microelectrode as the working electrode. The results show that NTA interfere with the measurements, but if the mercury iridium microelectrode is coated with different concentrations of agarose (0.75%, 1.5% and 15% w/v) this interference is greatly minimized. The best results were obtained with 1.5% agarose.

The adsorption on a mercury/water interface of a sample of humic acids from an Irish peat has been analyzed by a.c. voltammetry, using a hanging mercury drop electrode in an unstirred and/or stirred medium, at different pH values (pH = 2.5, 5 and 8) and two potential values (E = −0.5 and −0.9 V), in order to have a better insight into the kinetics and equilibrium adsorption which may drastically affect the voltammetric signal in trace metal speciation of natural waters. The results at pH 2.5 present a slower kinetic of adsorption but a higher coverage degree at the equilibrium when compared with pH 5 and/or 8 due to the charge of the molecules. At −0.5 V, close to the potential of zero charge, the kinetics of adsorption is discussed in terms of a diffusion mechanism, but at −0.9 V the experimental curves clearly show that the global adsorption process cannot be explained by a pure diffusion mechanism based on Frumkin isotherm.

In the present work, the applicability of thin mercury film on a rotating disk electrode (TMF-RDE), to assess the free metal ion concentration by the absence of gradients and Nernstian equilibrium stripping (AGNES), is evaluated. The thickness of the mercury film and several AGNES parameters has been optimized. A nominal 16 nm film is chosen due to the higher signal (faradaic current) relative to the value of the noise (capacitive current). Due to the smaller volume to area ratio, the deposition time needed to reach a certain preconcentration factor ( Y ) is much shorter than in larger electrodes, like the HMDE. The limit of detection (3 σ ) for lead(II) is 7.4 × 10 −9 M and 7.2 × 10 −8 M for a Y of 5000 (deposition time of 150 s) and 1000 (deposition time of 100 s), respectively. A specific mathematical treatment is developed in order to subtract a corrected blank taking into account the degradation of the thin film (presumably, falling down of drops). The couple TMF-RDE/AGNES is successfully applied for speciation purposes in the systems Pb(II)–latex nanospheres and Pb(II)–IDA (iminodiacetic acid), where the stability constants calculated for both systems agree with values reported in the literature.

Rigorous computation of the metal flux crossing a limiting surface of a system that contains a mixture of 1 : 1 metal complexes under steady-state planar diffusion in a finite domain and under excess of ligand conditions predicts, for some cases, an enhancement of the metal flux with respect to that expected in a system with independent complexes. Indeed, the coupling of the dissociation kinetics of both complexes can yield higher metal fluxes than expected with important environmental implications. By using the voltammetric techniques AGNES and stripping chronopotentiometry, this paper provides experimental evidence of this enhancement for two systems: Cd/NTA/glycine and Cd/NTA/citric acid. The flux measured in both cases is in good agreement with the flux computed for the global system, exhibiting maximum enhancement ratios above 20%. Theoretical discussion of the flux enhancement factors and of the conditions for this enhancement are also provided.

The heterogeneity of natural organic ligands in the presence of a metal ion affects its concentration profile in the diffusion layer. In fact, owing to heterogeneity, the ratio of complexed metal to free metal increases in the diffusion layer with proximity to the electrode surface. The diffusing species are then decelerated because the diffusion coefficient of the macromolecular complex is lower than that of the metal ion, and so the total amount of reduced metal is decreased compared with that obtained in a homogeneous system with the same bulk speciation. Therefore, the voltammetric current, which is directly related to the concentration profile in the diffusion layer, is affected by heterogeneity. In the work reported in this paper the concentration profile in the diffusion layer was determined on the basis of the diffusion process and a heterogeneity model describing the concentration of the different complexing sites and their complexing affinity for the metal. The mean stability parameters for the complexation of natural organic ligands with a metal ion were determined from this profile, simulating the value that should be determined from voltammetric currents. The difference between the simulated value and the mean stability parameter related to bulk speciation was analysed.