Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) is a recently suggested electroanalytical technique designed for the determination of the free concentration of heavy metals (such as Zn, Cd or Pb) which is here developed and applied to seawater samples. A key improvement for the implementation of AGNES with complex matrices is the development of a new blank, called the shifted blank (presented in this work for the first time), which can be applied to the same sample where the measurement is intended. The careful selection of the required parameters for the determination of the free Zn concentration (or activity) at the nanomolar level is described in detail. The methodology has been validated with a synthetic solution containing Zn and nitrilotriacetic acid (NTA) and then applied, as a first case, to two coastal seawater samples taken close to Barcelona and Tarragona (Catalonia, North-Eastern Spain) finding values in the range of 1-3nM, representing around 25% of total Zn. This technique can, in the near future, be crucial in helping to elucidate the role of the free zinc(II) concentration in natural waters.

The effective distribution of affinities (Conditional Affinity Spectrum, CAS) seen by a metal ion binding to a humic substance under natural water conditions is derived and discussed within the NICA-Donnan model. Analytical expressions for the average affinity of these distributions in general multi-ion mixtures are reported here for the first time. These expressions enable a simple evaluation of the effect of all interfering cations on the affinity distribution of a given one. We illustrate this methodology by plotting the affinity spectra of a generic fulvic acid for 14 different cations in the presence of major inorganic ions and trace metals at pH and concentration values representative of a river water. The distribution of occupied sites and their average affinity at the typical freshwater conditions are also reported for each ion. The CAS allows usto distinguish three groups of cations: (a) Al, H, Pb, Hg, and Cr, which are preferentially bound to the phenolic sites of the fulvic ligand; (b) Ca, Mg, Cd, Fe(II), and Mn, which display a greater effective affinity for carboxylic sites, in contrast to what would be expected from their individual complexation parameters; and (c) Fe(III), Cu, Zn, and Ni, for which phenolic and carboxylic distributions are overlapped.

The new electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) has been applied to follow Pb2+ complexation to Purified Aldrich Humic Acid. A refined methodology of AGNES, allowing considerably larger gains, reached free metal ion concentrations down to subnanomolar values in a reasonable deposition time due to the lability and mobility of these complexes. Further insights into the meaning of the binding data, fitted to a NICA (Non Ideal Competitive Adsorption) isotherm, can be obtained with the concept of conditional affinity spectrum (CAS). For this purpose, we present the analytical expression for the CAS of NICA isotherm and show the CAS distributions for the Pb binding at fixed pH. Results reveal that the underlying spectra of each elementary distribution of the bimodal NICA evolve with pH yielding different overlapping and nonsymmetrical distributions. A non-negligible occupation of phenolic and carboxylic sites by Pb2+ takes place in the range of 4 < pH < 9.

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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.