Ligia Maria Moretto

Determination of Trace Mercury in Saltwaters at Screen-Printed Electrodes Modified with Sumichelate Q10R

The results of a study on the determination of trace levels of mercury in natural saltwaters using screen-printed carbon electrodes modified with Sumichelate Q10R, a chelating resin containing dithiocarbamate groups, which binds mercury strongly and selectively are reported. Cyclic voltammetric measurements indicate that mercury(II) is preconcentrated by complexation at the modified electrode, giving peak currents which are about one order of magnitude higher than those obtained at an unmodifed screen-printed electrode. The voltammetric pattern is characterized by a broad reduction peak to which a sharp and well-resolved reoxidation peak is associated. Trace (subnanomolar) concentrations of mercury can be determined at the screen-printed modified electrode by adopting a complexation-anodic stripping procedure with detection via differential pulse voltammetry. The method is characterized by a low detection limit (3j blank/sensitivity) of 12 pM with 20 min complexation and 5 min reduction at ‐0.5 V. Excess copper interferes in the analysis of mercury only when present in more than hundred-fold excess. Data concerning the application of modified screen-printed electrodes to the analysis of mercury in water samples from the channels of Venice (Italy) are presented and discussed.

Ionomer-coated electrodes and nanoelectrode ensembles as electrochemical environmental sensors: Recent advances and prospects

This Minireview focuses on recent advances in the use of ionomer-coated electrodes and nanoelectrode ensembles for the determination of redox ions at trace concentration level with special stress on applications for environmental monitoring. After an overview on the most recent developments of ionomer-coated electrodes, attention is directed towards their use for speciation analysis in environmental samples. In particular, some examples such as the selective determination of inorganic and metallorganic species of different toxicity and the redox state speciation of multiply charged ions are discussed. The second part of the Minireview deals with the development and electroanalytical use of a new kind of nanostructured electrode, named nanoelectrode ensembles, which are prepared by template synthesis within the pores of microporous guest membranes. Advantages, peculiarities, and limits of such devices are discussed together with perspectives for their use in electroanalytical sensing at trace and ultratrace concentration levels in environmental samples.

Voltammetric determination of trace mercury in chloride media at glassy carbon electrodes modified with polycationic ionomers

Abstract The perfluorinated polycationic polymer Tosflex ® is used for preparing polymer modified electrodes able to preconcentrate and detect the anionic complex HgCl 2− 4 , which is the prevailing inorganic Hg(II) species in sea water and other chloride media. The study of the ion-exchange voltammetric behaviour of the tetrachloromercurate(II) complex at Tosflex-modified electrodes indicates the efficient incorporation of the analyte into the polymeric coating to which a remarkable positive shift in the reduction potential of HgCl 2− 4 to Hg 0 is associated. The partition and selectivity coefficient values for the ion-exchange equilibrium involved are calculated from voltammetric data. When using an ion-exchange preconcentration step under open circuit conditions followed by a cyclic voltammetry detection step, a detection limit in the 10 −8 M range is achieved, the exact figure depending on the scan rate employed. The application of a reducing potential during the preconcentration (i.e., using a mixed ion-exchange + Faradaic preconcentration regime) and the use of differential pulse voltammetry, lower the relevant detection limit by about three orders of magnitude. Results concerning optimization of the experimental parameters as well as lowering of the influence of possible interferents, such as copper and competing anions, are reported. Finally, an analytical procedure for exploiting the peculiar properties of Tosflex-modified electrodes for the determination of mercury in coastal waters samples is proposed and experimentally tested.

Ion-Exchange Voltammetry at Polymer Film-Coated Nanoelectrode Ensembles

Ensembles of nanoscopic disk-shaped electrodes have been shown to offer enhancements in electroanalytical detection limits relative to electrodes of macroscopic dimensions (e.g., disk electrodes with diameters of ∼1 mm). Enhancements in electroanalytical detection limits have also been observed at macroscopic electrodes that have been coated with films of ion-exchange polymers. In this paper we combine these two concepts. We demonstrate that a nanoelectrode ensemble (NEE) that has been coated with a thin film of the Kodak ion-exchange polymer AQ 55 shows enhanced electroanalytical detection limits relative to the uncoated NEE and to the coated macroscopic electrode. To our knowledge, this is the first investigation of the electrochemistry, and the electroanalytical advantages, of polymer film-coated NEEs.

Determination of mercury in process and lagoon waters by inductively coupled plasma-mass spectrometric analysis after electrochemical preconcentration: comparison with anodic stripping at gold and polymer coated electrodes

Abstract A combined electrochemical-inductively coupled plasma-mass spectrometry (EC-ICP-MS) method for the determination of trace mercury in water samples is presented. It takes advantage of the electrochemical preconcentration of mercury onto a gold spiral electrode followed by ICP-MS analysis after the electrochemical reoxidation of deposited mercury in pure supporting electrolyte. The advantages of the EC-ICP-MS approach with respect to conventional ICP-MS, are the increased sensitivity and the elimination of the effect of interfering substances eventually present in the sample. EC-ICP-MS is applied to the determination of nanomolar and subnanomolar concentrations of mercury(II) ions in real samples such as process waters (from a chlor-alkali plant) and lagoon waters (from Venice channels). Analytical performances obtained by EC-ICP-MS are discussed and compared with those obtained by anodic stripping voltammetry at gold and at Tosflex-coated glassy carbon electrodes.

Iron(II) and iron(III) determination by potentiometry and ion-exchange voltammetry at ionomer-coated electrodes

This paper examines the possibility to combine potentiometric and voltammetric measurements, both performed at ionomer coated glassy carbon electrodes, in order to determine the concentration and gain information on the redox state of electroactive ions even when they are present at trace (submicromolar) concentration levels. As a model case, the speciation of iron(II) and iron(III) is studied both for the cations Fe 2+ and Fe 3+ and for the anions Fe(CN)6 4− and Fe(CN)6 3− . For the first situation, electrodes coated with Nafion are employed while for the latter electrodes coated with the anion exchanger Tosflex are examined. The ratios between oxidised and reduced species are measured at trace levels thanks to the ion-exchange preconcentration capabilities of the coatings. Relevant equations ruling the potentiometric responses at the coated electrodes have been derived by proper adjustment of the Nernst equation. Ion-exchange voltammetry at the coated electrodes allowed the measurement of parameters, such as ion-exchange distribution coefficients, which are relevant for the experimental testing of the suitability of these equations. Voltammetry at the coated electrodes allowed also the sensitive determination of the total concentration of both ions of the same redox couple. The applicability of the method for practical purposes has been checked determining redox potential and concentration profiles for iron and reduced sulphur species in the pore-waters of sediments of the lagoon of Venice, Italy.

Trace Iron Determination by Cyclic and Multiple Square-Wave Voltammetry at Nafion Coated Electrodes. Applicationto Pore-Water Analysis

Nafion coated glassy carbon electrodes (NCE) are employed for preconcentrating and detecting Fe2+ and Fe3+ cations from aqueous solutions. The influence of the supporting electrolyte composition and of the redox state on the analyte partitioning within the Nafion coating are examined. By using cyclic voltammetry, the ion-exchange voltammetric determination of iron in the μM concentration range is achieved from voltammetric peak currents relevant to the reversible redox process: Fe3++e⇋Fe2+. Depending on the starting potential of the voltammetric scan, all the iron is initially converted to the Fe(III) or Fe(II) redox state, so that, from the voltammogram, the overall concentration of iron is always obtained. However, the simple measurement of the open circuit potential at the NCE before starting the scan allows one to get information on the ratio between Fe(III) and Fe(II) incorporated in the Nafion coating. From relevant ion-exchange distribution coefficients, open circuit potential values can be related to the Fe(III)/Fe(II) concentration ratio in the sample. The use at the NCE of a new electroanalytical technique named multiple square-wave voltammetry in the double differential mode allows the significative lowering of detection limits pushing the operative range of the method in the nM range. Application to analysis of Fe(II) in the pore-waters of sediments of the lagoon of Venice (Italy) is presented.

Ion-exchange voltammetry of copper ions in chloride media at glassy carbon electrodes modified with polycationic ionomers

Abstract Perfluorinated polycationic polymers (Tosflex) were applied for preparing polymer-modified electrodes able to preconcentrate anionic complexes of Cu + electrogenerated in situ at the electrode/polymer interface by reducing CU 2+ solutions in the presence of chloride ions. The species incorporated into the polymeric coating, namely CUCl − 2 , was detected by a suitable linear sweep voltammetric reoxidation scan. The dependence of the reoxidation peak current on experimental parameters such as amount of modifier, electrogeneration-preconcentration time and solution concentrations of Cu 2+ and Cl − was studied. The possibility of applying Tosflex-modified electrodes to copper speciation studies was also examined. In particular, the influence on the ion-exchange voltammetric responses of some amino acids such as histidine, glycine and glutamic acid which, at the pH employed in this study, carry positive, neutral and negative ionic charges, respectively, is discussed. Some of the advantages and limitations of Tosflex-modified electrodes compared with Nafion-coated electrodes and the hanging mercury drop electrode are critically evaluated.

Ion-exchange voltammetry and electrocatalytic sensing capabilities of cytochrome c at polyestersulfonated ionomer coated glassy carbon electrodes.

The incorporation and electrochemical behaviour of cytochrome c (Cyt C) at glassy carbon electrodes modified with the polyestersulfonated ionomer Eastman AQ 55 are examined. The presence of the polyelectrolytic coating allows the preconcentration of the protein within the polymer and the observation of the direct electrochemistry of Cyt C at the modified electrode without addition of promoters or mediators in the solutions. The dependence of voltammetric signals on typical parameters such as solution pH and nature or concentration of the supporting electrolyte supports the ion-exchange nature of the incorporation process. The relevant role of the permselectivity of the polymeric modifier is highlighted also by the study of electrocatalytic processes which take place at the modified electrode loaded with Cyt C. No electrocatalytic effect is observed when the electrogenerated dication (ferricenylmethyl)trimethylammonium is present as possible oxidant. On the contrary, electrocatalytic current enhancements are observed for anionic substrates such as Fe(CN)(6)(3-) (oxidant) and ascorbate (reductant). Catalytic currents increase with the substrate concentration, with higher sensitivity for Fe(CN)(6)(3-). Due to ionic repulsion, the reaction with anions occurs at the polymer-solution interface. In the case of chemically unstable substrates, such as superoxide anion, ionic repulsion slow down the approach rate of the substrate so that spontaneous decomposition can prevail over the reaction with Cyt C incorporated in the coating.

Electroanalytical study on the ion-exchange voltammetric behaviour of Hg(II) at Tosflex®-coated glassy carbon electrodes

Abstract The cyclic voltammetric behaviour of HgCl 4 2− at glassy carbon electrodes coated with a thin film of Tosflex® IE-SA 48 (a perfluorinated anion exchanger) is studied. Concerning the reduction of HgCl 4 2− preconcentrated in the coating, a relevant positive shift in reduction peak potentials is observed, both when increasing the HgCl 4 2− solution concentration and decreasing the scan rate. The analysis of the voltammograms indicates that both when increasing the HgCl 4 2− solution concentration and decreasing the scan rate. The analysis of the voltammograms indicates that an irreversible reduction process is occurring. A remarkable positive shift in peak potentials with respect to bare electrodes is also observed. All experimental evidence obtained agrees with the formation of a metal phase, namely Hg 0 , whose deposition is highly favoured by the presence of the coating, as a consequence both of preconcentration effects and of the decrease in diffusion coefficients related to the ion-exchange incorporation. As far as the reoxidation of deposited Hg 0 is concerned, the study of the effect of the scan rate, the analyte solution concentration and the loading of chloride anions in the coating indicates that the reversible oxidation to an insoluble calomel deposit occurs at high Hg 0 and low Cl − loadings. The formation of calomel is confirmed also by ex situ X-ray diffractometric measurements. On the contrary, low Hg 0 and high Cl − loadings within the coating favour the disproportionation of the calomel formed and the complete regeneration of the modified electrode.