Gian Antonio Mazzocchin

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.

Analysis of Roman age wall paintings found in pordenone, Trieste and Montegrotto

The aim of the present work is the study of many fragments of wall painting from archaeological excavations in three different Roman age sites dating back to the I Century before Common Era: Pordenone (località Torre); Trieste (Crosada) and Padova (Montegrotto). The techniques used were optical microscopy, scanning electron microscopy (SEM), equipped with a EDS microanalysis detector, X-rays powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Fourier transform Raman spectroscopy (FT-Raman) and electron paramagnetic resonance (EPR) spectroscopy. The identified pigments were: cinnabar, hematite, celadonite, glauconite, cuprorivaite (Egyptian blue), yellow and red ochre, calcite, limonite, coal black. In general, the mortar preparation did not correspond to the complex procedure suggested by Vitruvius (De Architectura), but generally showed a porous layer, with crushed grains under the pigment layer. In some cases, two superimposed pigment layers were found: yellow superimposed on both red and pink, black on pink, green on black. The slight differences we found in the use of the pigments in the three studied sites might show that the same technology, culture and taste spread all over the Roman Empire in North Eastern Italy (X(a) Regio Venetia et Histria).

Determination of heavy metals in real samples by anodic stripping voltammetry with mercury microelectrodes : Part 1. Application to Wine

Differential-pulse anodic stripping voltammetry with a mercury microelectrode is used for the determination of zinc, cadmium, lead and copper in wine at its natural pH without pretreatment. The effects of the matrix on the stripping peaks are studied in detail by varying the concentration of the metals. Intermetallic (CuZn) interferences and the effects of oxygen are described. The results obtained for the labile metal contents varied from 2 μg l−1 for cadmium to 148 μg l−1 for zinc; standard addition plots were linear over about two orders of magnitude above these levels, demonstrating the negligible effect of organic matter. Acidification of the sample with hydrochloric acid to pH 1 allowed the total metal contents to be determined. The reliability of the method was tested by comparison with the results obtained with atomic absorption spectrometry; the differences were within 10–20%.

Determination of heavy metals in real samples by anodic stripping voltammetry with mercury microelectrodes : Part 2. Application to Rain and Sea Waters

Differential-pulse anodic stripping voltammetry at a mercury microelectrode is applied to determine labile and total zinc, cadmium, lead and copper in samples of rain and sea water. The low ohmic drop associated with microelectrodes permits reliable measurements in rain water without addition of supporting electrolyte. The values found in a typical sample were 0.95 μg l−1 Cu, 0.38 μg l−1 Pb, 0.01 μg l−1 Cd and 0.95 μg l−1 Zn, with relative standard deviations in the range 4–18%. The small effects of organic matter at microelectrodes, compared with those at a hanging mercury drop electrode, allow sensitive and reliable measurements of labile metals in surface sea water. Total metal concentrations are determined after acidification to pH 1.5 with hydrochloric acid. The results are compared with those obtained with atomic absorption spectrometry and with differential-pulse anodic stripping voltammetry at conventional mercury electrodes. Satisfactory results were obtained for a reference sea water.

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.

Stripping analysis at mercury microelectrodes in the absence of supporting electrolyte

Anodic stripping voltammetry of cadmium and lead in pure water, of conductivity as low as 0.6 μS, was studied at a mercury microelectrode obtained by ex situ deposition of mercury on a 10-μm radius platinum disc electrode. The anodic stripping peaks were investigated in detail to verify the influences of the solution resistance and migration on peak position (E p ), peak width at half-height (W 1/2 ) and peak current. The results showed that the presence of a small ohmic drop leads to larger W 1/2 values than those obtained in the presence of supporting electrolyte, while the peak position varied slightly as a consequence of a change in the liquid junction potential

A Study on the Suitability of Carbon Disk Microelectrodes for Trace Analysis of Lead and Copper by ASV

The performance of carbon microdisk electrodes for the simultaneous trace analysis of lead and copper by anodic stripping voltammetry is investigated. The evaluation of the two metal ions in a single step, i.e., by employing a plating potential at which both cations are reduced simultaneously, gives unpredictable results due to the interaction between lead and copper deposited onto the electrode surface. In particular, the calibration plots of lead are not linear, while the copper stripping peak is quite broad and irreproducible. A two-steps deposition procedure, based on a preelectrolysis of copper performed at a potential at which lead ion is not reduced, followed by a simultaneous reduction of the two cations at large enough negative potentials, provides, instead, linear calibration graphs for lead. Copper can be determined without interference by plating the metal at a negative potential where lead ion is not reduced. The reliability of the two-steps procedure for the determination of lead and copper is verified in rain and wine samples.

Seasonal and depth variability of reduced sulphur species and metal ions in mud-flat pore-waters of the Venice lagoon.

An in-situ sampler was employed to monitor seasonal changes in the composition of pore-waters at different depths in a mud-flat located in the central part of the Lagoon of Venice, in proximity of the industrial area of Porto Marghera. The use of voltammetric methods of analysis allowed the simultaneous determination of concentrations of reduced S( − II), thiosulphate, Fe(II) and Mn(II) as well as zinc, lead, cadmium and copper ions. Other parameters such as the concentration of sulphate, Eh, pH and conductivity were also measured. Seasonal increases of S( − II) and Fe(II) concentrations in summer and autumn samples were observed, with the highest concentrations located at about 25 and 55 cm depth, respectively. An opposite seasonal dependence was observed for Mn(II) and S2O32− concentrations, which displayed higher values in winter: the highest concentrations were observed at a depth of 45 cm for Mn(II) and of 65 cm for thiosulphate. The concentrations of Cu, Cd, Pb and Zn ions showed a less evident seasonal variability. Top and bottom samples revealed high concentration values for zinc and copper ions, with the highest values in the 1,000–1,600 nM and 100–300 nM ranges, respectively. Cadmium and lead ions were present at lower concentrations, always less than 70 nM and 30 nM, respectively.

Linear sweep and cyclic voltammetry for metal deposition at solid and mercury microelectrodes from solutions with and without supporting electrolyte

Abstract The experimental behaviour under linear sweep and cyclic voltammetry at low scan rates of the reduction of Pb 2+ and Cd 2+ at solid and mercury microelectrodes is reported in the presence and in the absence of supporting electrolyte. The major effects observed on decreasing the concentration of supporting electrolyte are an increase in the steady-state limiting current and more drawn out curves during the direct scan, broader and asymmetric stripping peaks on the reverse scan. These effects are rationalized in terms of the contributions of migration associated with diffusion in the mass transport and ohmic polarization which adds to concentration polarization when the concentration of supporting electrolyte is decreased. The experimental results are also compared with theoretical predictions.

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.