Browsing by Author "Rocha, L. S."
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- Evaluation of nanometer thick mercury film electrodes for stripping chronopotentiometryPublication . Rocha, L. S.; Pinheiro, J. P.; Carapuça, H. M.In this work, the performance and applicability of the thin mercury film electrode (TMFE) in the heavy metal speciation, by stripping chronopotentiometry (SCP), were exploited. The TMFE thickness was optimized and a 7.6 nm mercury film was selected. This TMFE was mechanically stable and able to perform 60 SCP consecutive measurements, with no significant variation in the analytical signal of lead(II) (RSD less than 2%). Due to the small electrode thickness the measurements were performed under conditions of complete depletion over a wide oxidation current (Is) range, i.e., within the interval [75–500] × 10−9 A. The limit of detection (3σ) for lead(II) was 2.4 × 10−9 M for a deposition time of 40 s and an oxidation current of 75 × 10−9 A. The TMFE was successfully applied to the construction of SSCP experimental waves, which were in conformity to those predicted by the theory. The stability constant calculated (K′) for the Pb(II)-carboxylated latex nanospheres system using a TMFE, agreed with the one obtained using the HMDE, for identical experimental conditions.
- Evaluation of poly(sodium 4-styrenesulfonate) film coating in thin mercury film electrodes for lead determinationPublication . Silva, C. P.; Carapuça, H. M.; Rocha, L. S.; Pinheiro, J. P.In this work, polyelectrolyte coatings assembled on glassy carbon/thin mercury film electrodes, as an adsorbed layer of poly(sodium 4-styrenesulfonate) (PSS), were studied. The goal was to search for the best conditions for the production of stable PSS-coated electrodes that could present high negative charge densities within the thin polymeric film, providing a fast and significant electrostatic cation accumulation. Square-wave anodic stripping voltammetry was applied to measure the amount of incorporated lead used as the reference cation. The influence of the composition of the PSS solution, the amount of deposited PSS, molecular weight and of the ionic strength of the electrolyte solution, on the features of the PSS coatings for ion-exchange voltammetry was studied. The PSS films morphology was assessed by SEM. The best PSS coating performance was found for the electrodes prepared from water solutions with a molecular weight of at least 70,000, and a mass loading of ca. 8 μg mm−2. In these conditions a three fold increase was observed in low ionic strength (0.0032 M) media. However, this improvement was not sufficient for the application of direct ion-exchange voltammetry, thus different strategies to obtain polymer films with higher charge densities have to be envisaged.
- Evaluation of thin mercury film rotating disk electrode to perform absence of gradients and Nernstian equilibrium stripping (AGNES) measurementsPublication . Rocha, L. S.; Companys, E.; Galceran, J.; Carapuça, H. M.; Pinheiro, J. P.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 (3σ) 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.
- Ion-exchange voltammetry with nafion/poly(sodium 4-styrenesulfonate) mixed coatings on mercury film electrodes: Characterization studies and application to the determination of trace metalsPublication . Rocha, L. S.; Pinheiro, J. P.; Carapuça, H. M.This work aimed to produce improved polymer coatings for the modification of thin mercury film electrodes (TMFEs). The goal is to obtain sensitive, reproducible, mechanically stable and antifouling devices suitable for the determination of trace metal cations in complex media. Therefore, novel mixed coatings of two sulfonated cationexchange polymers of dissimilar characteristicssNafion (NA) and poly(sodium 4-styrenesulfonate) (PSS)swere produced by solvent evaporation onto glassy carbon electrodes. The effect of the mass ratio (NA:PSS) on the film morphology was studied by scanning electron microscopy, revealing the formation of biphasic polymer systems, where PSS bead-shaped clusters appeared randomly dispersed into a uniform and compactNAenvironment. The permselectivity/ ion-exchange features of the mixed films onto glassy carbon were evaluated using cathecol, urate, and dopamine. To allow trace metal analysis, thin mercury films were plated through the NA/PSS coatings, being the reproducibility and ion-exchange features of the mixed coatings-TMFE evaluated using lead ions. The best NA/PSS coating was found for the mass ratio of 5.3. Analytical performance of the NA/PSS-TMFE yielded a detection limit of 5.5 nM (3ó), and the application of this modified electrode to an untreated polluted estuarine water sample produced significant improvements in the quality of the signal compared with that for a bare TMFE.
- Performance of ex situ bismuth film rotating disk electrode in trace metal analysis by stripping chronopotentiometry: definition of the depletion regime and optimization of experimental parametersPublication . Rocha, L. S.; Pereira, E.; Duarte, A. C.; Pinheiro, J. P.The potentiality of the ex situ deposited bismuth film electrode, allied to the rotation of a glassy carbon disk electrode (BiFE-RDE), was exploited in trace metal analyses of lead(II) and cadmium(II) by stripping chronopotentiometry (SCP). A single BiFE (6.2 nm film thickness) can be used for a 1-day term with no significant variation in the analytical signal. The limit of detection (3s) for a deposition time of 40 s and an oxidation current of 15 10 9 A was 1.5 10 8 M for Pb(II) and 3.0 10 8 M for Cd(II). BiFE-RDE was successfully applied to the direct SCP determination of lead(II) in a fresh water certified material.