Helena M. Carapuça

Kinetic approach to heavy metal mobilization assessment in sediments: choose of kinetic equations and models to achieve maximum information.

Studies of trace metal mobilization in sediments are generally performed using sequential extraction schemes at equilibrium. In the present work, a kinetic fractionation of trace metals in sediments has been developed to assess that information. The extraction rate data have been obtained using a single extraction scheme with EDTA and following a protocol previously optimized. Two kinetic equations and two kinetic models were used to fit the experimental data. The two constants equation fits well the extraction rate data used in this work but does not present any physico-chemical meaning. The diffusion model and the two first-order reactions model allow determining which parameter (the reaction between the metal M and the EDTA or the diffusion of the complex M/EDTA) is rate limiting in the trace metal extraction by EDTA. It appears that the two first-order reactions model is more efficient than the diffusion model to fit the present extraction rate data so it can be deduced that the diffusion of the complex M/EDTA is not the limiting step of the trace metal extraction by EDTA in estuarine sediments. In a second part, relationships between the fraction of metals determined with the two first-order reactions model and the sediments composition were established.

Mixed polyelectrolyte coatings on glassy carbon electrodes: Ion-exchange, permselectivity properties and analytical application of poly-l-lysine-poly(sodium 4-styrenesulfonate)-coated mercury film electrodes for the detection of trace metals.

The present work describes the preparation, optimization and characterization of mixed polyelectrolyte coatings of poly-l-lysine (PLL) and poly(sodium 4-styrenesulfonate) (PSS) for the modification of thin mercury film electrodes (MFEs). The novel-modified electrodes were applied in the direct analysis of trace metals in estuarine waters by square-wave anodic stripping voltammetry (SWASV). The effects of the coating morphology and thickness and also of the monomeric molar ratio PLL/PSS on the cation-exchange ability of the PLL-PSS polyelectrolyte coatings onto glassy carbon (GC) were evaluated using target cationic species such as dopamine (DA) or lead cation. Further, the semi-permeability of the PLL-PSS-coated electrodes based both on electrostatic interactions and on molecular size leads to an improved anti-fouling ability against several tensioactive species. The analytical usefulness of the PLL-PSS-mixed polyelectrolyte coatings on thin mercury film electrodes is demonstrated via SWASV measurements of trace metals (lead, copper and cadmium at the low nanomolar level; accumulation time of 180s) in estuarine waters containing moderate levels of dissolved organic matter, resulting in a fast and direct methodology requiring no sample pretreatment.

Ion-exchange and permselectivity properties of poly(sodium 4-styrenesulfonate) coatings on glassy carbon: application in the modification of mercury film electrodes for the direct voltammetric analysis of trace metals in estuarine waters

The present work describes the optimisation and characterization of poly(sodium 4-styrenesulfonate)-coated thin mercury film electrodes (PSS-TMFE) for the direct analysis of trace metals in estuarine waters by square-wave anodic stripping voltammetry (SW-ASV). The morphology, thickness and ion exchange ability of the poly(sodium 4-styrenesulfonate) coatings onto glassy carbon were evaluated and these features particularly favoured the incorporation of cationic species, such as dopamine or lead cation. For the case of the heavy metal cations, a simple, sensitive and very reproducible methodology for their SW-ASV analysis could be developed. In fact, with the PSS-TMFE, a significant increase in the sensitivity of the ASV determination of lead was obtained compared both to the uncoated TMFE (ca. 82%) as well as to Nafion-coated electrodes of similar thickness (ca. 43-49%). Furthermore, the permselectivity of the poly(sodium 4-styrenesulfonate) coatings, based both on electrostatic interaction and molecular size, leads to an improved anti-fouling ability against surfactant species. The analytical usefulness of the poly(sodium 4-styrenesulfonate)-coated thin mercury film electrodes is demonstrated by application to the direct ASV determination of trace heavy metals at the low nanomolar level, in estuarine waters with moderate contents of dissolved organic matter, where the uncoated TMFE failed due to fouling.

Comproportionation and disproportionation reactions in the electrochemical reduction of nitroprusside at a hanging mercury drop electrode in acidic solution

Abstract The appearance of (two) cathodic peaks in the region of the second reduction process (E∼−0.55 V) during positive-going potential scans in cyclic voltammograms, at the HMDE, of nitroprusside (NP) (1×10−3 mol dm−3) at pH 5.5 is evidence of a comproportionation process involving adsorption. This is supported by the fact that these inverted peaks are eliminated on the addition of surface active Triton X-100. The evidence from the present study indicates that the well-characterised tetracyanonitrosylferrate(II) ion, [Fe(CN)4NO]2−, is the sole reductand responsible for the second one-electron reduction process of nitroprusside at all pH values, and that the [Fe(CN)5NOH]2− ion, the protonated form of the initial product ([Fe(CN)5NO]3−) of the first one-electron reduction of nitroprusside, is not formed. On the other hand, at lower (sub-micromolar) concentrations, the reaction taking place at the potentials of the second reduction step of NP follows different pathways depending on the pH. The voltammetric and adsorptive characteristics of the ion [Fe(CN)4NO]2− at pH 3.0 and 7.6 have been investigated by square-wave (SW) voltammetry and cyclic voltammetry (CV). Integration of the current under the CV peak at ∼−0.55 V allows the number of electrons involved in the reduction at this potential to be determined: one at pH 7.6 and three at pH 3.0. A surface regenerative process induced by H+ and based on the disproportionation of the product of the second reduction step of NP is proposed in order to explain this. The disproportionation is favoured at high [H+]/[NP] concentration ratios, and, under these solution conditions, the final four-electron reduction product of NP, [Fe(CN)4NH2OH]2−, is produced at this low potential, rather than at the much more negative potentials required polarographically. Bulk electrolysis experiments confirmed the proposed mechanism.

Simultaneous determination of copper and lead in seawater using optimised thin-mercury film electrodes in situ plated in thiocyanate media

In the present work the anodic stripping voltammetric (ASV) methodology using a thin mercury film electrode in situ plated in thiocyanate media was re-assessed in order to allow the simultaneous determination of copper and lead in seawater. Under previously suggested conditions [6], i.e. using a concentration of thiocyanate of 5mM, the ASV peaks of copper and lead overlapped due to the formation of a stable copper(I)-thiocyanate species, limiting the analytical determinations. Therefore, the best value for the thiocyanate concentration was re-evaluated: for 0.05mM a trade-off between good resolution of the copper and lead peaks and high reproducibility of the mercury film formation/removing processes was achieved. In this media, the ASV peaks for Pb and Cu occurred, separated by 140mV. Also, the in situ thin mercury film electrode was produced and removed with good repeatability, which was confirmed by the relative standard deviation values for the ASV determinations: 0.5% for Pb and 2.0% for Cu (10 replicate determinations in a solution with metal concentrations 1.5x10(-8)M for lead and 2.2x10(-8)M for copper). The optimised methodology was successfully applied to the determination of copper in the presence of lead, in certified seawater (NASS-5).

Evaluation of thin mercury film rotating disk electrode to perform absence of gradients and Nernstian equilibrium stripping (AGNES) measurements.

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 sigma) for lead(II) is 7.4 x 10(-9)M and 7.2 x 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.

Unusual electrochemical reduction of copper(II) to copper(I) in polyoxotungstates

The electrochemical behaviour of the copper-substituted Keggin-type and sandwich-type polyoxotungstate anions of the compounds α-[(C4H9)4N]4H[PW11CuIIO39] and α-B-[(C4H9)4N]7H3[CuII4(H2O)2(PW9O34)2] was studied by cyclic voltammetry in acetonitrile. In both cases two copper 1-electron reduction waves were detected in the cathodic scan. The first one was due to the reduction of one CuII to CuI in the polyoxoanion and the second one to the consecutive reduction of the preformed CuI to Cu0, with the consequent deposition/adsorption of the ejected metal atom at the glassy carbon electrode surface. In the anodic scan, Cu0 was re-oxidised with regeneration of the initial copper(II) complexes, via a CuI intermediate. The observed two-step reduction of copper(II) to copper(0) and the formation of intermediate species containing copper(I) is here reported for the first time for copper substituted polyoxotungstates. The co-ordination of the acetonitrile molecules to the copper ions must play a role in the formation of the copper(I) species, which are not detected in aqueous solution.

Synthesis and characterisation of ruthenium(II) complexes containing ferrocenyl-derived ligands

Ruthenium(II) complexes containing the macrocycle [14]aneS4 and pyridyl ligands with an end-capping ferrocene were prepared using [Ru([14]aneS4)(DMSO)Cl]Cl as the starting material. Substitution of the DMSO ligand by 4-ferrocenylpyridine (4-FP), ferrocenyl-4-pyridylacetylene (FPA) and pyridine (py) gave the complexes [Ru([14]aneS4)(L)Cl]Cl (L=4-FP, FPA, py). The acetonitrile complex [Ru([14]aneS4)(NCMe)2](BF4)2 was also prepared starting from [Ru([14]aneS4)(DMSO)Cl]Cl. UV/Vis absorption spectroscopy and cyclic voltammetry indicate low electronic communication between the metal centres in the heterobimetallic complexes. The crystal structure of [Ru([14]aneS4)(4-FP)Cl]Cl was determined by X-ray diffraction. Crystal data: C25.780H37.134Cl2FeNO1.288S4Ru, M=737.82, monoclinic, space group P2/n, a=13.192(1), b=7.6662(6), c=28.549(2) A, β=100.341(2)°, V=2840.4(4) A3, Z=4.

Electrochemical studies of nitroprusside in the presence of copper(II): formation of Cu(I) reduced nitroprusside species

Abstract Cu 2+ is reduced in the presence of nitroprusside to form two Cu(I) reduced nitroprusside species at about +0.050 V (pH 7.6). These species are reduced further at about −0.60 V. The two species are formed by an EC mechanism, and the species are believed to be [Cu I Fe(CN) 4 NO] − , which predominates in acidic solution, and [Cu I Fe(CN) 5 NO] 2− , which predominates in alkaline solution. These conclusions are supported by cyclic voltammetric and bulk electrolysis/coulometric experiments.

Electrochemistry of the nitroprusside ion. From mechanistic studies to electrochemical analysis

The electrochemical reduction of the nitroprusside ion (NP) was examined by differential-pulse polarography and square-wave voltammetry. The effect of pH on the electrochemical behaviour of NP was studied and experimental evidence for the occurrence of adsorption of the products of the first one-electron reduction of NP is shown. Owing to the adsorption of these products, which are the reactants in the second reduction step of NP, the peak current of the second process is highly enhanced compared with that of the first process, especially at low concentration levels (< 1 × 10–5 mol dm–3). For acidic solutions the enhancement is considerably higher than at pH 7. A surface reaction leading to the regeneration of the reactants of the second reduction process can be advanced as a possible explanation. Square-wave cathodic stripping voltammetry with adsorptive accumulation was used for the determination of NP. The detection limit depends on the square wave parameters and can be as low as 2.3 nmol dm–3 for a delay time of 60 s.