M.L.S. Simões Gonçalves

Complexation study of humic acids with cadmium (II) and lead (II)

A set of experiments on the complexation of cadmium and lead with humic acids were performed at pH 5.0 using normal- and reverse-pulse polarography and d.c. and differential-pulse anodic stripping voltammetry. The concentration of deprotonated groups at pH 7.0, the average stability parameter, K, the differential equilibrium function, f(KDEF), and a parameter, Γ, proportional to the metal buffer capacity of the system and related to the heterogeneity of the ligand, were calculated. It was concluded that the complexes are labile within the experimental error for the time scale of the techniques; that lead complexes are more stable than cadmium complexes and that the ligand has a more heterogeneous behaviour with respect to lead than to cadmium.

Effect of plants on sulphur geochemistry in the Tagus salt-marshes sediments

Depth distributions of salinity, pH, Eh, sulphate (SO 4 2- ), thiosulfate (S 2 O 3 2- ), sulphite (SO 3 2- ), hydrogen sulphide (H 2 S) and acid volatile sulphides (AVS) were measured seasonally in pore waters of vegetated and non-vegetated sediments of a Tagus salt-marsh colonized by Spartina maritima. Near-bottom water was also analyzed. The vertical distributions obtained in both sites were compared. The vegetated site was more acid (6.0-7.0) and oxidative (reaching + 360 mV vs. H), and presented higher SO 4 2- concentrations (10-31 mM). At depth of higher root density, AVS and H 2 S levels were low (0.1-53 μmol g -1 and < 0.1-0.5 μM, respectively) in comparison to vegetated sediments (30-225 μmol g -1 and 0.5-17 μM). S 2 O 3 2- was only detected in anoxic non-vegetated sediments (11-54 μM) and concentrations increased with depth. Differences of sulphur speciation between the two sites were recorded during all seasons as result of Spartina activity almost the entire year.

Adsorption of fulvic-like organic ligands and their Cd and Pb complexes at a Mercury electrode

Benzyliminodiacetic acid, pyridine-2-carboxylic acid and pyridine 2,6-dicarboxylic acid were used as model compounds of the components of natural pedogenic fulvic acids to study the most important factors related to their adsorbing and complexing properties which may affect the behaviour of their complexes at a mercury electrode. In the absence of metal ions, their adsorption properties were studied by alternating current polarography, at different pHs and ligand concentrations. The adsorption of their Pb and Cd complexes was studied by normal pulse polarography, in a low concentration range. Comparison of all the experimental data shows that adsorption of the free ligand and the complexes is always primarily due to the hydrophobic part of the ligand which tends to accumulate on the Hg surface. Any electrically charged group of the molecule (either positive or negative) increases its hydrophilicity and decreases its adsorbability. However, this effect plays a significant role only when the distance between the charged and hydrophobic groups is small. For instance, it was found to be much less important with the benzyliminodiacetic ligand than with the other two ligands. For all the ligands and complexes, 10−6M is the approximate limit below which the adsorption is small in the normal time-scale of a dropping mercury electrode, due to the small adsorption equilibrium constants of free or protonated ligands and to the slowness of the overall adsorption rate of the complexes. These results are compared to those obtained with pedogenic fulvic compounds.

Electrochemical studies on small electron transfer proteins using membrane electrodes

Membrane electrodes (ME) were constructed using gold, glassy carbon and pyrolytic graphite supports and a dialysis membrane, and used to study the electrochemical behavior of small size electron transfer proteins: monohemic cytochrome c522 from Pseudomonas nautica and cytochrome c533 as well as rubredoxin from Desulfovibrio vulgaris . Different electrochemical techniques were used including cyclic voltammetry (CV), square wave voltammetry (SW) and differential pulse voltammetry (DP). A direct electrochemical response was obtained in all cases except with rubredoxin where a facilitator was added to the protein solution entrapped between the membrane and the electrode surface. Formal potentials and heterogeneous charge transfer rate constants were determined from the voltammetric data. The influence of the ionic strength and the pH of the medium on the electrochemical response at the ME were analyzed. The benefits from the use of the ME in protein electrochemistry and its role in modulating the redox behavior are analyzed. A critical comparison is presented with data obtained at non-MEs. Finally, the interactions that must be established between the proteins and the electrode surfaces are discussed, thereby modeling molecular interactions that occur in biological systems. # 2002 Elsevier Science B.V. All rights reserved.

Voltammetric and potentiometric investigations on the complexation of Cd(II) by glycine in seawater

Abstract Stability constants of Zn-glycine complexes have been determined by two independent methods, i.e. potentiometry and differential pulse polarography at the ionic strength of seawater in 0.7 M NaClO4. Moreover, these stability constants have been determined in artificial seawater and in seawater from the open Pacific at the low level of 10−6 M Zn by differential pulse polarography. Also, the stability constants of complexes of Ca and Mg needed in the calculations have been determined potentiometrically at the ionic strength of 0.7 M. The specific influences of alkaline earth metals and of traces of other heavy metals present in seawater and competing for the ligand have been elucidated. The potentialities of both methods are compared with respect to sensitivity, accuracy and possible interferences. Fairly good agreement of the values of the stability constants obtained by the two electrometric methods ensures the accuracy of the results. The stability constants are practically the same in artificial seawater and in natural seawater, indicating that all relevant complexation equilibria existing in seawater have been taken into account. From the measured stability constants of zinc-glycine complexes and stability constants of other complex equilibria relevant for Zn speciation in the sea, the distribution of Zn among the various species in seawater was evaluated. It has been concluded that in seawater the Zn-glycine complexes will only be detectable, i.e. present at 2%, at total glycine concentrations above 10−4 M. As the stability constants of the complexes of Zn with other amino acids do not differ substantially from those of glycine, the distribution presented will be generally applicable for the Zn speciation in seawater containing amino acids as the only natural organic ligand. Bearing in mind that the concentration of total DOM in seawater is usually lower than 1 mg kg−1, amino acids like glycine cannot contribute significantly to the speciation of heavy metals in most parts of the open sea. However, for other types of natural waters containing smaller concentrations of competitive ions, especially chloride ions, the limiting glycine concentration for a detectable complexation of Zn lies much lower at 10−6 M.

Voltammetric studies of purine bases and purine nucleosides with copper

Anodic stripping voltammetry with a hanging mercury drop electrode has been used to investigate the interaction of cooper with the purines adenine, hypoxanthine, xanthine and purine nucleosides adenosine, guanosine and inosine at an ionic strength of 0.1 M in KNO3 and in the pH range 3.5–5.5. In all cases stabilisation of copper(I) occurs suggesting that the oxidation of copper(0) in the presence of excess ligand proceeds in two one-electron steps. Adsorption onto the electrode has been analysed and conditions where this is negligible were chosen for complexation studies. From the shift of the peak potential corresponding to Cu(0)/Cu(I) oxidation with increasing ligand concentration the stoichiometry of the complexes and their formation constants have been determined. The values obtained are discussed in terms of the ligand structure.

Flow injection-assisted optical sensor for determination of iron(II) and iron(III) in natural water

Abstract An approach for speciation of iron (as Fe(II) and Fe(III)) based on integration of retention of the Fe(III)-SCN complex with detection using a conventional spectrophotometer is proposed here. The device (namely, a flow-cell packed with an exchange resin) has been coupled to a flow-injection manifold with inner-coupled injection valves which enables discrimination between Fe(III) and Fe(II) taking advantage of a redox minicolumn housed in the loop of one of the valves. The method thus proposed has good selectivity with a determination limit of 80 μgl −1 and affords a determination range of 80–500 μgl −1 for Fe(II) plus Fe(III). Application to synthetic and real samples containing both the oxidation states of iron lead to average recoveries of 103% and 101% for Fe(II) and total iron ( Fe ( II )+ Fe ( III )), respectively, thus showing the usefulness of the overall approach.

Voltammetry of labile heterogeneous complexes with low diffusion coefficients

Abstract The heterogeneity of natural organic ligands in the presence of a metal ion affects its concentration profile in the diffusion layer. In fact, owing to heterogeneity, the ratio of complexed metal to free metal increases in the diffusion layer with proximity to the electrode surface. The diffusing species are then decelerated because the diffusion coefficient of the macromolecular complex is lower than that of the metal ion, and so the total amount of reduced metal is decreased compared with that obtained in a homogeneous system with the same bulk speciation. Therefore, the voltammetric current, which is directly related to the concentration profile in the diffusion layer, is affected by heterogeneity. In the work reported in this paper the concentration profile in the diffusion layer was determined on the basis of the diffusion process and a heterogeneity model describing the concentration of the different complexing sites and their complexing affinity for the metal. The mean stability parameters for the complexation of natural organic ligands with a metal ion were determined from this profile, simulating the value that should be determined from voltammetric currents. The difference between the simulated value and the mean stability parameter related to bulk speciation was analysed.

THE PH EFFECT IN THE DIFFUSION COEFFICIENT OF HUMIC MATTER : INFLUENCE IN SPECIATION STUDIES USING VOLTAMMETRIC TECHNIQUES

Abstract The dynamic aggregation properties of humic matter and the influence of intramolecular electrostatic interaction lead to an increase of polydispersity at pH > 5 in two humic samples studied (“Purified Peat Humic Acid”, PPHA and “Laurentian Fulvic acid”, LFA). The smaller entities formed at pH increasing above 5 are responsible for the increase of the diffusion coefficient as “seen” by voltammetry. The voltammetric results show that the diffusion coefficient of the humic complex inmplex increases from 5 × 10−12 m2 s−1 (pH≤5) up to 2 × 10−11 m2 s−1 (pH=6) for Cd PPHA and from 6 × 10−11 m2 s−1 (pH Pb LFA . Systematic shifts in voltammetric results with changing ligand concentrations often attributed to adsorption of humic matter on the electrode and/or heterogeneity effects, may also be due to incorrect diffusion coefficient values used in speciation calculations. The aim of this paper is to show that the diffusion coefficient of metal-humic complexes is a function of pH. It is necessary to take this into account to obtain more accurate speciation data from voltammetric results and interpret the physicochemical background of the metal humic interaction.

VOLTAMMETRIC BEHAVIOUR OF COPPER COMPLEXES WITH CYTOSINE AND ITS NUCLEOSIDE

Abstract The interaction of divalent metal ions such as copper ions with basic constituents of nucleic acids may help to gain an insight into the mechanisms with DNA itself. In this work the complexation of copper by pyrimidine bases and their nucleosides, namely cytosine and cytidine, is analysed at the ionic strength of 0.10 M and in the pH range 3.5–6.5, using anodic stripping voltammetry with the hanging mercury drop electrode. In the presence of cytosine and cytidine, stabilisation of copper(I) oxidation state occurs due to complex formation, and so copper(II) redox reaction proceeds in two one-electron steps. From the peak potential shifts with increasing ligand concentration, the stoichiometry and formation constants of the complexes with copper(II) and copper(I) have been determined. The values obtained were analysed in terms of ligand structure.