Aldaléa Lopes Brandes Marques

Surface Complexation of Copper(II) with Alizarin Red S Adsorbed on a Graphite Electrode and Its Possible Application in Electroanalysis

Alizarin red S (ARS) has been used previously as a spectrophotometric reagent for several metals. It has been also employed as an electrode modifier agent for voltammetric analysis. With the purpose of studying the electrochemistry of the adsorbed ARS ligand and the surface ARS-copper(II) complex, on the pyrolytic graphite electrode, cyclic voltammetric experiments were carried out in open and closed circuits. The results indicate that ARS is strongly and irreversibly adsorbed on the electrode surface. This adsorption process was employed to immobilize, by complexation, copper ions from the solution, and for a possible application of the ARS modified electrode in trace copper electroanalysis are explored in this article. The optimization for trace copper determination in aqueous solution was accomplished by anodic stripping voltammetric technique (differential pulse mode); the preconcentration potential was –0.40 V; pH was 5.3 and accumulation time was 2 minutes. Under these conditions, the detection limit was 1.7×10–8 mol L–1, and a linear range from 1.0×10–7 to 4.0×10–6 mol L–1.

Poisoning effect of SCN−, H2S and HCN on the reduction of O2 and H2O2 catalyzed by a 1:1 surface complex of Cu: 1,10-phenanthroline adsorbed on graphite electrodes, and its possible application in chemical analysis

Abstract The copper complex with a single 1,10-phenanthroline ligand can be irreversibly adsorbed on graphite electrodes, catalyzing the reduction of both O 2 and H 2 O 2 . The electrocatalytic kinetics of both substrate reductions were studied by cyclic voltammetric and rotating disk electrode methods. The addition of a very small quantity of a species such as SCN − , H 2 S or HCN in the test solution poisons the electrocatalytic activity for O 2 and H 2 O 2 reduction. A theoretical model is proposed to describe this poisoning effect based on the coordination equilibrium between poisoning species and surface adsorbed catalyst, the inner-sphere mechanism of substrate reduction, and Koutecky-Levich theory. The model is supported by experimental results. The surface behaviour of the adsorbed [Cu(phen)] 2+ ads in the presence of the poison species clearly shows the formation of a new surface complex with [Cu(phen)] 2+ ads . These new “poisoned” surface complexes are electrocatalytically inactive towards O 2 and H 2 O 2 reduction. The possible application of this poisoning effect for the analysis of trace SCN − , H 2 S and HCN was explored.

Elimination of the copper-zinc interference in anodic stripping voltammetry by addition of a complexing agent.

The addition of cyanide and Triton X-100 suppress the formation of the CuZn intermetallic compound in ASV making it possible to determine traces of zinc(II) in the presence of an excess of copper ions. The precision of the method (5%) and the accuracy (error of 1.4%) in sea water are satisfactory.

Graphite electrodes modified by 8-hydroxyquinolines and its application for the determination of copper in trace levels

Surface modification by 8-hydroxyquinoline-5-sulfonic acid (8-HQS) or 8-hydroxyquinoline (8-HQ) on a graphite electrode through irreversible adsorption is reported in this paper. Cyclic voltammetry was used to characterize the surface behavior. The modified surface exhibited an affinity to chelating Cu(II) in the solution, forming a Cu(II) complex, which was employed for Cu(II) trace analysis. Of the metals Zn, Ni, Pb, Co, and Cd, none presented interference until excess concentration of 10 times. Significant interference could be observed from Co(II), Cd(II) and Fe(II) for an excess concentration of 100 times on the analyte. A differential pulse voltammetry, combined with a preconcentrating-stripping process and a standard addition method was used for the analysis. A detection limit for trace copper determination in water, such as 5.1´10-9 mol L-1, was obtained.

Quantum chemical DFT study of the interaction between molecular oxygen and FeN4 complexes, and effect of the macrocyclic ligand

Density functional theory (DFT) was used to examine the interaction between molecular oxygen (O2) and macrocyclic iron complexes of the type FeN4 during the formation of FeN4–O2 adducts. In order to understand how this interaction is affected by different macrocyclic ligands, O2 was bonded to iron-tetraaza[14]annulene (FeTAA), iron-tetramethyl-tetraaza[14]annulene (FeTMTAA), iron-hexamethyl-tetraaza[14]annulene (FeHMTAA), iron dibenzotetraaza[14]annulene (FeDBTAA), and two iron-tetramethyl-dibenzotetraaza[14]annulene complexes (FeTMDBTAA1, FeTMDBTAA2). The ground state for FeN4-O2 adducts was the open-shell singlet. Analysis of the factors influencing the O2 bonding process showed that different macrocyclic ligands yielded adducts with differences in O-O and Fe-O2 bond lengths, total charge over the O2 fragment, O-O vibrational frequency, and spin density in the O2 fragment. A smaller energy gap between the α-HOMO of the FeN4 complexes and the β-LUMO of O2 increased the interaction between the complex and the O2 molecule. The order of activity was FeDBTAA < FeTMDBTAA2 < FeTMDBTAA1 < FeTAA < FeTMTAA < FeHMTAA.

Electrochemical behavior of ruthenium-hexacyanoferrate modified glassy carbon electrode and catalytic activity towards ethanol electrooxidation

3- ion and three ruthenium forms (Ru(II), Ru(III) and Ru(IV)), characteristic of ruthenium oxide compounds. The modified electrode displayed excellent electrocatalytic activity towards ethanol oxidation in the potential region where electrochemical processes Ru(III)-O-Ru(IV) and Ru(IV)-O-Ru(VI) occur. Impedance spectroscopy data indicated that the charge transfer resistance decreased with the increase of the applied potential and ethanol concentration, indicating the use of the RuHCF modified electrode as an ethanol sensor. Under optimized conditions, the sensor responded linearly and rapidly to ethanol concentration between 0.03 and 0.4 mol L -1 with a limit of detection of 0.76 mmol L -1 , suggesting an adequate sensitivity in ethanol analyses.

Application of electrochemical impedance spectroscopy: A phase behavior study of babassu biodiesel-based microemulsions.

Microemulsions are thermodynamically stable systems of two immiscible liquids, one aqueous and the other of organic nature, with a surfactant and/or co-surfactant adsorbed in the interface between the two phases. Biodiesel-based microemulsions, consisting of alkyl esters of fatty acids, open a new means of analysis for the application of electroanalytical techniques, and is advantageous as it eliminates the required pre-treatment of a sample. In this work, the phase behaviours of biodiesel-based microemulsions were investigated through the electrochemical impedance spectroscopy (EIS) technique. We observed thatan increase in the amount of biodiesel in the microemulsion formulation increases the resistance to charge transfer at the interface. Also, the electrical conductivity measurements revealed that a decrease or increase in electrical properties depends on the amount of biodiesel. EIS studies of the biodiesel-based microemulsion samples showed the presence of two capacitive arcs: one high-frequency and the other low-frequency. Thus, the formulation of microemulsions plays an important role in estimating the electrical properties through the electrochemical impedance spectroscopy technique.

Novo método espectrofotométrico para determinação de Hg (II) em amostras de peixe

A simple, sensitive and selective spectrophotmetric method is presented for the rapid determination of mercury (II) at trace levels using 2-mercapto-5-methylbenzimidazole (MMBI) as a new spectrophotometric reagent (lambdamax = 320 nm) in a slightly basic (0.04 M Britton-Robinson (BR) buffer, pH 8.0) aqueous solution. The reaction is instantaneous and the absorbance remains stable for over 24 hours. The average molar absorption coefficient was found to be 2.71 x 104 L mol-1 cm-1. The stoichiometric composition of the chelate is 1:1 (Hg:MMBI). Considerable excesses of copper, zinc, lead and cadmium do not interfere in the determination. The method developed allowed the determination of mercury in the range of 2 x 10-6 to 4 x 10-5 mol.L-1 with good precision, accuracy and the detection limit was 9.9 x 10-7 mol.L-1 of Hg. The method was successfully applied to fish samples and the results were evaluated and compared with classic atomic absorption spectrometry (AAS). The relative standard deviations for the samples analyzed were between 7.2 and 33% (n = 5), while the errors were between 1.63 and 11.6. The method was found to be sensitive, selective and was used for the determination of mercury in fish with satisfactory results.

Distribution and Fractionation of Metals in Mangrove Sediment from the Tibiri River Estuary on Maranhão Island

Sediment quality guidelines and the sequential chemical extraction protocol of the European Commission were applied for the assessment of Cd, Cr, Cu, Ni, Pb and Zn in mangrove sediments from the Tibiri River on Maranhao Island. Metals concentrations were determined using the inductively coupled plasma optical emission spectrometry (ICP OES) technique. The results demonstrated that metal levels were below TEL (Threshold effect level) and ERL (effect range low), range associated with occasional adverse biological effects for the aquatic life. Metal mobility in decreasing order was: Ni (24%) > Zn (20.6%) > Cu (14.5%) > Pb (1.9%). Except by Ni and Zn, other metals were not found in the exchangeable/acid-soluble fraction (F1). Only Ni, Zn and Cu were present in the reducible fraction (F2). The major metal percentage was associated to oxidizable fraction (F3) and residual fraction (F4). In the latter, metal percentage ranged from 76 to 100%, which suggests an inexpressive impact of anthropogenic activities.

Avaliação integrada da qualidade de águas superficiais: grau de trofia e proteção da vida aquática nos rios Anil e Bacanga, São Luís (MA)

This work proposes two indexes to assess the quality of surface waters where domestic sewage or similar pollution source prevails: the Trophic State Index (IETP) and the index for protection of aquatic life (IQApva). Both were applied to Anil and Bacanga rivers located on Maranhao Island. Although the general quality of the water indicates human presence, the Anil River stands out by its high trophic level and poor conditions for aquatic life. The results of applying these indexes allow easily assess of the degradation state of the ecosystem. By being connected directly to basic sanitation, these indices are an important management tool for governmental and non-governmental stakeholders.