Antonio S. Santos

Solid-phase extraction system for Pb (II) ions enrichment based on multiwall carbon nanotubes coupled on-line to flame atomic absorption spectrometry

The present paper proposes the application of multiwall carbon nanotubes (MWCNTs) as a solid sorbent for lead preconcentration using a flow system coupled to flame atomic absorption spectrometry. The method comprises the preconcentration of Pb (II) ions at a buffered solution (pH 4.7) onto 30mg of MWCNTs previously oxidized with concentrated HNO(3). The elution step is carried out with 1.0molL(-1) HNO(3). The effect of the experimental parameters, including sample pH, sampling flow rate, buffer and eluent concentrations were investigated by means of a 2(4) full factorial design, while for the final optimization a Doehlert design was employed. Under the best experimental conditions the preconcentration system provided detection and quantification limits of 2.6 and 8.6mugL(-1), respectively. A wide linear range varying from 8.6 up to 775mugL(-1) (r>0.999) and the respective precision (relative standard deviation) of 7.7 and 1.4% for the 15 and 200mugL(-1) levels were obtained. The characteristics obtained for the performance of the flow preconcentration system were a preconcentration factor of 44.2, preconcentration efficiency of 11min(-1), consumptive index of 0.45mL and sampling frequency estimated as 14h(-1). Preconcentration studies of Pb (II) ions in the presence of the majority foreign ions tested did not show interference, attesting the good performance of MWCNTs. The accuracy of the method was assessed from analysis of water samples (tap, mineral, physiological serum and synthetic seawater) and common medicinal herbs submitted to the acid decomposition (garlic and Ginkgo Biloba). The satisfactory recovery values obtained without using analyte addition method confirms the feasibility of this method for Pb (II) ions determination in different type of samples.

Climatology of F region zonal plasma drifts over Jicamarca

[1] We use extensive incoherent scatter radar observations made at the Jicamarca Radio Observatory between 1970 and 2003 to study and model empirically the equatorial zonal plasma drifts near the F region peak using Bernstein polynomials as base functions. Our quiet-time model results confirm that the daytime drifts are westward and are nearly season and solar cycle independent. The nighttime drifts are eastward, have larger magnitudes, and increase strongly with solar flux, particularly near equinox and December solstice. Enhanced geomagnetic activity drives small eastward perturbation drifts during the day and much larger westward disturbance drifts at night. The nighttime drift perturbations are largest near midnight and increase strongly with solar flux near equinox and December solstice but are essentially absent near June solstice. The Jicamarca zonal disturbance drifts can be largely accounted for by disturbance dynamo electric fields with a dominant time delay of about 3-15 hours following enhanced geomagnetic activity. In the postmidnight sector, there are also smaller westward disturbance drifts associated with time delays of about 15-24 hours and perhaps even longer. Our results strongly suggest that the longitudinal dependence of both the quiet and disturbed equatorial nighttime zonal drifts varies with season.

Tendências em modificação de eletrodos amperométricos para aplicações eletroanalíticas

The most relevant advances on the analytical applications of chemically modified electrodes (CME) are presented. CME have received great attention due to the possibility of electrode surface modification including chemisorption, composite generation and polymer coating. In recent years, the interest in CME has increased overall to improve the sensitivity and selectivity of the electroanalytical probes, considering the electron mediator incorporation and the new conducting polymers development. The general procedures employed for the electrode modification and the operational characteristics of some electrochemical sensors are discussed.

Electrocatalytic NADH oxidation using an electrode based on Meldola blue immobilized on silica coated with niobium oxide

The immobilization of meldola blue (MB) on silica gel coated with niobium oxide (SN) and electrochemical investigation of its behavior as well as the electrocatalytical oxidation of nicotinamide adenine dinucleotide in reduced form (NADH) by this modified electrode are described. The modified silica containing 1.4 mmol g(-1) of niobium oxide dispersed on its surface adsorbed 55 mumol g(-1) of MB. SN containing MB adsorbed on its surface (SNMB) was mixed with carbon paste to prepare modified electrodes. Cyclic voltammetry of this modified electrode showed a quite reversible redox couple with a formal potential of -60 mV (vs. SCE) at pH 7.0. assigned to the immobilized MB. This formal potential is 60 mV more positive than that observed for MB in solution and this shift was attributed to the acidity of SN. The formal potential was independent of the solution pH in the range between 4 and S. A linear response range between 1.0 x 10(-5) and 7.50 x 10(-4) mol L-1 NADH concentration at pH 7.0 was observed. The electrocatalytical properties of the electrode to oxidize NADH was investigated and showed an interesting behavior. The heterogeneous electron transfer rate. k(cbp) was 1852 M(-1)s(-1) and the Michaelis-Menten constant, was 0.27 mM at pH 7,0 with the electrode coverage of about 4.4 x 10(-9) mol cm(-2), evaluated from the rotating disk electrode (RDE) experiments. The slight increase in the reaction rate with the solution pH was assigned to the thermodynamic driving force. This behavior can confirm a possible formation of a charge transfer complex between meldola blue and coenzyme. (Less)

o-Phenylenediamine adsorbed onto silica gel modified with niobium oxide for electrocatalytic NADH oxidation

Abstract A study of a modified carbon paste electrode employing o -phenylenediamine (PDA) adsorbed onto silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of nicotinamide adenine dinucleotide (NADH) is described. The species adsorbed on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential ( E 0′ ) of the adsorbed PDA was −140 mV vs. SCE (saturated calomel electrode). The electrochemical behavior of the adsorbed PDA, compared to that of PDA dissolved in aqueous solution, was completely different. In solution, pH between 3.0 and 8.0, E 0′ remained almost constant and the response was very stable. A linear response range for NADH between 4.0×10 −5 and 8.0×10 −4 mol l −1 , at pH 7.0, was observed for the electrode, with an applied potential of −50 mV vs. SCE. The formation of an intermediate charge transfer complex is proposed for the charge transfer reaction between NADH and adsorbed PDA. The heterogeneous electron transfer rate, k obs , was 5480 mol −1 l s −1 and the apparent Michaelis–Menten constant, 1.04×10 −4 mol l −1 at pH 7.0, evaluated with rotating disk electrode (RDE) experiments with an electrode with a coverage of PDA of 5.7×10 −9 mol cm −2 . The slight increase in the reaction rate with the solution pH was assigned to the thermodynamic driving force.

RMFinder 2.0: An Improved Interactive Multi-Criteria Scenario Reduction Methodology

This paper presents an extension of the RMFinder technique, previously proposed to identify representative models (RMs) within the decision-making process in oil fields. As there are several uncertainties associated with this decision-making process, a large number of scenarios are supposed to be analyzed, so that high-quality production strategies can be defined. Such broad analysis is often unfeasible, so techniques to automatically identify RMs are particularly relevant. The original RMFinder does not consider the individual probability of each RM, which may not be accurate when the risk curves of the problem are estimated. Therefore, a mechanism to calculate the individual probability of each RM was developed here, together with a graphical way to visualize different proposals of RMs. To automatically identify the optimal probability of each RM, this new version of RMFinder minimizes the deviation between the risk curves generated with the selected RMs and the original risk curves of the problem. The graphical approach automatically exhibits, in a single page per solution, the RM dispersion in the scatter plots, the resulting risk curves and the differences between attribute-level distributions. This helps the decision makers to visualize and compare different sets of RMs. The proposed methodology was applied to a small synthetic problem and to three reservoir models based on real-world Brazilian fields: (i) UNISIM-I-D, a benchmark case based on the Namorado field; (ii) UNISIM-II-D, a benchmark case based on a highly fractured pre-salt carbonate reservoir; and (iii) ST001a, a highly heterogeneous heavy oil offshore field. The obtained sets of RMs were evaluated by experts and considered appropriate to the studied problems, being adopted as the standard models in the following steps of the decision-making process to define the production strategies under uncertainties. Introduction One of the main goals of the decision making process in oil fields is a proper definition of the production strategy to be implemented. The basis of the production strategy definition procedure is the geological model of the field, which is used by computational simulators to estimate the behavior of the reservoir during the concession time (given a production strategy). Therefore, given the behavior of the reservoir, it is possible to evaluate the quality of the production strategy. Notice that the production strategies can be evaluated according to different criteria, such as the Net Present Value (NPV), for example. In this context, geological models of oil fields usually contain 1 The production strategy is composed by the infrastructure of wells, pipes, platforms and manifolds, for example. All manuscripts will be sent through an XML tagging process that will standardize the look of the paper and create links for figures, equations, and references. Figures and tables should be placed directly after the first paragraph they are mentioned in. The XML tagging will not alter the technical content of the paper.

Aplicação do Extrato De Amêndoa de Cacau ( Theobroma Cacao. L ) como um Novo Indicador em Titulações Ácido-Base

Neste trabalho, foi proposto a utilizacao de um indicador natural obtido das sementes do Theobroma cacao L. (cacau) que e uma planta encontrada em abundância no sul da Bahia, Brasil. O extrato que contem os corantes a partir de sementes de cacau foi aplicado como indicador em titulacoes acido / base, e os resultados foram comparados com os valores obtidos na titulacao potenciometrica e utilizando a fenolftaleina como um indicador de referencia. As metodologias que envolvem acidos fracos e fortes foram aplicadas com os alunos de pos-graduacao de Quimica da Universidade Estadual de Santa Cruz, em Ilheus, Bahia, Brasil. DOI: 10.5935/1984-6835.20150081