Química Nova, Volume: 23, Issue: 3, Published: 2000
  • Editorial

    Vieira, Paulo Cezar
  • Iron extraction from Brazilian smectite by dithionite-citrate-bicarbonate method Artigo

    Leite, Sidnei Quezada M.; Colodete, Carlos Henrique A.; Dieguez, Lídia Chaloub; San Gil, Rosane A. S.

    Abstract in English:

    A natural clay from Campina Grande region (Paraíba, Brazil), with 8.57% of Fe2O3, was used to study the most appropriate condition to carry out the iron extraction, without altering the clay structure in a significant way. Samples were treated with the Dithionite-Citrate-Bicarbonate method (DCB) for 30 and 120 minutes (pH=9.1), and also with citric acid (pH=1.8; time=15min), at 75°C. Conductivity measurements, X-ray fluorescence, X-ray diffraction, energy-dispersive spectrometry, electron-diffraction with transmission electron microscopy and textural evaluation by nitrogen adsorption were done. The treatment in a basic medium was more selective for iron removal than in acid condition. The time of 30 minutes, with 1.6 g Na2S2O4/10 g clay, was the best condition for the iron extraction.
  • Toluene disproportionation on mordenite type zeolites <FONT FACE=Symbol>-</FONT> activity and selectivity in the xylenes obtention Artigo

    Escobar, Flávia A. Malacrida; Batista, Marcelo da Silva; Urquieta-González, Ernesto A.

    Abstract in English:

    A sodium mordenite zeolite (Na-MOR) was synthesized and modified by dealumination with chloridric acid solution (H-MOR). X-Ray Diffraction (XRD), Inductive Coupled Plasm (ICP) and Scanning Electron Micrograph (SEM) techniques were used for sample characterization. The zeolite catalytic behavior was evaluated through toluene disproportionation at 435°C. It was verified that mordenites were very selective for the disproportionation reaction and the samples with higher aluminum content showed larger initial activity, however, these samples showed too a higher deactivation velocity due to a blockage of the unidimensional porous system of the zeolite by coke deposits. The selectivity to xylene isomers was practically not influenced by the Si/Al ratio and changed with the time on stream, due to coke formation. Transition state shape selectivity inside the mordenite pores is also discussed.
  • Separation and identification of polar constituents from the branches of Porcelia macrocarpa Artigo

    Chaves, Mariana H.; Freitas, Alexander de; Roque, Nídia F.; Cavaleiro, Alberto José

    Abstract in English:

    This paper describes a methodology to separate and identify polar compounds as flavonoid and lignan glucosides, amino acids, salt of quaternary ammonium amino acid and carbohydrates from branches of Porcelia macrocarpa (Annonaceae).
  • Direct determination of selenium in coconut water and coconut milk using graphite furnace atomic absorption spectrometry Artigo

    Aleixo, Poliana C.; Nóbrega, Joaquim A.; Santos Júnior, Dário; Muller, Regina C. S.

    Abstract in English:

    Selenium is both essential and toxic to man and animals, depending on the concentration and the ingested form. Most fruits and vegetables are poor sources of selenium, but coconut can be a good selenium source. Samples were suspended (1 + 4 v/v) in a mixture of tertiary amines soluble in water (10% v/v CFA-C). This simple sample treatment avoided contamination and decreased the analysis time. The standard additions method was adopted for quantification. The action of the autosampler was improved by the presence of the amines mixture in the suspension. A Varian model AA-800 atomic absorption spectrometer equipped with a graphite furnace and a GTA 100 autosampler was used for selenium determination in coconut water and coconut milk. Background correction was performed by means of the Zeeman effect. Pyrolytically coated graphite tubes were employed. Using Pd as chemical modifier, the pyrolysis and the atomization temperatures were set at 1400 and 2200ºC, respectively. For six samples, the selenium concentration in coconut water varied from 6.5 to 21.0 mug L-1 and in coconut milk from 24.2 to 25.1 mug L-1. The accuracy of the proposed method was evaluated by an addition-recovery experiment and all recovered values are in the 99.5-102.3% range. The main advantage of the proposed method is that it can be directly applied without sample decomposition.
  • Effect of adsorbed polyaniline on the thermal stability of iron and arsenic oxides

    Farias, Robson Fernandes de

    Abstract in English:

    Iron and arsenic oxide grains are coated with the conducting organic polymer polyaniline. The obtained samples were characterized by infrared spectroscopy, SEM, conducting measurements and thermogravimetry. The thermal stability of both oxides are increased. For As2O3 the sublimation temperature is increased from 165ºC in the pure oxide to 206ºC in the polymer modified sample. The pure Fe3O4 sample exhibits sublimation at 780ºC whereas the polyaniline coated oxide is stable until at least 1000ºC.
  • The determination of the stoichiometry of the mixed complex of vanadium with hydrogen peroxide and with 4-(2-Pyridilazo) Resorcinol

    He, Xiwen; Tubino, Matthieu; Rossi, Adriana Vitorino

    Abstract in English:

    The present study reports details of the stoichiometric characterization of the mixed complex system, V(H2O2)PAR, formed when vanadium adequately reacts with hydrogen peroxide and with 4-(2-Pyridilazo)Resorcinol. Also the presence of polynuclear species was investigated in order to elucidate about unambiguous assignment of the molar absorptivity, stability constant and composition of the complex. Two mathematical treatments methods of the experimental results were employed. From the results it can be concluded that this system corresponds to a mononuclear complex with 1:1:1 stoichiometry.
  • Effect on the reactive diluent PGE on the cure kinetics of an epoxy resin used in structural composites Artigo

    Costa, Michelle Leali; Rezende, Mirabel Cerqueira; Pardini, Luiz Claudio

    Abstract in English:

    Phenil glycidyl ether (PGE), a monofunctional diluent, has been used in epoxy resins formulations in order to increase the toughness of the epoxy molded composite. In a systematic study concerning its influence in the cure kinetics of the epoxy resin, it was used in concentrations of 2,5; 5,0; 10 and 20% in relation to a diglycidyl ether bisphenol-A (DGEBA)/diamino diphenil-sulfone (DDS) base matrix. Dynamic and isothermal scanning analysis were carried out using a differential scanning calorimety (DSC) equipment. For all the concentrations of PGE, a n order kinetics was observed, with n varing between 0,35 -- 0,91 as a function of the increase in the PGE concentration.
  • Obtention of heavy metal ions adsorbent material by 2-mercaptobenzimidazole electropolimerization at reticulated vitreous carbon surface Artigo

    Sousa, Maria de Fátima Brito; Dallan, Émerson José; Bertazzoli, Rodnei

    Abstract in English:

    Electrochemical methods applied to organic species transformation has been used as excellent synthesis tools. C-C bonds can be established, making possible polymer synthesis by both anodic and cathodic reactions of suitable monomer species at the working electrode surface. In this study, anodic procedure was used to electropolymerization of 2-mercaptobenzimidazole at reticulated glassy carbon (RGC) surface. 2-mercaptobenzimidazole presents ligand sites towards Hg2+, Ag+ and Cu2+ ions. The obtained material has been able to adsorb the above mentioned ions in aqueous solution.
  • Catalytic and electrocatalytic oxidation of organic substrates. Cerium as oxidant Revisão

    Carrijo, Rosa Maria C.; Romero, José Ricardo

    Abstract in English:

    This paper reviews the catalytic and electrocatalytic oxidation of organic compounds using cerium (IV) salts. Direct and indirect methods and those using modified electrodes are described.
  • Carbonium ions Revisão

    Mota, Claudio J. A.

    Abstract in English:

    Carbonium ions are carbocations with a pentacoordinated carbon atom, where the electronic octet is maintained. They possess a three center two electron bond in order to keep the tetravalence of the carbon atom. This paper reviews the concept of carbonium ions, their formation, stability and reactions.
  • The wide synthetic versatility of five membered rings containing phosphorus

    Caliman, Vinicius

    Abstract in English:

    The chemistry of cyclopentadiene rings has been widely studied. This review article deals with a similar chemistry of new compounds containing from 1 to 5 phosphorus atoms on the ring substituting the carbon atoms. The neutral rings containing one, two and three phosphorus atoms can be used as building blocks for the synthesis of new organic compounds containing phosphorus. These rings plus the anionic ones also show great potential as ligands in coordination chemistry. The aim of this article is to show how important this new area is and how diverse the chemistry related to a single type of ring can be.
  • Safrole and eugenol: study of the chemical reactivity and use in the synthesis of biologically active natural products and its derivatives Divulgação

    Costa, Paulo R. R.

    Abstract in English:

    The chemical reactivity of safrole, eugenol, piperonal, vanillin and derivates toward ozone, aluminium chloride, brominating agents and butyl lithium was investigated. The synthesis of naturally occuring anthraquinones, furonaphthoquinones, naphthoquinones, lignans and pterocarpans from these easily available staring materials is also discussed.
  • State of the art of high temperature high resolution gas chromatography Divulgação

    Pereira, Alberto dos Santos; Aquino Neto, Francisco Radler de

    Abstract in English:

    The developments in stationary phase synthesis and capillary column technology, have opened new perspectives in analysis of high molecular mass compounds (<FONT FACE="Symbol">³</FONT>600 daltons) and thermolabile organic compounds by High Temperature High Resolution Gas Chromatography (HT-HRGC). HT-HRGC is a new analytical borderline and its application to the analysis of high molecular mass compounds is still in its infancy. The apolar and medium polar gum phases can now be operated at temperatures up to 400-480ºC, being used for the analysis of n-alcanes up to C-100, lipids, oligosaccharides, industrial resins, polyglycerols, cyclodextrins, porphyrins, etc. This technique should play a leading role as a powerful tool, for many different analysis types, in multidisciplinary fields of Science.
  • Molecular and biomolecular conductance Divulgação

    Gama, A. Arnóbio S. da

    Abstract in English:

    The concept of molecular conductance is discussed in terms of the propagation of an electronic interaction, between electron donor and acceptor groups, through the bonds of a molecular structure where these groups are embedded. The electronic interaction propagation is described by a Green's function matrix element, in a donor-bridge-acceptor molecular system reduced to a two-level representation.
  • The use of mass spectrometry in electrochemical measurements - the DEMS technique Divulgação

    Souza, José Pio I. de; Queiroz, Salete Linhares; Nart, Francisco Carlos

    Abstract in English:

    The combination of cyclic voltammetry and on line mass spectrometry, called differential electrochemical mass spectrometry (DEMS), is already a well established method for on line identification and quantitative detection of the products and intermediates of electrochemical reactions. This article aims to show the principles of the method and how this technique have been used to study electrochemical problems such the electrocatalytic oxidation of organic molecules, as methanol and ethanol, at noble metal electrodes.
  • Studies of the underpotential deposition of metals Divulgação

    Santos, Mauro C.; Machado, Sergio A. S.; Avaca, Luis A.; Mascaro, Lucia H.

    Abstract in English:

    This work reviews recent studies of underpotential deposition (UPD) of several metals on Pt and Au substrates performed in the Grupo de Materiais Eletroquímicos e Métodos Eletroanalíticos (IQSC -- USP, São Carlos). The UPD Cu, Cd and Pb on Pt were analysed in terms of their influence in the oxygen evolution reaction. Partial blockage of surface active sites, promoted by Pb ad-atoms, resulted in a change from water to hydrogen peroxide as the final product. The Ag UPD on Pt and Au substrates was also discussed in this work. A detailed model of charge calculation for Ag monolayer was developed and confirmed by the rotating ring-disk data. The partial charge transfer in UPD studies was analysed in the Cd/Pt and Cd/Au systems and a value of 0.5 was found for the adsorption electrovalence of Cd ad-ions. The Sn/Pt UPD systems were studied from the point of view of the valences of metallic ions in solution. The deposition from Sn(IV) generates a full monolayer with a maximum occupation of approximately 40% of the surface active sites (340 µC cm-2) plus 105 µC cm-2 of Hads (half monolayer). Changing the metallic ion for Sn(II), it was possible to deposit a full monolayer (210 µC cm-2) without any detectable Hads. Finally, the effect of anions was discussed in the Zn/Pt and Zn/Au systems. Here, the hydrogen evolution reaction (her) and the hydrogen adsorption/desorption were used in order to investigate the maximum coverage of the surface with Zn ad-atoms. The full monolayer, characterised by the complete absence of Hads, was achieved only in 0.5 M HF solutions.
  • Exploring the motivation to the chemistry study Educação

    Cardoso, Sheila Pressentin; Colinvaux, Dominique

    Abstract in English:

    This work intends to identify the reasons why people either like or dislike chemistry classes, based on a written questionnaire answered by 157 students of private and public schools. It has a main question - "Do you like to study chemistry? Why?" - and along with others that complement it, we attempt to explore both pupils' school and personal experiences which help them to increase their learning capacities. The answers have given us a very rich piece of discussing material about the like-and-dislike on the study of chemistry mainly about the social interactions in teaching not only this but all other subjects.
  • Buffer capacity evaluating <FONT FACE=Symbol>-</FONT> a collaborative experiment Educação

    Silva, César Ricardo; Simoni, José de Alencar

    Abstract in English:

    The aim of this work is to show an experiment from which students can learn some of the main characteristics of buffer solutions. A mixture of some acid-base indicators, named as Yamada's indicator, can be used to estimate pH values in an acid-base titration of a buffer, with good approximation. In the experiment it is also possible to verify the relationship between the buffer capacity and the concentrations and the molar ratio of the components of a NH3 / NH4+ buffer solution. The shortage of experiments associated with the relative small importance given to many aspects of buffer solutions, is now explored with simplicity. In the proposed experiments, students prepare buffer solutions by themselves, calculate the pH, understand how acid-base indicators act and learn how buffer solutions work through graph constructed by sharing experimental data.
  • Humic substances: a sequential device for fractionation by ultrafiltration with molecular size basis Nota técnica

    Rocha, Julio Cesar; Zara, Luiz Fabricio; Rosa, André Henrique; Sargentini Junior, Ézio; Burba, Peter

    Abstract in English:

    A sequential system for fractionation by ultrafiltration (SSFU) equipped with advanced membranes filters (molecular size cut-off: 5, 10, 30, 50 and 100 kDalton) of the polyethersulfone type was developed for analytical fractionation of humic substances (HS) extracted from aquatic systems or soils. The device consists of five membrane filters (Sartocon® Micro) operated by a multi-channel peristaltic pump, enabling an easy handling, working in a closed system and with simple collection of the six obtained fractions (F1>100; F2: 100-50; F3: 50-30; F4:30-10; F5: 10-5 and F6 <5 kDalton). Then, the HS sample (250 mL solution 1.0 mg/mL, pH 5.0 to 6.0) to be fractionated is pumped by pump through the series of membrane filters with a tangential flow of 85 mL/min, initial pressure 0.2 to 0.3 bar and permeation flux through the membranes of 0.8 to 1.4 mL/min. The overall time for fractionation and cleaning of the device is about 10 h and 25 mL of each fraction is obtained.
  • The Fourier transform (FFT) in basic Nota técnica

    Constantino, Mauricio Gomes; Silva, Gil Valdo José da

    Abstract in English:

    In this paper we describe three computer programs in Basic language about the Fourier transform (FFT) which are available in the Internet site http://artemis.ffclrp.usp.br/SoftwareE.htm (in English) or http://artemis.ffclrp.usp.br/softwareP.htm (in Portuguese) since October 1998. Those are addresses to the Web Page of our Laboratory of Organic Synthesis. The programs can be downloaded and used by anyone who is interested on the subject. The texts, menus and captions in the programs are written in English.
  • Wilhelm Michler, a scientifical adventure in the tropics Assuntos gerais

    Santos, Nadja Paraense dos; Pinto, Angelo C.; Alencastro, Ricardo Bicca de

    Abstract in English:

    Wilhelm Michler lived in Brazil for seven years from 1882 to 1889, when he died. Here, he published several articles on natural products, and became a professor of Industrial Chemistry at the Escola Politécnica, in Rio de Janeiro. He was respected by colleagues and students. This article addresses this poorly known period of the life of a scientist, world-famous by the aromatic ketone that carries his name.
  • The 200 years of the electric pile Assuntos gerais

    Chagas, Aécio Pereira

    Abstract in English:

    In celebration of the 200th anniversary of the electric pile, invented by Alessandro Volta, a synopsis of his letter to The Royal Society (London) and of his biography is presented.
  • Carta ao o editor Assuntos gerais

    Nascimento, Alexandre Rodrigues do
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