Scielo RSS <![CDATA[Química Nova]]> http://www.scielo.br/rss.php?pid=0100-404220010002&lang=en vol. 24 num. 2 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.br/img/en/fbpelogp.gif http://www.scielo.br <link>http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200001&lng=en&nrm=iso&tlng=en</link> <description/> </item> <item> <title><![CDATA[<b>The catalytic combustion of methane</b>: <b>statistical study of preparation and pretreatment conditions of palladium supported catalysts and their relationship with catalytic activity</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200002&lng=en&nrm=iso&tlng=en The catalytic combustion of methane on alumina supported palladium catalysts was studied. It has been reported that the activity of the catalyst increases with its time on line, despite of an increase of the palladium particle size. However, different preparation, pretreatment and testing conditions can be the reason for the observed different results. An experimental design, which allows to verify the influence of several parameters at the same time with a good statistical quality, was used. A Plackett-Burman design was selected for the screening of the variables which have an effect on the increase of the catalyst activity. <![CDATA[<B>Solid-phase extraction (SPE) and solid-phase microextraction of pyrethroids in water</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200003&lng=en&nrm=iso&tlng=en The pyrethroids bifenthrin, permethrin, cypermethrin and deltamethrin were extracted by solid phase extraction (SPE) and solid phase microextraction (SPME). The analysis were performed on a gas chromatograph with electron capture detection (GC-ECD). Octadecil Silano-C18, Florisil and Silica stationary phases were studied for SPE. Better results were obtained for Florisil which gave recoveries from 80% to 108%. Pyrethroids extraction by SPME showed a linear response and a detection limit of 10 pg ml-1. Although the data showed that the two extraction methods were able to isolate the pesticide residues from water samples, the best results were obtained by using SPME which is more sensitive, faster, cheeper, being a more useful technique for the analysis of pyrethroids in drinking water. <![CDATA[<B>Comparison between on-column injection and dynamic headspace in the determination of Benzene, Toluene and Xylene (BTX) in water</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200004&lng=en&nrm=iso&tlng=en The analysis of water samples containing volatile organic compounds has become an important task in analytical chemistry. Gas chromatography has been widely used for the analysis of volatile organic compounds in water. The headspace analysis shows as a principal characteristic the possibility of determination of the volatile components in drinking water. Benzene, Toluene and Xylene (BTX) are important compounds usually present in drinking water, from contamination by petroleum derivatives. Since they are toxic compounds even when present in low concentration levels, their determination is important in order to define the quality of the water. The sampling technique using headspace, coupled with gas chromatography as the separation method, showed to be suitable for BTX analysis in several samples at the mug/L (ppb) level. <![CDATA[<b>Combination of low- and high-pressure flow systems for the determination of Mg, Ca and Sr in brines by flame atomic absorption spectrometry</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200005&lng=en&nrm=iso&tlng=en This article describes the combination of low- and high-pressure flow systems for the determination of Magnesium, Calcium and Strontium by flame atomic absorption spectrometry (FAAS). In the low-pressure system a short C-18 RP column (length 0,5 cm) was utilized for the preconcentration/matrix separation step, xylenol orange was used as chelating agent and tetrabutylamonium acetate for ion pair formation. The hydraulic high pressure nebulization (HHPN) was used for sample transport and sample introduction in the high pressure system. The repeatabilities and detection limits for Mg, Ca and Sr were determined and compared with those obtained by pneumatic nebulization (PN). The results show that the detection limits obtained using the HHPN for Mg, Ca and Sr are between 1.5 to 2 times better than those obtained by PN when the signal transient was measured in area. The system presented a sampling frequency of 130 h-1 for direct determination of Mg, Ca or Sr in samples of saturated sodium chloride used in the production of chlorine and sodium hydroxide. <![CDATA[<b>Activity-guided isolation of the constituents from bark of stem of <i>Duguetia glabriuscula </i>- Annonaceae, using Brine Shrimp Lethality test (BSL)</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200006&lng=en&nrm=iso&tlng=en The extract obtained from stem bark of Duguetia glabriuscula - Annonaceae was evaluated by Brine Shrimp Lethality test (BSL). The bioactive compounds, oxobufoline and lanuginosine, two oxoaporphine alkaloids were isolated by activity-guided fractionation. In addition, the compounds asaraldehyde, (+)-allo-aromadendrane-10beta, 14-diol, and two aporphine alkaloids, polyalthine and oliveridine were also obtained. <![CDATA[<B>Flow injection spectrophotometric system for hydrogen peroxide monitoring in photo-Fenton degradation processes</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200007&lng=en&nrm=iso&tlng=en A flow injection spectrophotometric system was projected for monitoring hydrogen peroxide during photodegradation of organic contaminants in photo-Fenton processes (Fe2+/H2O2/UV). Sample is injected manually in a carrier stream and then receives by confluence a 0.1 mol L-1 NH4VO3 solution in 0.5 mol L-1 H2SO4 medium. The product formed shows absorption at 446 nm which is recorded as a peak with height proportional to H2O2 concentration. The performance of the proposed system was evaluated by monitoring the consumption of H2O2 during the photodegradation of dichloroacetic acid solution by foto-Fenton reaction. <![CDATA[<B>Flow system for spectrophotometric urea determination in animal blood plasma employing leguminous plant as natural <I>urease </I>enzyme source</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200008&lng=en&nrm=iso&tlng=en A spectrophotometric flow injection analysis (FIA) procedure employing natural urease enzyme source for the determination of urea in animal blood plasma was developed. Among leguminous plants used in the Brazilian agriculture, the Cajanus cajan specie was selected as urease source considering its efficiency and availability. A minicolumn was filled with leguminous fragments and coupled to the FIA manifold, where urea was on-line converted to ammonium ions and subsequently it was quantified by spectrophotometry. The system was employed to determine urea in animal plasma samples without any prior treatment. Accuracy was assessed by comparison results with those obtained employing the official procedure and no significant difference at 90 % confidence level was observed. Other profitable features such as an analytical throughput of 30 determinations per hour, a reagent consumption of 19.2 mg sodium salicylate, 0.5 mg sodium hipochloride and a relative standard deviation of 1.4 % (n= 12) were also obtained. <![CDATA[<b>Comparative study of analytical methods for iron and titanium determination in Portland cement</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200009&lng=en&nrm=iso&tlng=en In the present work four different analytical methodologies were studied for the determination of iron and titanium in Portland cement. The cement samples were dissolved with hot HCl and HF, being compared Fe and Ti concentrations through four analytical methods: molecular absorption spectrophotometry using the reagents 1,2-hydroxybenzene-3,5-disulfonic acid (Tiron) and the 5-chloro-salicylic acid (CSA), inductively coupled plasma atomic emission spectrometry (ICP-AES) and flame atomic absorption spectrophotometry (FAAS). In the spectrophotometric determinations were studied pH conditions, reagents addition order, interferences, amount of reagents, linear range and stability of the system. In the techniques of ICP-AES and FAAS were studied the best lines, interferences, sensibility and linear range. The obtained results were compared and the agreement was evaluated among the methods for the determination of the metals of interest. <![CDATA[<b>Metal-hexacyanoferrate films</b>: <b>a tool in analytical Chemistry</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200010&lng=en&nrm=iso&tlng=en Chemically modified electrodes based on hexacyanometalate films are presented as a tool in analytical chemistry. Use of amperometric sensors and/or biosensors based on the metal-hexacyanoferrate films is a tendency. This article reviews some applications of these films for analytical determination of both inorganic (e.g. As3+, S2O3(2-)) and organic (e.g. cysteine, hydrazine, ascorbic acid, gluthatione, glucose, etc.) compounds. <![CDATA[<b>Strategies for the syntheses of deoxinucleosides</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200011&lng=en&nrm=iso&tlng=en The number of agents available for the treatment of viral infections has increased dramatically during the past decade most of them belonging to the nucleoside class. The accomplishment of many syntheses for these nucleosides in recent years offers evidence that the field is very important for the organic synthesis. The discussion of several approaches for preparing deoxy and dehydronucleosides is reviewed in a simplified way. <![CDATA[<B>Gas chromatography coupled with atomic absorption spectrometry</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200012&lng=en&nrm=iso&tlng=en This review presents an updated overview of the trace element speciation by gas chromatography coupled with atomic absorption spectrometry. <![CDATA[<b>Ultrathin polymer films produced by the self-assembly technique</b>: <b>preparation, properties and applications</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200013&lng=en&nrm=iso&tlng=en The self-assembly technique is a powerful tool to fabricate ultrathin films from organic compounds aiming at technological applications in molecular electronics. This relatively new approach allows molecularly flat films to be obtained on a simple and cheap fashion from various types of material, including polyelectrolytes, conducting polymers, dyes and proteins. The resulting multilayer films may be fabricated according to specific requirements since their structural and physical properties may be controlled at the molecular level. In this review we shall comment upon the evolution of preparation methods for ultrathin films, the process of adsorption and their main properties, as well as some examples of technological applications of layer-by-layer or self-assembled films. <![CDATA[<b>Applications of combinatorial Chemistry to drug development</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200014&lng=en&nrm=iso&tlng=en Combinatorial Chemistry has become a very efficient methodology in drug research. Recent progress in combinatorial synthesis performed both in solid and solution phase have led to a change in the paradigm for the identification and optimization of lead compounds. This article gives an overview of the principal characteristics of combinatorial libraries and some examples of the application of this methodology in the identification of test compounds and lead compound optimization, either from synthetic or natural sources. <![CDATA[<B>Nickel-metal hydride batteries, an alternative for the nickel-cadmium batteries</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200015&lng=en&nrm=iso&tlng=en Nickel metal hydride (Ni-MH) batteries have emerged as an alternative for replacement of nickel-cadmium batteries, because of their more environmental compatibility and high energy capacity. In this article, we described the properties and applications for Ni-MH batteries, giving some emphasis on the metal-hydride electrode, including the description of composition, the charge storage capacity and the discharge profile. The key component of the nickel-metal hydride electrode is a hydrogen storage alloy whose composition is formulated to obtain a high stable material over a large number of charge-discharge cycles. <![CDATA[<b>The petrochemical industry in the next century</b>: <b>how to replace petroleum as raw material</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200016&lng=en&nrm=iso&tlng=en The world reserves of petroleum will finish in about 100 years. For a tropical country like Brazil, biomass will be the natural substitute for petroleum. For the best utilization of biomass, it first needs to be separated into its principal components: cellulose, hemicelluloses, lignins, vegetable and essential oils, non-structural carbohydrates, bark and foliage. All feedstocks for the chemical industry can be obtained from these biomass components, as shown in the first part of this paper. In the second part we discuss how the major products from petrochemicals can be obtained from the different biomass components. We show that Brazil can use different strategies, compared to other countries, to obtain petrochemical products, which could result in innovations. However, it is necessary that the government starts to invest immediately in order to keep the petrochemical industries competitive with foreign industries, so that they continue to be one of Brazil's major employers. <![CDATA[<b>Considerations on the hydrogen peroxide electrogeneration</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200017&lng=en&nrm=iso&tlng=en This paper presents some results that may be used as previous considerations to a hydrogen peroxide electrogeneration process design. A kinetic study of oxygen dissolution in aqueous solution is carried out and rate constants for oxygen dissolution are calculated. Voltammetric experiments on vitreous carbon cathode shown that the low saturation concentration drives the oxygen reduction process to a mass transfer controlled process which exhibits low values of limiting currents. Results have shown that the hydrogen peroxide formation and its decomposition to water are separated by 400 mV on the vitreous carbon surface. Diffusion coefficients for oxygen and hydrogen peroxide are calculated using data taken from Levich and Tafel plots. In a series of bulk electrolysis experiments hydrogen peroxide was electrogenerated at several potential values, and concentration profiles as a function of the electrical charged passed were obtained. Data shown that, since limiting current plateaus are poorly defined onto reticulated vitreous carbon, cathodic efficiency may be a good criterion for choosing the potential value in which hydrogen peroxide electrogeneration should be carried out. <![CDATA[<b>Construction and optimization of a low cost reactor for the photodegradation of organic matter from natural waters and its application for the study of copper speciation by voltammetry</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200018&lng=en&nrm=iso&tlng=en This work describes the construction of a home-made low-cost reactor, using easily available materials, capable of destroying efficiently dissolved organic matter. Just 30 minutes of irradiation were sufficient to destroy more than 99% of the humic acids present in a solution of 4 mg C L-1. Copper speciation was evaluated in natural waters of different salinities to test the reactor's efficiency in destroying organically complexed metal species. The effect of the organic matter concentration, salinity, dissolved oxygen and temperature in the photo-oxidation process is discussed. <![CDATA[<b>Dropping out of the Chemistry course at the University of Brasilia</b>: <b>students reasons to leave the course</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200019&lng=en&nrm=iso&tlng=en This study aimed the investigation of the reasons that led undergraduate students of the Chemistry course at Universidade de Brasília to abandon the course before its conclusion. From the former students' point of view, the reasons that led them to leave the course were related to aspects of their academic life prior to university entrance, expectations which failed to be met by the course, the intricate nature of the academic system, personal and social experiences. Abandonment is a consequence of the problems he encounters during the course and takes on a connotation of protest, more than of failure. <![CDATA[<B>Women in science and technology</B>: <B>restricted success</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200020&lng=en&nrm=iso&tlng=en In contemporary times, women can choose freely to enter any professional field. Along the way they come across stumbling blocks that make their progress difficult. Most of these difficulties are not gender-specific, yet women encounter them more consistently than do men. It is remarkably true for the areas of Science and Technology. However, it is not straightforward to evaluate and to obtain an accurate measure of the effects of gender bias. The factors and consequences associated with the phenomena are multiple, with many shades of regionalism as we look at different countries. Despite of the absence of detailed studies regarding the situation in Brazil, it seems unlikely we would be an exception to a world pattern. In this article, some causes as well as current actions around the world to fight gender bias are presented. <![CDATA[<B>The physician George Thomson and the early developments of the concept of gas</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000200021&lng=en&nrm=iso&tlng=en The word gas was coined by the "chemical philosopher" Joan Baptista Van Helmont (1579 -- 1644) to name a very broad concept in his chemico-medical system. Eventually, some physicians who followed Helmontian ideas adopted the concept. The present paper aims to analyze the reception of the original idea of gas by an English Helmontian physician, George Thomson (1619 -- 1677). Thomson wrote that the "material cause" of the plague was a gas, and compared it to the "Gas of sulphur". He also related the human archeus to a gas, and explained some observations in the laboratory in terms of production of gases. We observe, however, that Thomson was not as interested as Van Helmont in discussing details about the structure of the matter. Thus, gas did not have the same relevance in Thomson's work as it had in Van Helmont's.