Scielo RSS <![CDATA[Química Nova]]> http://www.scielo.br/rss.php?pid=0100-404219990004&lang=en vol. 22 num. 4 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=S0100-40421999000400001&lng=en&nrm=iso&tlng=en</link> <description/> </item> <item> <title><![CDATA[<b>Selective reduction for the determination of mercury in fish</b>: <b>an adaptation of the Magos method</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400002&lng=en&nrm=iso&tlng=en In the selective reduction procedure proposed by Magos SnCl2 was used as reductant for inorganic mercury while total mercury was determined after reduction with a mixture of SnCl2 and CdCl2. The difference between total mercury and inorganic mercury determines the content of organic mercury. The procedure of the present work differs of Magos in that the mercury vapour is carried to the absorption cell after magnetic stirring of the solution in the reaction flask; in the Magos procedure, mercury vapour is carried by bubbling the gas in to the solution. In contrast to the Magos procedure this slight modification overcame the necessity of at calibration by analyte addition, saving time and gainning accuracy. <![CDATA[<b>Caracterization of humic acids from vermicompost of cattle manure composting by 3 and 6 months</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400003&lng=en&nrm=iso&tlng=en The main objective of this research was the characterization of the humic fractions isolated from vermicomposting, originating from cattle manure and treated with Eisenea foetida or Lumbricus rubellus, during 3 and 6 months. Elemental analysis and Infrared and UV-vis spectroscopy were used for their characterizations. The results obtained shown that both humic acids are very similar, but six-month humic acid shown lower percentage of organic material than three month humic acid. The spectroscopy analysis shown that the humic acid studied can be compared with other humic acids reported in the literature. By comparing both vermicomposts, the one produced in three months presents a great potential as fertilizer and it is more economical than the vermicompost produced during a six month period. <![CDATA[<b>The presence of carbonyl compounds in indoor air of São Paulo city</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400004&lng=en&nrm=iso&tlng=en The presence of low carbonyl compounds (C1-C4) and glutaraldehyde was observed in 14 work-locations in São Paulo city, Brazil, during January to July 1997 period. The quantification of other carbonyls was not possible due to a sampling artifact caused probably by undesirable reactions between the ambient ozone and the organic substract of C18 cartridge used to collect carbonyls. High indoor concentrations, compared to outdoor levels, were observed for all compounds. Formaldehyde was the most abundant species (29 ppb) followed by acetone and acetaldehyde (25 and 17 ppb) with a small contribution of propanal, crotonaldehyde and C4 isomers (0.7 to 1.5 ppb) when related to the total indoor carbonyls level. Glutaraldehyde was observed only in two different occupational locations in a hospital at high concentrations (121 ppb). In general, high individual carbonyl levels as well as total carbonyls levels found in several locations indicated an unpropitious air quality for the occupants of these non-industrial sites. <![CDATA[<b>Laboratory study of the effect of the calcium and casein concentration, pH and temperature on the incorporation of dairy proteins to the coagulum obtained by enzimatic action</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400005&lng=en&nrm=iso&tlng=en The effect of casein concentration, Ca2+ concentration, temperature and pH on the amount and size of protein aggregates (fines) in the whey produced by enzimic coagulation of nonfat milk was studied in laboratory conditions. Casein concentrations about 0.3 g/L showed a minimal amount of caseins in the whey, with presence of small aggregates of casein micellles. Ca2+ concentrations higher than 5 mM were neccesary to reduce the whey protein to a minimum constituted by protein particles smaller than casein micelles. The coagulation temperature, in the 35 - 45oC range, produced almost no variations in the whey proteins. The obtention of a minimum amount of whey proteins was possible only in a narrow pH range around 6.4. These results pointed to casein concentration and pH as important variables to be controlled in connection with the process yield. <![CDATA[<b>Ab-initio study of <FONT FACE="Symbol">a</font>-alanine in aqueous medium</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400006&lng=en&nrm=iso&tlng=en Ab initio Hartree-Fock (HF), Density Functional (B3LYP) and electron correlation (MP2) methods have been used to caracterize the aqueous medium intramolecular hydrogen bond in <FONT FACE="Symbol">a</font>-alanine. The 6-31G* and 6-31++G** were taken from Gaussian94 library. We were concerned on the structure of three conformers of <FONT FACE="Symbol">a</font>-alanine, in their neutral form plus on the structure of the zwitterionic form (Z). The Z structure is a stationary point at the HF/6-31G* level but it is not when diffuse functions and electron correlation are included. This results shows that the Z form does not exist in the gas phase. The inclusion of solvent effects changed significantly the results obtained in gas phase, therefore this inclusion make the Z form a stationary point within all level of theory, and the relative energy depends dramatically on the level of calculation. <![CDATA[<B>Determination of organochlorinated present in river from the region of Viçosa, MG</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400007&lng=en&nrm=iso&tlng=en In this work the contamination, by organochlorinated, of a drinking water source located in the region of Viçosa, MG, was evaluated. The identification and quantification of the analytes was carried out using a gas-chromatograph equipped with an electron capture detector after liquid-liquid extraction and concentration. Four insecticides, Aldrin, Epoxy heptachloride, Endrin and op'- DDT were detected in concentrations that are higher than the safety limits established in the Brazilian legislation. <![CDATA[<b>Estimative by DSC data of the sublimation enthalpies for ethyleneurea and propyleneurea</b>: <b>some empirical correlations with amides and tioamides</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400008&lng=en&nrm=iso&tlng=en By using DSC data, the enthaplies of sublimation for ethyleneurea and propyleneurea, are calculated as 84 and 89 kJ mol-1 respectively. Using the vaporization enthalpy values for dimethylethyleneurea and dimethylprophyleneurea, as obtained from literature, the empirical relation: <FONT FACE="Symbol">D</font>crg Hmo (1)/ <FONT FACE="Symbol">D</font>crg Hmo (2) = <FONT FACE="Symbol">D</font>lg Hmo (1)/ <FONT FACE="Symbol">D</font>lg Hmo(2) = constant, that relate sublimation or vaporization enthalpies of two different substances and of its methylated derivatives, is obtained. Correlations like that are found to another ureas and thioureas. <![CDATA[<b>Determination of Cd, Ni and Zn by inductively coupled plasma emission atomic spectrometry after separation and preconcentration in column packed with 5-(4-dimethylaminobenzylidene) rhodanine adsorbed on silica gel</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400009&lng=en&nrm=iso&tlng=en A column packed with 5-(4-dimethylaminobenzylidene)rhodanine adsorbed on silica gel was used for the preconcentration of Cd, Ni and Zn at different spiked solutions prior to their determination by inductively coupled plasma atomic emission spectrometry. This column allowed recoveries over than 98% for the above elements and accurate analyses of coastal sea-water certified reference material were also achieved. <![CDATA[<b>Steady-state and laser flash photolysis of 1 - benzocyclanones and their <FONT FACE="Symbol">a</font></b><strong>,<FONT FACE="Symbol">a</font> </strong><b>-dimethyl derivatives</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400010&lng=en&nrm=iso&tlng=en Laser excitation of 0.01 M solutions of 1-indanone (Ia), 1-tetralone (Ib), 1-benzosuberone (Ic), and their <FONT FACE="Symbol">a</font>,<FONT FACE="Symbol">a</font> -dimethyl derivatives IIa-c, respectively, in benzene, produced transients with maximum absorption at 425 nm, and lifetimes ranging from 62 ns (IIa) to 5.5<FONT FACE="Symbol">m</font>s (Ic). Quenching studies using well known triplet quenchers such as 1,3-cyclohexadiene and oxygen demonstrated the triplet nature of these transients. In the presence of hydrogen donors, such as 2-propanol, the triplet state decay of the ketones Ia-c leads to the formation of the corresponding ketyl radicals, i.e. IIIa-c, which show absorption spectra very similar to the parent ketone, with <FONT FACE="Symbol">l</font>max at 430 nm and lifetime in excess of 20 <FONT FACE="Symbol">m</font>s. Steady state irradiations show that the <FONT FACE="Symbol">a</font>,<FONT FACE="Symbol">a</font> -dimethyl ketones IIa and IIc form ortho-alkyl benzaldehydes probably derived from an initial a-cleavage of the corresponding triplet excited states. <![CDATA[<b>Diffuse reflectance technique applied to a fluorescence study of n-alkyl substituted 2,3-naphthalimides included in <FONT FACE="Symbol">b</font>-ciclodextrin and adsorbed in microcrystalline cellulose</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400011&lng=en&nrm=iso&tlng=en The solution fluorescence of N-alkyl-2,3-naphthalimides (1-4) in polar protic and aprotic solvents was compared to the emission from solid samples resulting from the imide complexation with <FONT FACE="Symbol">b</font>-cyclodextrin or adsorption on the surface of microcrystalline cellulose. Solid samples of the inclusion complex 2,3-naphthalimides/<FONT FACE="Symbol">b</font>-cyclodextrin show maximum for fluorescence emission significantly different to the observed in methanolic solution. Beside this, a clear effect on the alkyl chain length could be observed for these samples which is probably due to differences in probe location inside the cyclodextrin cavity. The constancy for fluorescence quantum yield and fluorescence lifetime for the imides 1 - 4 adsorbed on microcrystalline cellulose suggests that, independently of the polarity of the solvent used for sample preparation, the probe is preferentially located on the cellulose surface. An increase of fluorescence quantum yield and fluorescence lifetime for solid samples, when compared to the values obtained in solution for the different solvents employed in this study (acetonitrile, methanol and water), is fully in accordance with a decrease of the probe mobility due to inclusion in <FONT FACE="Symbol">b</font>-cyclodextrin or to adsorption on cellulose. <![CDATA[<b>Kinetic and mechanism of S(IV) autoxidation reaction catalysed by transition metal ions</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400012&lng=en&nrm=iso&tlng=en The oxidation process of sulfur (IV) species (SO2, HSO3- e SO32-) by oxygen, catalysed by trace metal ion and complexes, can play an important role in atmospheric, analytical and bioinorganic chemistry. An overview of the most important reactions in these fields is presented. A fascinating redox cycling of the metal ions and complexes during such autoxidation process was revealed by the combination of kinetics and coordination chemistry studies. <![CDATA[<b>Twenty years of SERS</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400013&lng=en&nrm=iso&tlng=en The Surface Enhanced Raman Scattering (SERS) effect was observed for the first time in 1974, but it was only considered a new effect three years later, hence, nearly twenty years ago. Since its discovery, a significant amount of investigations have been performed aiming at to clarify the nature of the observed enhancement, to improve the surface stability and to establish applications which nowadays range from the study of biomolecules to catalysis. Some of the more relevant aspects of this effect which have been examined across the last two decades are summarized in this paper which presents the introductory aspects of SERS alongside with several of its applications. <![CDATA[<b>Characterization of polymorphism in pharmaceuticals by solid state nuclear magnetic resonance</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400014&lng=en&nrm=iso&tlng=en This review aims to present some features about solid state NMR and its application in the field of pharmaceutical chemistry, for the characterization of polymorphism of pharmaceutical molecules. <![CDATA[<b>Ultrasensitive analytical methods</b>: <b>thermal lens and related techniques</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400015&lng=en&nrm=iso&tlng=en Among the emergent laser based spectrometric methods, thermal lensing and other photothermal techniques present a great potential for solving a variety of problems in the fields of chemistry, physics and biology. Their main advantages are high concentration sensitivity, sensibility to physical-chemical properties of the medium, excellent spatial resolution and noninvasive characteristics. In this article, theoretical principles, main applications and practical hints as well as fundamental limitations of these techniques will be carefully described. It is hoped that this will give the reader a clear picture of this field of investigation as well as provide to the ones who are not specialists in the area, the necessary background to understand, implement and use photothermal techniques. In the final sections the development frontiers of photothermal spectrometry will be discussed. <![CDATA[<b>Statistical thermodynamics of liquids using the Monte Carlo method. II. Liquid chloroform</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400016&lng=en&nrm=iso&tlng=en Thermodynamic properties and radial distribution functions for liquid chloroform were calculated using the Monte Carlo method implemented with Metropolis algorithm in the NpT ensemble at 298 K and 1 atm. A five site model was developed to represent the chloroform molecules. A force field composed by Lennard-Jones and Coulomb potential functions was used to calculate the intermolecular energy. The partial charges needed to represent the Coulombic interactions were obtained from quantum chemical ab initio calculations. The Lennard-Jones parameters were adjusted to reproduce experimental values for density and enthalpy of vaporization for pure liquid. All thermodynamic results are in excelent agreement with experimental data. The correlation functions calculated are in good accordance with theoretical results avaliable in the literature. The free energy for solvating one chloroform molecule into its own liquid at 298 K and 1 atm was computed as an additional test of the potential model. The result obtained compares well with the experimental value. The medium effects on cis/trans convertion of a hypotetical solute in water TIP4P and chloroform solvents were also accomplished. The results obtained from this investigation are in agreement with estimates of the continuous theory of solvation. <![CDATA[<B>Biological functions of nitric oxide</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400017&lng=en&nrm=iso&tlng=en Nitric Oxide has been shown to be involved in many important biological functions. It serves as a vasodilator and a neurotransmitter in the brain and peripheral nervous system. It is also involved in the bacteria-killing process by macrophages. In this review we will discuss progress in nitric oxide chemistry and we will also attempt to explain its action in the cardiovascular, nervous and immune systems. <![CDATA[<strong>Carbon fibers</strong>: <strong>electroanalytical applications as electrodic material</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400018&lng=en&nrm=iso&tlng=en The use of carbon fibers to develop new sensors and biosensors has received great attention due to its characteristics and electrochemical properties. A brief presentation about history, properties, characteristics, composition and structure of the carbon fibers are shown in this paper. Several applications of the carbon fibers in electroanalytical chemistry for determination of metals and organic molecules in environmental and clinical samples are also described. <![CDATA[<strong>Production of electrolytic manganese dioxide for usage in lithium batteries</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400019&lng=en&nrm=iso&tlng=en The <FONT FACE="Symbol">e</font> phase of electrolytic manganese dioxide (EMD) is the structural form most easily converted in the LiMn2O4 spinel used as cathode in lithium batteries. Thus, employing titanium as anode, a study of electrolysis parameters was carried out in order to determine the best conditions to produce an <FONT FACE="Symbol">e</font>-EMD suitable for that spinel preparation. The influence of solution temperature (65oC and 90oC) and current density (between 1 mA/cm2 and 17.5 mA/cm2) on the anode potential and the EMD properties was investigated using an aqueous 2.0 mol/L MnSO4 + 0.30 mol/L H2SO4 solution. In any of the electrolysis conditions tested only the e-EMD structure was obtained, but its specific surface area varied with the applied current density and temperature. Drying the <FONT FACE="Symbol">e</font>-EMD at temperatures between 60oC and 120oC did not cause any phase changes. To produce a suitable EMD at the highest current density possible without passivation of the titanium anode, the best electrolysis parameters were determined to be 90oC and 15 mA/cm2. The <FONT FACE="Symbol">e</font>-EMD thus obtained had a specific surface area (BET) of ca. 65 m2/g. <![CDATA[<strong>Experimental organic Chemistry</strong>: <strong>integration of theory, experiment and analysis</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400020&lng=en&nrm=iso&tlng=en A new approach for teaching in basic experimental organic chemistry is presented. Experimental work goes on parallel to theoretical lectures leading to an immediate application of theoretical concepts transmitted therein. One day/week is dedicated exclusively to the organic laboratory. Reactions are proposed as problems to be solved; the student has to deduce the structure of the product on the basis of his observations, the analytical data and his mechanistical knowledge. 70 different experiments, divided in 7 thematical chapters, are presented. All experiments require the analysis and discussion of 1H and 13C NMR, IR and UV spectra. Additional questions about each reaction have to be answered by the student in his written report. Laboratory safety is garanteed by the exclusion or substitution of hazardous and toxic reagents. Microscale preparations are adopted in most cases to lower the cost of materials and the amount of waste. Recycling of many reaction products as starting materials in other experiments reduces the need for commercial reagents and allows the execution of longer reaction sequences. Only unexpensive standard laboratory equipment and simple glassware are required. All experiments include instructions for the save treatment or disposal of chemical waste. <![CDATA[<strong>Evaluating approaches employed in Physical Chemistry teaching</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400021&lng=en&nrm=iso&tlng=en The approaches are part of the everyday of the Physical Chemistry. In many didactic books in the area of Chemistry, the approaches are validated starting from qualitative and not quantitative approaches. We elaborated some examples that allow evaluating the quantitative impact of the approaches, being considered the mistake tolerated for the approximate calculation. The estimate of the error in the approaches should serve as guide to establish the validity of the calculation, which use them. Thus, the shortcut that represents a calculation approached to substitute accurate calculations; it can be used without it loses of quality in the results, besides indicating, as they are valid the adopted criterions. <![CDATA[<strong>The influence of Chemistry on XVIIIth century academic and practical medical knowledge in Portugal and Brazil</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400022&lng=en&nrm=iso&tlng=en At the dawn of the eighteenth century chemistry was establishing itself as a physical science on its own right, after a long ancillary relationship with medicine and pharmacy, which had began two centuries before. This association, and the many changes that came along the scientific revolution spread into many walks of life. The Luso-Brazilian world, apparently so removed from the new developments, could not help to be touched by them, as this study shows, in which two contemporary medical authors are analysed. Both were Portuguese who had long lived in Brazil; both practised and wrote extensively on Medicine; both felt the influence of the new times, albeit in quite different ways. <![CDATA[<strong>Parameters and symbols for use in Nuclear Magnetic Resonance (IUPAC recommendations 1997)</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400023&lng=en&nrm=iso&tlng=en NMR is now frequently the technique of choice for determination of chemical structure in solution. Its uses also span structure in solids and mobility at the molecular level in all phases. The research literature in the subject is vast and ever-increasing. Unfortunately, many articles do not contain sufficient information for experiments to be repeated elsewhere, and there are many variations in the usage of symbols for the same physical quantity. It is the aim of the present recommendations to provide simple check-lists that will enable such problems to be minimised in a way that is consistent with general IUPAC formulation. The area of medical NMR and imaging is not specifically addressed in these recommendations, which are principally aimed at mainstream use of NMR by chemists (of all sub-disciplines) and by many physicists, biologists, material scientists and geologists etc. working with NMR. <![CDATA[<strong>In memorian</strong>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40421999000400024&lng=en&nrm=iso&tlng=en NMR is now frequently the technique of choice for determination of chemical structure in solution. Its uses also span structure in solids and mobility at the molecular level in all phases. The research literature in the subject is vast and ever-increasing. Unfortunately, many articles do not contain sufficient information for experiments to be repeated elsewhere, and there are many variations in the usage of symbols for the same physical quantity. It is the aim of the present recommendations to provide simple check-lists that will enable such problems to be minimised in a way that is consistent with general IUPAC formulation. The area of medical NMR and imaging is not specifically addressed in these recommendations, which are principally aimed at mainstream use of NMR by chemists (of all sub-disciplines) and by many physicists, biologists, material scientists and geologists etc. working with NMR.