O ensino de aspectos históricos e filosóficos da Química e as teorias ácido-base do século XX

In this paper, an overview about philosophy of science, the concept of "theory" and some characteristics of a"good"theory (1- ordination and explanation of the facts, 2- proposal of problems, and 3- simplicity and funcionality) are initialy introduced. Following, some historical landmarks of acidity and basicity and a summary of the main acid-base theories of the XX century (Arrhenius, solvent systems, protonic, electronic, Lux, Usanovich and ionotropic) are presented. Historical and conceptual relations between these theories are discussed and the three characteristics of a "good" theory are applied. The results showed that the protonic and eletronic theories are the "best". Some discussion of the implications to chemical education are presented too.

Determinacion de la configuracion E-Z de los acidos Fumarico y Maleico. Un experimento orientado a incentivar el desarrollo de la investigacion cientifica en alumnos de Pregrado

In this work we intend to eliminate the idea that laboratory exercises seem like cookbooks. That is, exercises shall be presented as a problematic situation. Based on observation and experimentation, the students should determine the E-Z configuration of maleic and fumaric acids. The basis of this laboratory exercise is the acid-catalyzed isomerization of maleic acid to fumaric acid. Students are given the starting material, reagents and the experimental procedure. They are told that the starting material is a dicarboxylic acid containing a C=C double bond of formula C4H4O4. Students determine melting points, solubilities, acidity and chromatographic patterns for both the starting material and the product, so that a configuration of each acid can be proposed. This type of experiment yields excellent results, because the students are left to deduce that maleic acid is less stable than fumaric acid. Additionally, they conclude that maleic acid is the "Z" isomer and fumaric acid is the "E" isomer. Finally, this laboratory exercise allows the students to develop simultaneously their critical-thinking skills with the respective laboratory techniques and not to see chemistry as recipes to be followed.

Estudo semi-empírico de ácidos hidroxâmicos: ácido formoidroxâmico e derivados do aleloquímico dimboa

Open chain hydroxamic acid (Hx) can exist as Z and E diastereomers of two tautomers, hydroxamic acid and hydroximic acid. The conformational stability of the formohydroxamic acid isomers evaluated by PM3 compared better to ab initio results from the literature than AM1 results. Structural data of the cyclic Hx 2,4-dihydroxy-7-metoxy-2H-1,4-benzoxazin-3(4 H)-one (DIMBOA) obtained by both semiempirical methods compared well to ab initio results. pKa data from the literature for derivatives of the aldolic isomer of DIMBOA were compared to the stability of the anions resulting from the loss of protons of their phenol and hydroxamic acid groups, determined as the difference in heat of formation between anionic and neutral forms, calculated by AM1 and PM3 methods. Good correlations between theoretical and experimental data were obtained for both semiempirical methods.

Tensoatividade de ácidos húmicos de procedências distintas

The surface activity of humic acids (HA) is a parameter which might be considered in the evaluation of all types of aggregation of these substances. In this work the surface tension of aquatic and terrestrial-HA aqueous solutions was observed under varied conditions of pH, concentration and ionic strength. Results showed that HA present surface activity for a extent range of concentration. The surface tension decreases with HA concentration increasing and, for all samples exist a concentration value above which a reduction in the rate of the solute migration to the solution surface is observed. This value is sensibly reduced and sharpened with the increasing of the ionic strength. Surface tension is also reduced with the acidity increasing, being a minimum reached between pH 3 and 5. Observed effects are explained in view of conformational arrangements of HA which tend to agglomerate in micelle-like domains in aqueous solutions.

Uso de métodos analíticos convencionados no estudo da autenticidade do óleo de copaíba

The acidity and ester index are suggested, as simple and cheap methods, adequate to be employed in the study of the authenticity of copaíba oil by small drugstores and export firms. The acidity index permits both the detection and the evaluation of the content of adulteration of copaíba oil. The ester index permits to know if the adulterant either contain ester compounds,as the case of fatty oils, or contain only non saponifiable compounds as is the case of ethyl alchool or mineral oil. The results of the aplication of the methods to eight commercial samples, of copaiba oil, where three samples were adultered, are coherent with those obtained by more sophisticated instrumental methods.

Alcóxidos como precursores na síntese de novos materiais através do processo sol-gel

An overview about the role of alkoxides in the most recent uses of the sol-gel process in the synthesis of new materials is presented. Special attention is focused on the uses of silicon, aluminum, zirconium and titanium alkoxides. This review shows that the alkoxides enable the synthesis of new matrices with controlled surface area, acidity and porosity, as well as some unusual properties. The property associated with the solubility of metal alkoxides opens enormous possibilities of combining them for the synthesis films of powders with a very large range of metal compositions.

Composição iônica majoritária de águas de chuva no centro da cidade de São Paulo

The chemical composition of rainwater samples collected from March 2002 to February 2003 in downtown São Paulo city (Universidade Presbiteriana Mackenzie) is presented. Potentiometric and conductimetric measurements were used to evaluate the [H+] and the total ionic content. Major anions and cations were determined by ion chromatography with conductivity detection. The rainwater was acidic with a mean pH of 4.99. The volume weighted means, VWM, of the anions nitrate, acetate and sulfate were, respectively, 21.2, 16.9 and 12.4 µmol L-1. Ammonium was the most abundant ion with a VWM of 37.6 µmol L-1. The contribution of each anion to the free acidity potential decreases in the following order: SO4(2-) (28.8%), CH3COO- (24.7%), NO3- (22.8%), Cl- (13.4%), HCOO- (7.7%) and C2O4(2-) (2.5%). The relative contribution of the weak organic acids to the free acidity was significant, 34.9%.

Efeito do cromo nas propriedades catalíticas da MCM-41

The effect of chromium on the catalytic properties of MCM-41 was evaluated in order to develop new catalysts for the trimethylbenzene transalkylation with benzene to produce ethylbenzene, a high-value aromatic in the industry. It was found that chromium decreases the specific surface area but increases the acidity, turning MCM-41 into an active and selective catalyst for ethylbenzene and toluene production. The coke produced on the catalyst is hydrogenated and mainly located outside the pores and thus can be easily removed. The catalyst is more active and selective than mordenite, a commercial catalyst, and thus more promising for commercial applications.

Ação adsorvente da bauxita sobre óleo isolante novo

Samples of new insulating mineral oil, after contact with bauxite, were analyzed by visible spectrophotometry, impedance spectroscopy and their total acidity index was measured. The results of these analyses were compared to samples of new insulating mineral oil, which had not been in contact with bauxite. The comparison demonstrated that the bauxite didn't reduce the insulating capacity of the mineral oil and thus could be used to treat the oil in situ during the operation of an electric transformer.

Microesferas de quitosana reticuladas com tripolifosfato utilizadas para remoção da acidez, ferro(III) e manganês(II) de águas contaminadas pela mineração de carvão

Considerable attention has been paid to chitosan and derivatives as efficient adsorbents of pollutants such as metal ions and dyes in aqueous medium. Nevertheless, no report can be found on the remedial actions of chitosan microspheres crosslinked with tripolyphosphate to control acidity, iron (III) and manganese (II) contents in wastewaters from coal mining. In this work, chitosan microspheres crosslinked with tripolyphosphate were used for the neutralization of acidity and removal of Fe (III) and Mn (II) from coal mining wastewaters. The study involved static and dinamic methods. The neutralization capacity of the surface of the static system was 395 mmol of H3O+ per kilogram of microspheres, higher than that of the dynamic one (223 mmol kg-1). The removal of Fe(III) in wastewater was of 100% and that of Mn(II) was 90%.

Cinética e caracterização físico-química do fermentado do pseudofruto do caju (Anacardium occidentale L.)

The production of cashew apple wine has the purpose of minimizing the wastage in the Brazilian cashew production. Knowing that the cashew apple fermentation produces a good cashew wine, a study of alcoholic fermentation kinetics of the cashew apple and the physico-chemical characterization of the product were made. The cashew wine was produced in an stirred batch reactor. The results of the physico-chemical analysis of volatiles, residual sugars, total acidity and pH of cashew wine showed that their concentrations were within the standard limits established by the Brazilian legislation for fruit wines.

O pKa de indicadores ácido-base e os efeitos coloidais

An experiment is proposed that can be included in undergraduate courses of chemistry. The subject is the acidity of organic compounds, which are employed as pH indicators, particularly in acid-base titrations. The indicators used are methyl orange, bromophenol blue and bromocresol green in aqueous medium. The influence of colloidal systems on the equilibrium is evaluated by the pKa. The colloids employed are surfactants like sodium dodecyl sulfate, cetyl-trimethylammonium bromide and a polymeric non-ionic F127 (pluronics). The effect of stabilization promoted by the system on the acidic or basic structureof the indicator establishes the action mechanism of the colloid on the pKa values.

De Svante Arrhenius ao peagâmetro digital: 100 anos de medida de acidez

This work describes the establishment of the concept of pH and the evolution of its measurement. The origin of the pH definition can be found in the development of the chemistry of aqueous solutions during the XIXth century. The electrolytic dissociation theory by Svante Arrhenius played a central role. After the proposal of the pH scale by Sörensen, many years were necessary for the acceptance of this new parameter among chemists in general. Its importance was first recognized in biochemistry and related areas. Twenty years after, its importance had been recognized in many industrial and laboratorial practices. The previous methods were based on colorimetric and electrometric methods, but both suffered from many problems. Acceptance of pH in Chemistry was only possible after the development of experimental trustable measurements. The invention of the pH meter was the primordial step.

Physicochemical parameters of Amazon Melipona honey

Stingless bees produce a honey that is different from the Apis honey in terms of composition. There aren't enough data to establish quality control parameters for this product, mainly due to lack of research results. The aim of this work is to evaluate some physicochemical parameters that can be used for the characterization and for the quality control of the Meliponinae honey. Four different samples were collected in the Amazon region of Brazil in 2004 (Melipona compressipes manaoense bee and Melipona seminigra merribae bee). Honey analyses were performed as described by the official methods. The mean results were: moisture (30.13%), pH (3.65), acidity (24.57 mEq/kg), water activity (0.75), fructose (31.91%), glucose (29.30%) and sucrose (0.19%). These results reinforce the need for a specific regulation for stingless bee honey. This will only be feasible when enough data is available to establish upper and lower limits for the physicochemical parameters used for quality control.

Teores de carbamato de etila e outros componentes secundários em diferentes cachaças produzidas em três regiões do estado de Minas Gerais: Zona da mata, sul de minas e vale do jequitinhonha

Minas Gerais is the highest producer of handmade spirits. Research has been carried out aiming to contribute to progress. Thus, the goal of this work is to evaluate the quality of spirits in three different regions of Minas Gerais state. We analyzed ethyl carbamate, alcoholic grade, volatile acidity, higher alcohols, aldehyde, esters, methanol, furfural and copper. The values obtained for ethyl carbamate varied from undetectable to 643.5 µg L-1, with only one region not following the legislation (150 µg L-1). Most of the compounds in the spirits were present within the limits estabilished by the legislation.