Resumo: As atividades experimentais representam um papel importante no processo de ensino e aprendizagem de Química, porque motivam os estudantes e promovem a correlação entre atividades desenvolvidas em sala de aula e situações cotidianas. Baseado neste contexto, este trabalho apresenta uma proposta experimental para a síntese de FeSO4.7H2O utilizando palha de aço, visando trabalhar conceitos como estequiometria, reações de óxido-redução, lei dos gases ideais, interações intermoleculares, solubilidade, volumetria e equilíbrios em meio aquoso, destacando os equilíbrios ácido-base, de precipitação e de complexação. O experimento é direcionado tanto para estudantes do ensino médio, como para alunos de graduação, visando auxiliá-los no desenvolver conceitos importantes na química geral, química inorgânica e química analítica. Em contrapartida, a atividade também poderá auxiliar na reflexão sobre reaproveitamento de materiais.

Abstract: Experimental activities play an important role in the teaching and learning process in chemistry, because it motivates students and promotes the correlation between the activities in the classroom and everyday situations. Based on this context, this work presents an experimental proposal for the synthesis of FeSO4.7H2O using steel wool, aiming at the presentation of concepts such as stoichiometry, oxidation-reduction reactions, the ideal gas law, the intermolecular interactions, solubility, volumetry and equilibrium in aqueous medium, emphasizing the acid-base, precipitation and complexation equilibria. The experiment is designed for both high school students and undergraduate students, with the goal of helping them to develop key concepts in General Chemistry, Inorganic Chemistry and Analytical Chemistry. Moreover, activity may also assist in reflection on reuse of materials.

O ácido fítico é um composto natural muito utilizado como despigmentante em emulsões cosméticas magistrais. No entanto, observou-se experimentalmente que o ácido fítico, quando aquecido a 150 °C durante cerca de uma hora, mostra evidências de decomposição térmica e que poucos estudos envolvendo essa espécie, isoladamente quanto a sua estabilidade, estão disponíveis na literatura. Esse fato motivou o estudo de caracterização e de comportamento térmico dessa espécie, empregando a análise térmica (TG/DTG e DSC) e associando os resultados obtidos com aqueles de análise elementar (AE) e espectroscopia de absorção na região do infravermelho. As curvas TG/DTG e DSC permitiram avaliar o comportamento térmico da amostra de ácido fítico e, com isso, foi possível estudar, pelo método termogravimétrico não isotérmico, a cinética dos três principais eventos de perda de massa, o de desidratação I, desidratação II e de decomposição térmica. A associação das técnicas de espectroscopia de absorção no infravermelho e análise elementar permitiu avaliar os produtos intermediários da decomposição térmica do ácido fítico. Os espectros no infravermelho de amostras isoladas durante o aquecimento evidenciaram a diminuição de intensidade da banda de absorção relativa ao estiramento do grupo O-H como consequência do processo de desidratação. Também, os resultados de análise elementar indicaram que nas temperaturas de 95, 150 e 263 ºC houve aumento no teor de carbono e diminuição do teor de hidrogênio. Visualmente, observou-se o escurecimento do material a 150 ºC, indicando que a decomposição térmica do material iniciou-se nessa temperatura. Na temperatura de 380 ºC, a diminuição do teor de carbono e hidrogênio foi devido ao avanço do processo de decomposição térmica. Os resultados da termogravimetria juntamente com aqueles da análise elementar permitiram concluir que há concordância entre as percentagens de ácido fítico encontrado na solução aquosa. O estudo cinético pelo método termogravimétrico não isotérmico mostrou que o processo de desidratação ocorreu em duas etapas. Na etapa I da desidratação ocorreu um processo de vaporização de água (ordem de reação igual a zero). Na etapa II da desidratação foi encontrada uma ordem de reação igual a cinco. Essa mudança na ordem de reação foi atribuída à perda de moléculas de água quimicamente ligada, ao ácido fítico ou à presença de substâncias voláteis. Finalmente, na etapa de decomposição térmica foi observado que a ordem da reação foi igual a um. Para as outras etapas de perda de massa não foi possível a realização do estudo cinético.

Phytic acid is a natural compound widely used as depigmenting agent in galenic cosmetic emulsions. However, we have observed experimentally that phytic acid, when heated to 150 ºC for around one hour, shows evidence of thermal decomposition. Few studies investigating this substance alone with regard to its stability are available in the literature. This fact prompted the present study to characterize this species and its thermal behavior using thermal analysis (TG/DTG and DSC) and to associate the results of these techniques with those obtained by elemental analysis (EA) and absorption spectroscopy in the infrared region. The TG/DTG and DSC curves allowed evaluation of the thermal behavior of the sample of phytic acid and enabled use of the non-isothermal thermogravimetric method to study the kinetics of the three main mass-loss events: dehydration I, dehydration II and thermal decomposition. The combination of infrared absorption spectroscopy and elemental analysis techniques allowed evaluation of the intermediate products of the thermal decomposition of phytic acid. The infrared spectra of samples taken during the heating process revealed a reduction in the intensity of the absorption band related to O-H stretching as a result of the dehydration process. Furthermore, elemental analysis results showed an increase in the carbon content and a decrease in the hydrogen content at temperatures of 95, 150, 263 and 380 °C. Visually, darkening of the material was observed at 150 °C, indicating that the thermal decomposition of the material started at this temperature. At a temperature of 380 °C, thermal decomposition progressed, leading to a decrease in carbon and hydrogen. The results of thermogravimetry coupled with those of elemental analysis allow us to conclude that there was agreement between the percentages of phytic acid found in aqueous solution. The kinetic study by the non-isothermal thermogravimetric method showed that the dehydration process occurred in two stages. Dehydration step I promoted a process of vaporization of water (reaction order of zero), whereas dehydration step II showed an order of reaction equal to five. This change in reaction order was attributed to loss of chemically bonded water molecules of phytic acid or to the presence of volatile substances. Finally, the thermal decomposition step revealed an order of reaction equal to one. It was not possible to perform the kinetic study for other stages of mass loss.

Hydrated compounds prepared in aqueous solution by reaction between amidosulfonic acid [H3NSO3] and suspensions of rare earth hydroxycarbonates [Ln2(OH)x(CO3)y.zH2O] were characterized by elemental analysis (% Ln, % N and % H), infrared spectroscopy (FTIR) and thermogravimetry (TG). The compounds presented the stoichiometry Ln(NH2SO3)3.xH2O (where x = 1, 5, 2.0 or 3.0). The IR spectra showed absorptions characteristic of H2O molecules and NH2SO3 groups. Degree of hydration, thermal decomposition steps and formation of stable intermediates of the type [Ln2(SO4)3] and (Ln2O2SO4), besides formation of their oxides, was determined by thermogravimetry.

O comportamento térmico de duas formas polimórficas da rifampicina foi estudado por DSC e TG/DTG. Os resultados termoanalíticos mostraram claramente as diferenças entre as duas formas cristalinas. O polimorfo I é a forma mais estável termicamente, a curva DSC não mostrou a fusão dessa espécie e o processo de decomposição térmica ocorreu próximo a 245 ºC. A curva DSC do Polimorfo II apresentou dois eventos consecutivos, um endotérmico (Tpico = 193,9 ºC) e outro exotérmico (Tpico = 209,4 ºC), devido à fusão seguida de recristalização, a qual foi atribuída à conversão da forma II à forma I. Métodos termogravimétricos isotérmicos e não-isotérmicos foram empregados para determinar os parâmetros cinéticos do processo de decomposição térmica. Para experimentos não-isotérmicos, a energia de ativação (Ea) foi obtida a partir do gráfico de Log β vs 1/T, e os valores 154 e 123 kJ mol-1 foram encontrados, respectivamente, para os polimorfos I e II. Para os experimentos isotérmicos, a Ea foi obtida a partir do gráfico de lnt vs. 1/T a um nível de conversão constante. O valor médio encontrado foi 137 e 144 kJ mol-1, respectivamente, para a forma I e forma II.

The thermal behavior of two polymorphic forms of rifampicin was studied by DSC and TG/DTG. The thermoanalytical results clearly showed the differences between the two crystalline forms. Polymorph I was the most thermally stable form, the DSC curve showed no fusion for this species and the thermal decomposition process occurred around 245 ºC. The DSC curve of polymorph II showed two consecutive events, an endothermic event (Tpeak = 193.9 ºC) and one exothermic event (Tpeak = 209.4 ºC), due to a melting process followed by recrystallization, which was attributed to the conversion of form II to form I. Isothermal and non-isothermal thermogravimetric methods were used to determine the kinetic parameters of the thermal decomposition process. For non-isothermal experiments, the activation energy (Ea) was derived from the plot of Log β vs 1/T, yielding values for polymorph form I and II of 154 and 123 kJ mol-1, respectively. In the isothermal experiments, the Ea was obtained from the plot of lnt vs 1/T at a constant conversion level. The mean values found for form I and form II were 137 and 144 kJ mol-1, respectively.

A determinação da pureza química, a faixa de fusão e a variação de entalpia envolvida no processo de caracterização de fármacos é um dos principais requisitos avaliados no controle de qualidade em indústrias farmacêuticas. Neste trabalho é feita uma breve abordagem sobre o método de determinação de pureza utilizando DSC, assim como a aplicação desta técnica para avaliação de amostras comerciais de zidovudina (AZT) (matéria-prima) fornecida por diferentes laboratórios. Para tal, foram analisadas amostras de seis diferentes laboratórios (A,B,C,D,E e F) e a substância química de referência (R) da United States Pharmacopeia (USP). As curvas DSC foram obtidas na faixa de temperatura entre 25 a 200 ºC, sob atmosfera dinâmica de N2 (50 mL min-1), β=2 ºC min-1, utilizando cápsula de Al contendo aproximadamente 2 mg de amostra. De acordo com os resultados, pode-se observar que a substância química de referência apresentou teor igual a 99,83% e que as amostras de AZT apresentaram uma faixa de variação entre 97,59 e 99,54%. Pode-se verificar, ainda, que a substância química de referência apresentou uma temperatura onset de fusão igual a 122,80 °C. Para as amostras dos princípios ativos fornecidos pelos diferentes laboratórios, pode-se verificar uma faixa de variação entre 118,70 e 122,87 °C. No que se refere ao ΔHm, as amostras apresentaram valor médio de 31,12 kJ.mol-1.

The determination of chemical purity, melting range, and variation of enthalpy in the process of characterizing medicines is one of the principal requirements evaluated in quality control of the pharmaceutical industry. In this study, the method of purity determination using DSC was outlined, as well as the application of this technique for the evaluation of commercial samples of zidovudine (AZT) (raw material) supplied by different laboratories. To this end, samples from six different laboratories (A, B, C, D, E, and F) and the standard reference (R) from the United States Pharmacopeia (USP) were analyzed. The DSC curves were obtained in the temperature range of 25 to 200 ºC under the dynamic atmosphere of N2 (50 mL min-1), heating rate of β=2 ºC min-1, using an Al capsule containing approximately 2 mg of sample material. The results demonstrated that the standard reference presented a proportion of 99.83% whereas the AZT samples presented a variation ranging from 97.59 to 99.54%. In addition, the standard reference was found to present a temperature of onset of melting point of 122.80 °C. Regarding the samples of active agents provided by the different laboratories, a variation ranging from 118.70 to 122.87 °C was measured. In terms of ΔHm, the samples presented an average value of 31.12 kJ mol-1.

The purpose of this study was to evaluate the flexural strength of a direct composite, for indirect application, that received heat treatment, with or without investment. One indirect composite was used for comparison. For determination of the heat treatment temperature, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed, considering the initial weight loss temperature and glass transition temperature (Tg). Then, after photoactivation (600 mW/cm² - 40 s), the specimens (10 x 2 x 2 mm) were heat-treated following these conditions: 170ºC for 5, 10 or 15 min, embedded or not embedded in investment. Flexural strength was assessed as a means to evaluate the influence of different heat treatment periods and investment embedding on mechanical properties. The data were analyzed by ANOVA and Tukey's test (α = 0.05). TGA showed an initial weight loss temperature of 180ºC and DSC showed a Tg value of 157°C. Heat treatment was conducted in an oven (Flli Manfredi, Italy), after 37°C storage for 48 h. Flexural strength was evaluated after 120 h at 37°C storage. The results showed that different periods and investment embedding presented similar statistical values. Nevertheless, the direct composite resin with treatments presented higher values (178.7 MPa) compared to the indirect composite resin (146.0 MPa) and the same direct composite submitted to photoactivation only (151.7 MPa). Within the limitations of this study, it could be concluded that the heat treatment increased the flexural strength of the direct composite studied, leading to higher mechanical strength compared to the indirect composite.

The thermal decomposition of hydroxyl-terminated polybutadiene (HTPB)/ammonium nitrate (AN) based propellants, so called smokeless formulations, and raw materials were investigated by differential scanning calorimetry (DSC) and thermogravimetry (TG). The thermoanalytical profile of different components and of propellant were evaluated and the Arrhenius parameters for the thermal decomposition of the propellant sample were determined by the Ozawa method. The kinetic parameters of the thermal decomposition of propellant samples were determined by DSC measurements. The values obtained for activation energy (Ea) and pre-exponential factor were 163 kJ mol-1 and 1.94x10(6) min-1.

Este artigo apresenta a análise térmica para os estudos das propriedades físicas e químicas de compostos e de suas misturas no contexto da área de cosméticos. Os autores relatam alguns trabalhos da literatura em que diferentes técnicas termoanalíticas são usadas na caracterização de matérias-primas, produtos cosméticos e amostras de cabelo.

This paper presents the thermal analysis for the studies of the physical and chemical properties of compounds and their mixtures in the cosmetic area. The authors relate some papers in wich different thermoanalytic techniques have been used for characterization of raw material, cosmetic product and samples of hair.

Bovine pericardium bioprosthesis has become a commonly accepted device for heart valve replacement. Present practice relies on the measurement of shrinkage temperature, observed as a dramatic shortening of tissue length. Several reports in the last decade have utilized differential scanning calorimetry (DSC) as an alternative method to determine the shrinkage temperature, which is accompanied by the absorption of heat, giving rise to an endothermic peak over the shrinkage temperature range of biological tissues. Usually, freeze-drying microscope is used to determine collapse temperature during the lyophilization of solutions. On this experiment we used this technique to study the shrinkage event. The aim of this work was to compare the results of shrinkage temperature obtained by DSC with the results obtained by freeze-drying microscopy. The results showed that both techniques provided excellent sensitivity and reproducibility, and gave information on the thermal shrinkage transition via the thermodynamical parameters inherent of each method.

O desempenho do eletrodo de pasta de carbono (EPC) modificado com a sílica SBA-15 nanoestruturada organofuncionalizada com benzotiazol-2-tiol na determinação de íons Cd(II) em águas naturais é descrito, baseado no processo de DPASV. O Cd(II) apresentou um pico de oxidação por volta de -0,85 V (vs. ECS) em tampão fosfato (pH 3,0). Os melhores resultados obtidos foram com as seguintes condições otimizadas: amplitude de pulso 100 mV, 2 min de tempo de acumulação, velocidade de varredura 10 mV s-1 em solução tampão fosfato pH 3,0. Usando tais parâmetros observou-se uma resposta linear para o Cd(II) no intervalo de 1,00 a 10,0 <FONT FACE=Symbol>´</FONT> 10-6 mol L-1 com limite de detecção de 4,5 <FONT FACE=Symbol>´</FONT> 10-7 mol L-1. Cd(II) foi adicionado em amostras de águas naturais e determinado com recuperação média de 96,4% em concentrações da ordem de 10-6 mol L-1.

The performance of a carbon paste electrode (CPE) modified with SBA-15 nanostructured silica organofunctionalised with 2-benzothiazolethiol in the determination of Cd(II) ions in natural water is described, based in a DPASV procedure. The Cd(II) oxidation peak was observed around -0.85 V (vs. SCE) in phosphate buffer (pH 3.0). The best results were obtained under the following optimised conditions: 100 mV pulse amplitude, 2 min accumulation time, 10 mV s-1 scan rate in phosphate buffer pH 3.0. Using such parameters a linear dynamic range from 1.00 to 10.0 <FONT FACE=Symbol>´</FONT> 10-6 mol L-1 Cd(II) was observed with limit of detection 4.5 <FONT FACE=Symbol>´</FONT> 10-7 mol L-1. Cd(II) spiked in a natural water sample was determined with 96.4% mean recovery at µmol L-1 level. The results indicate that this electrode is sensitive for determination of Cd(II). Interference of some divalent cations were also evaluated.

Paullinia cupana Kunth, vegetal popularmente conhecido como guaraná, é uma planta da família Sapindaceae predominante na região amazônica. A baixa qualidade deste produto pode ser atribuída a espécies e variedades menos nobres, técnicas de colheita e/ou de processamento inadequado ou, ainda, devido à adição de substâncias que não fazem parte de sua composição natural. Neste trabalho objetivou-se avaliar, a partir da termogravimetria (TG)/termogravimetria derivada (DTG) e calorimetria exploratória diferencial (DSC), o comportamento térmico de amostras comerciais de guaraná em pó existentes no Brasil e estabelecer um estudo comparativo entre os métodos convencionais e termogravimétrico para determinação dos teores de umidade e cinzas. A similaridade entre os perfis das curvas TG/DTG e DSC indicam que não há diferenças entre as amostras. A utilização da TG mostrou que é possível reduzir o tempo de análise, utilizar menor quantidade de amostra, assim como, fazer a determinação simultânea dos teores de umidade e cinzas. Em relação ao método convencional, erros de análise inerentes à manipulação das amostras são minimizados. As técnicas termoanalíticas mostram-se como ferramentas potenciais para a obtenção de parâmetros tecnológicos, em controle de qualidade, torrefação e condições adequadas de armazenamento.

Paullinia cupana Kunth a vegetable popularly known as guarana, belongs to the Sapindaceas plant family and is predominant in the Amazon region. The low quality of this product can be attributed to the species and less noble varieties, harvest techniques and/or inadequate processing or, yet due to addition of substances that are not included in its natural composition. The main goal of this work was to evaluate from thermogravimetry (TG)/derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC), the thermal behavior of powdered guarana samples in the Brazilian market in order to evaluate the thermal behavior of guaraná powder samples commercialized in Brazil and to establish a comparative study between conventional and TG methods for humidity contents determination and ashes. In general, the samples did not present significant differences in the DSC curves profiles and TG/DTG. The thermogravimetry showed that it is possible to decrease the time of the analysis using less amount of sample and allowed to determine humidity and ashes contents. In relation to conventional method the errors of analysis inherent of the samples manipulation were decreased. The thermoanalytical techniques have shown potential applications in the determination of technological parameters, such as: quality control, toasting and adequate stock conditions.

The results suggest that the two variables studied are significant and that they may be independently optimized. The material of the sample holder interferes with the incineration process only due to the amount of heat transfered. The sample holder volume aids diffusion of the atmosphere to the honey sample, minimizing the foaming effect. According to the results, for the thermogravimetric analysis of ash content in honey, sample holders of platinum or alumina of 150 µL are indicated.

In the last decade, many method has been developed to obtain oxysufides. However, theses materials were obtained by reaction involved gaseous toxics, CO, CS2, H2S and S. In the present work, the synthesis of lanthanum oxysufides actived by europium (III) through an alternative method has been made. This method involve the rare earth sulfate reduction under an atmosphere of argon contained 10% hydrogen using the thermogravimetric technique. The results showed the formation of the phase TR2O2S (TR = La and Eu) at temperatures which depend upon the heating rate, respectively 650 - 830ºC at 5ºC min-1 and 680 - 800ºC at 10ºC min-1. The oxysufides obtained are characterized by infrared spectroscopy. The method developed is more economic than the usual industrial methods and the environmental problems during the synthesis are also better controled.

The effects of the heating rate on a TG curve of a sample have been widely discussed in the literature. This paper shows the influence of heating rate (beta) in thermogravimetry results when stoichiometry determinations are studied. For this purpose the compound Sm(CH3SO3)3.2(3-picNO) was considered beta of 2,5; 5; 10; 20 and 40°C min-1 in dynamic air atmosphere were obtained. The results are in agreement with proposed stoichiometry for beta equal to 2,5 and 5°C min-1. However, using a higher beta the same results were not obtained resulting in false stoichiometry determinations.