Luiz Antônio Ramos

Determination of atenolol at a graphite-polyurethane composite electrode

A bare graphite-polyurethane composite was evaluated as an alternative electrode in the determination of atenolol (ATN) in pharmaceutical formulations. Using a DPV procedure, a linear analytical curve was observed in the 4-100mumolL(-1) range with a LOD=3.16mumolL(-1), without need of surface renewing between successive runs, with recoveries between 95.5 and 108%. Interference of other antihypertensive drugs was observed, but not from the usual components of tablets. The results of the proposed method agreed with HPLC ones within 95% confidence level.

Development of graphite-polymer composites as electrode materials

Graphite powder was mixed to polyurethane, silicon rubber and Araldite® (epoxy) in order to prepare composite materials to be used in the preparation of electrodes. Results showed that voltammetric response could be obtained when at least 50% of graphite (w.w-1) is present in the material. SEM and thermogravimetry were also used in the characterization of the composites.

Determination and fractionation of barium in Brazil nuts

Diversas estrategias para preparo de amostra, determinacao e fracionamento, tais como espectrometria de absorcao atomica com atomizacao eletrotermica em forno de grafite (GF AAS) e espectrometria de emissao optica com plasma acoplado indutivamente com configuracao axial (ICP OES), foram empregadas para o fracionamento de bario em Castanha-do-Para. Esse alimento e amplamente apreciado tanto pelo valor nutricional, quanto pelo sabor agradavel. A analise quimica de Castanha-do-Para nao e trivial devido a matriz complexa. O fracionamento de bario em Castanha-do-Para foi estudado devido a toxicidade desse elemento e a correlacao entre a forma quimica e a absorcao. Os teores totais de bario nas amostras de Castanha-do-Para variaram entre 860 e 2084 mg kg-1. Extracoes sequenciais foram feitas com base na solubilidade em diferentes meios e composicao quimica envolvendo lipidios, proteinas e compostos de baixa massa molecular (LMW). Teores mais elevados de bario foram determinados nas fracoes LMW e insoluvel em agua que variaram entre 778-1606 e 551-1520 mg kg-1, respectivamente. Esses resultados indicaram a indisponibilidade de bario presente nesse alimento ao organismo humano. Baseando-se nos teores de bario e enxofre nas diferentes fracoes e em calculos estequiometricos para as possiveis reacoes envolvidas pode-se inferir que bario se encontra principalmente na forma de BaSO4. Experimentos termogravimetricos tambem confirmaram essa suposicao.

Determinação enzimática de dopamina em formulações farmacêuticas utilizando sistema de análise por injeção em fluxo com extrato bruto de abacate (Persea americana)

In this work, a spectrophotometric flow injection analysis system using a crude extract of avocado (Persea americana) as a source of polyphenol oxidase to dopamine determination was developed. The substrates and enzyme concentrations from 2.4x10-7 to 5.3x10-4 mol L-1 and 28 to 332 units mL-1 were evaluated, respectively. In addition, the FIA parameters such as sample loop (50 to 500 µL), flow rate (1.4 to 4.3 mL min-1) and reactor length (100 to 500 cm) were also evaluated in a 0.1 mol L-1 phosphate buffer solution (pH 7.0). Dopamine solution concentrations were determined using 277 units mL-1 enzyme solution, 400 mL enzyme loop, 375 µL sample loop, 2.2 mL min-1 flow rate and a reactor of 350 cm. The analytical curve showed a linearity from 5.3x10-5 to 5.3x10-4 mol L-1 dopamine with a detection limit of 1.3x10-5 mol L-1. The analytical frequency was 46 h-1 and the RSD lower than 0.5% for 5.3x10-4 mol L-1 dopamine solution (n=10). A paired t-test showed that all results obtained for dopamine in commercial formulations using the proposed flow injection procedure and a spectrophotometric procedure agree at the 95% confidence level.

Aplicação de pigmentos de flores no ensino de química

In the present work, the pH dependent colour change of the crude flower extracts is suggested as subject in teaching chemical or acid-base equilibria, visual indicators and some aspects of spectrophotometric concepts and applications. The vegetal species used are commonly found in Brazil, and the extraction methodology proposed is inexpensive and easy to perform in secondary schools and in general chemistry or instrumental undergraduate courses. A bibliographic review about the use of vegetal extracts in chemical education and a discussion of the flower colour are also presented.

A Solid Fe2O3 Based Carbon–Epoxy Electrode for Potentiometric Measurements of pH

A solid Fe2O3 based carbon-epoxy composite electrode was investigated for use as a potentiometric pH sensor. The electrode was constructed using a mixture of sulfated iron(III) oxide, carbon power, and epoxy resin, which was deposited directly onto a glass tube. The effect of composition (Fe2O3 , carbon and epoxy resin) on the electrode response and its calibration curve (mV/pH) were investigated. The analytical behavior of the electrode in acid–base titrations was compared with that of a glass electrode. A linear response from pH 1.7 to 12.2 with a slope of –39.7 ± 0.6 mV/pH (at 25°C) was observed.

The use of magnesium silicate (talc) in a potentiometric sensor for hydrogen ions

The construction and analytical evaluation of a pH sensor based in a matrix containing 20% of [(Mg6Si8O20)](OH)4, a magnesium silicate, talc and 30% graphite dispersed in an epoxy resin (50%) is described. The data obtained from various acid–base titrations were compared with those using a glass electrode in the same conditions. The proposed electrode presented a linear response in the 1–12 pH range with a slope of � 39.9F0.3 mV/pH (at 25 jC) and ca. 15 s of response time. The lifetime of this electrode was higher than 8 months (ca. 1500 determinations) and it showed a good performance for pH determination and end-point indication in the potentiometric titrations of acids and bases.

Utilização do extrato bruto de frutos de solanum nigrum L no ensino de química

Anthocyanins, pigments of flavonoids class, are the most important chromophors found in blue, red and purple vegetal tissues, whose color changes depending on the pH. The goal of this work was to propose a simple and cheap experiment to chemical education, using vegetal species easily found in Brazil. Such class of comounds presents a potential interest to students and teachers alike, to be useful in illustrating chemical principles such as acid-base equilibria, pKa, light absorption, effects of changes in conjugated double bounds, etc. The specie used was Solanum nigrum L (maria-preta) whose fruits crude extract was used to obtain molecular absorption spectra, to verify the Lambert-Beer law and as an indicator in acid-base titrations. The end point results obtained with the crude extracts when compared to the glass electrode ones showed relative errors between 0,1 and 1,0%. The color changed from red to yellow between pH 4 and 10. To determine the highest absorption wavelenght and demonstrate the shape changing of spectra in function of the acidity, the absorption spectra in UV and visible regions were obtained in different pH values.

Titulações espectrofotométricas de sistemas ácido-base utilizando extrato de flores contendo antocianinas

ANTHOCYANINS. Considering the attraction of the students’ attention by the changes in the colors of vegetable crude extracts caused by the variation of the pH of the medium, the use of these different colors in order to demonstrate principles of spectrophotometric acid-base titrations using the crude extracts as indicators is proposed. The experimental setup consisted of a simple spectrophotometer, a homemade flow cell and a pump to propel the fluids along the system. Students should be stimulated to choose the best wavelength to monitor the changes in color during the titration. Since the pH of the equivalence point depends on the system titrated, the wavelength must be properly chosen to follow these changes, demonstrating the importance of the correct choice of the indicator. When compared with the potentiometric results, errors as low as 2% could be found using Rhododendron simsii (azalea) or Tibouchina granulosa (Glory tree, quaresmeira) as sources of the crude extracts.

UTILIZAÇÃO DE UM ELETRODO DE GRAFITE-EPÓXI RECOBERTO COM (Zn(FEN) 3 )(TETRATRIS(4- CLOROFENIL) BORATO) 2 SENSÍVEL A ZINCO(II) EM MEIO 1,10-FENANTROLINA COMO ELETRODO INDICADOR EM TITULAÇÕES POTENCIOMÉTRICAS DE PRECIPITAÇÃO

The construction and analytical evaluation of a coated graphite-epoxy electrode sensitive to the zinc-1,10-phenantroline complex based on the [Zn(fen)3][tetrakis(4-chlorophenyl)borate]2 incorporated into a poly(vinylchloride) (PVC) matrix are described. A thin membrane film of this ion-pair, dibutylphthalate (DBPh) and PVC were deposited directly onto an electrically conductive graphite-epoxy support located inside a Perspex® tube. The best PVC polymeric membrane contains 65% (m/m) DBPh, 30% (m/m) PVC and 5% (m/m) of the ion-pair. This electrode shows a response of 19.5 mV dec-1 over the zinc(II) concentration range of 1.0 x 10-5 to 1.0 x 10-3 mol L-1 in 1,10-phenantroline medium, at pH 6.0. The response time was less than 20 seconds and the lifetime of this electrode was more than four months (over 1200 determinations by each polymeric membrane). It was successfully used as an indicator electrode in the potentiometric precipitation titration of zinc(II) ions.