Andrea Boldarini Couto

Influência de alguns parâmetros experimentais nos resultados de análises calorimétricas diferenciais - DSC

A series of experiments were performed in order to demonstrate to undergraduate students or users of the differential scanning calorimetry (DSC), that several factors can influence the qualitative and quantitative aspects of DSC results. Saccharin, an artificial sweetner, was used as a probe and its thermal behavior is also discussed on the basis of thermogravimetric (TG) and DSC curves.

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.

The Catalytic Cycle of Oxidation of Iodide Ion in the Oxygen/Nitrous Acid/Nitric Oxide System and Its Potential for Analytical Applications

Abstract The oxidation of iodide to iodine by nitrous acid in aqueous acidic medium takes place catalytically in the presence of dissolved oxygen and can be followed spectrophotometrically at 288 and 352 nm. An indirect molar absorptivity for nitrite on the basis of I3 − formation can be as high as 8.5×105 L mol−1 cm−1 at 288 nm. Analytical curves were established. The iodine released in the catalytic cycle can also be titrated with thiosulfate. The reaction is pseudo–second‐order in oxygen consumed, with t1/2=15.7 min at 25°C. A rate determining step could be the NO · O2 as the activated species. Measurements of the iodine formed at catalytic conditions was used to determine nitrite in meat extracts and NOx in car exhausts.

Influência do eletrólito na eletrodeposição de nanopartículas de Cu sobre eletrodo de diamante dopado com boro

This paper presents the electrolyte influence on deposition and dissolution processes of Cu nanoparticles on boron doped diamond electrodes (BDD). Morphological, structural and electrochemical analysis showed BDD films with good reproducibility, quality and reversible in a specific redox system. Electrodeposition of Cu nanoparticles on DDB electrodes in three different solutions was influenced by pH and ionic strength of the electrolytic medium. Analyzing the process as function of the scan rate, it was verified a better efficiency in 0,5 mol L-1 Na2SO4 solution. Under the influence of the pH and ionic strength, Cu nanoparticles on DDB may be obtained with different morphologies and it was important for defining the desired properties.

Nitrate Removal By Electrolysis Using Cu/BDD Electrode Cathode

Nowadays, cleaner technologies are been required in the treatment of polluted water effluents. In this cont ext, electrochemical process has shown to be highly effe ctive and inexpensive alternative technique for this purp ose. In particular, nitrate has been considered an importan t environmental contaminant of soil and natural water s due to its capability of carrying not only on the surfa ce but it also penetrates the soil, thus contaminating large areas. Several groups have focused their efforts in prepar ing electroreduction (1). In this sense, boron doped di amond (BDD) has been reported to be an excellent electrod e t study nitrate reduction due to its outstanding electrochemical features such as: wide potential wi ndow in aqueous solutions and weak adsorption for most t ypes of molecules (2). It is known that its behavior can be improved from surface modifications with metallic particles deposition. In this context, Cu is consid ered one of the best electrocatalytic metals considering the kin tics for nitrate reduction process. The aim main of the present work is to modify the BDD surface with Cu particle using photoelectrodeposition as a method for obtaining Cu/BDD electrode, in order to analyze their perform ance as a cathode with respect to nitrate electroreducti on. Likewise, the influence of current density and pH i n the nitrate electrolysis were studied. The films were g own by hot filament-assisted chemical vapor deposition (HF CVD) technique with the following growth parameters: 780 C, 50 Torr, 14 h and gas mixture 1/99 % of CH 4/H2. Boron source was obtained by an additional hydrogen line passing through a bubbler containing B 2O3 dissolved in methanol. The doping level corresponds to the accep tor densities of around 10 21 atoms cm estimated from Ramans measurements. Electrochemical measurements were carried out using a standard three-electrode c ell. A platinum wire served as a counter electrode and Ag/AgCl/KCl(sat) was used as the reference electrode. Prior to the photoelectrodeposition, the films were exposed under UV irradiation during 2h to promote t h oxygen functional groups that serve as anchor point s for Cu deposited thus improving the interfacial adhesio n between BDD films and the metal coating. The photoelectrodeposition of Cu particles on diamond f ilms were performed under potentiostatic mode, at a fixe d potential of -0.6 V for a time of 10 min in a 50 mm ol l NaClO4 + 1 mmol l -1 CuSO4 aqueous solution, under ultra-violet (UV) irradiation. The light source was a deuterium lamp. For the nitrate electrolysis experi m nts, about 300 mL of the nitrate solution prepared with 150 ppm KNO3 + 0.1 mol L -1 K2SO4 was electrolyzed for 5 h. At different intervals, 1 mL samples were withdrawn from the electrochemical cell and it was analyzed by ion chromatography. Different current density was appli ed under galvanostatic control at 10, 20, 40, 60, 100 mA/cm. Three different pH solutions (acid, neutral and basic) were used. The Figure 1 shows the Raman ́s scattering spectrum of the BDD film. Figure 1: Raman spectrum of the BDD electrode.

Investigation of a Cu/Pd Bimetallic System Electrodeposited on Boron-Doped Diamond Films for Application in Electrocatalytic Reduction of Nitrate

The Cu/Pd bimetallic system electrodeposited on boron-doped diamond (BDD) films for application, as electrode material in the electrochemical reduction of nitrate was studied. The electrochemical behavior of Cu, Pd, and Cu/Pd bimetallic system was evaluated by cyclic voltammetry. From these results, the formation of the Cu/Pd composite was verified. In addition, Cu with different phases and a Cu/Pd phase in the composite were obtained. Morphological analysis by scanning electron microscopy (SEM) revealed a homogeneous distribution of Cu/Pd bimetallic particles with intermediary dimensions compared to those observed in Cu or Pd electrodeposits separately. These composites were tested as electrocatalysts for nitrate reduction in Britton-Robinson buffer solution (pH 9). Electrochemical measurements showed that composites with higher Cu content displayed the best electrocatalytic activity for nitrate reduction, and the Cu/Pd phase in the bimetallic system served to improve the Cu adherence on BDD electrode.