Éder Tadeu Gomes Cavalheiro

Thermal behavior of alginic acid and its sodium salt

Uma avaliacao da hidratacao e de decomposicao termica do HAlg e seu sal de sodio foi realizada usando termogravimetria (TG) e a calorimetria exploratoria diferencial (DSC). As curvas TG em N2 e ar, para o acido alginico apresentaram duas etapas de perda de massa correspondendo a saida de agua e posterior decomposicao do polimero. Ja para o alginato de sodio, a decomposicao termica ocorreu em tres etapas. A primeira corresponde a liberacao de agua, seguida da formacao de residuo carbonizado e formacao do carbonato de sodio. As curvas DSC concordam com os eventos observados em TG. Na regiao do infravermelho, o acido alginico apresentou bandas em 1730 e 1631 cm-1, enquanto o alginato de sodio apresentou duas bandas em 1614 e 1431 cm-1, evidenciando a presenca de grupos carboxilicos salificados.

Evaluation of a carbon paste electrode modified with organofunctionalised SBA-15 nanostructured silica in the simultaneous determination of divalent lead, copper and mercury ions

The performance of a carbon paste electrode (CPE) modified with SBA-15 nanostructured silica organofunctionalised with 2-benzothiazolethiol in the simultaneous determination of Pb(II), Cu(II) and Hg(II) ions in natural water and sugar cane spirit (cachaça) is described. Pb(II), Cu(II) and Hg(II) were pre-concentrated on the surface of the modified electrode by complexing with 2-benzothiazolethiol and reduced at a negative potential (-0.80 V). Then the reduced products were oxidised by DPASV procedure. The fact that three stripping peaks appeared on the voltammograms at the potentials of -0.48 V (Pb2+), -0.03 V (Cu2+) and +0.36 V (Hg2+) in relation to the SCE, demonstrates the possibility of simultaneous determination of Pb2+, Cu2+ and Hg2+. The best results were obtained under the following optimised conditions: 100 mV pulse amplitude, 3 min accumulation time, 25 mV s(-1) scan rate in phosphate solution pH 3.0. Using such parameters, calibration graphs were linear in the concentration ranges of 3.00-70.0 x 10(-7) mol L(-1) (Pb2+), 8.00-100.0 x 10(-7) mol L(-1) (Cu2+) and 2.00-10.0 x 10(-6) mol L(-1) (Hg2+). Detection limits of 4.0 x 10(-8) mol L(-1) (Pb2+), 2.0 x 10(-7) mol L(-1) (Cu2+) and 4.0 x 10(-7) mol L(-1) (Hg2+) were obtained at the signal noise ratio (SNR) of 3. The results indicate that this electrode is sensitive and effective for simultaneous determination of Pb2+, Cu2+ and Hg2+ in the analysed samples.

Caracterização de quitosanas comerciais de diferentes origens

Chitosan is a linear polysaccharide obtained from the deacetylation of chitin. The physico-chemical properties of chitosan dependent on the average degree of acetylation () and of the average molar mass (). Several applications of chitosan have been proposed in the literature, mainly in water treatment, cosmetic and drug manufacturing, food additives, semipermeable membranes and development of biomaterials. In this work, three commercial chitosan samples were characterized by proton nuclear magnetic resonance (1H NMR), infrared spectroscopy (IR), conductimetric titration and thermal analysis (TG and DSC) in order to compare commercial chitosans from different supplies. The average viscosimetric molecular weight Mv was estimated from the intrinsic viscosity. The results confirmed remarkable differences in relation to the mean degree of acetylation () (or desacetylation, ). The thermal analysis data permitted to follow the dehydration, decomposition, and glass transition temperature (Tg).

The use of a graphite–castor oil polyurethane composite electrode for the determination of hydroquinone in photographic developers

The performance of a new graphite-castor oil polyurethane composite electrode in the determination of hydroquinone in photographic developers in a differential pulse voltammetric (DPV) procedure is described. The 60% (graphite, w/w) composite electrode presented good stability, repeatability and accurate response. Limit of detection of 934 nmol L(-1) was observed for hydroquinone within a linear dynamic range from 66 to 530 nmol L(-1). The determination of hydroquinone in a photographic developer sample showed agreement with the label values in a 95% confidence level with maximum 2% relative error and recoveries between 100.1 and 100.4%. No need of sample preparation was required since the p-methylaminophenolsulfate (metol) present in the developer formulation did not interfered in the measurements.

Correlation between ionic radius and thermal decomposition of Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) diethanoldithiocarbamates

Diethanoldithiocarbamate complexes with Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) were prepared and characterised by elemental analysis, infrared spectroscopy and flame atomic absorption spectroscopy. Their thermal decomposition was investigated by TG/DTG and DSC techniques under dynamic air and nitrogen atmospheres. These studies showed that the smaller the metallic ionic radius the higher was their thermal stability and that the bidentate complex decomposed in a direct way, with no evidence of thyocyanate intermediates.

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.

Determination of vitamin B6 (pyridoxine) in pharmaceutical preparations by cyclic voltammetry at a copper(II) hexacyanoferrate(III) modified carbon paste electrode

A copper(II) hexacyanoferrate(III) (CuHCF) modified carbon paste electrode was used for the electroanalytical determination of pyridoxine (vitamin B6) in pharmaceutical preparations, using cyclic voltammetry. Diverse parameters were investigated for the optimization of the sensor response, such as composition of the electrode, electrolytic solution, effect of pH, scan rate of potential and interferences. The optimum conditions were found at an electrode composition of 20% CuHCF, 55% graphite and 25% mineral oil (m/m) in an acetate buffer (pH 5.5) containing 0.05 mol L-1 of NaCl. The range of determination of pyridoxine was from 1.2 x 10-6 to 6.9 x 10-4 mol L-1. The procedure was successfully applied to the determination of vitamin B6 in formulation preparations. The CuHCF modified carbon paste electrode gave results comparable to those obtained using spectrophotometry.

Use of Graphite Polyurethane Composite Electrode for Imipramine Oxidation—Mechanism Proposal and Electroanalytical Determination

Abstract This work is aimed at the elucidation of the oxidation mechanism for the tricyclic antidepressant imipramine using electrochemical and quantum chemical studies. The excellent response obtained with the use of a rigid graphite‐polyurethane composite electrode (GPU) provided the development of a new electroanalytical methodology, in 0.10 mol/L BR buffer (pH 7.0), employing square wave voltammetry. Detection and quantification limits of 4.60×10−9 mol/L(1.5 µg L) and 3.04×10−7 mol/L (96 µg L) were obtained. This methodology was tested in a commercial formulation of Tofranil® and excellent recoveries were achieved by electrochemical (97.60±0.90%) and spectrophotometrical (87.10±0.90%) methods.

Flow injection amperometric determination of dipyrone in pharmaceutical formulations using a carbon paste electrode

The behavior of a carbon paste electrode was investigated as an amperometric detector for the determination of dipyrone by flow injection analysis (FIA). The electrode presented low cost and easy construction by simple mixing of graphite powder and mineral oil. Initially, an electrochemical study of the dipyrone oxidation at a carbon paste electrode has been developed before its use in the FIA system. The oxidation currents monitored at +0.35 V versus Ag/AgCl, were proportional to the dipyrone concentrations. Experimental parameters, such as nature of supporting electrolyte, pH of the carrier solution, flow rate, sample volume injection and probable interferents were investigated. Under the best experimental conditions selected, the calibration curve for dipyrone was linear in the concentration range from 4.91 x 10(-6) to 2.50 x 10(-4) M l(-1) (I(anodic)/microA)=0.056+81.06 [dipyrone]) with a detection limit of 2.07 x 10(-6) M l(-1). Recoveries ranged from 93.8 to 100.8% and an analytical frequency of 130 h(-1) was achieved. The proposed flow procedure has been satisfactorily applied to the determination of dipyrone in several pharmaceutical formulations.

Using the organofunctionalised SBA-15 nanostructured silica as a carbon paste electrode modifier: determination of cadmium ions by differential anodic pulse stripping voltammetry

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 ´ 10-6 mol L-1 Cd(II) was observed with limit of detection 4.5 ´ 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.