André L. Santos

Copper determination in ethanol fuel by differential pulse anodic stripping voltammetry at a solid paraffin-based carbon paste electrode modified with 2-aminothiazole organofunctionalized silica.

Solid paraffin-based carbon paste electrodes modified with 2-aminothiazole organofunctionalized silica have been applied to the anodic stripping determination of copper ions in ethanol fuel samples without any sample treatment. The proposed method comprised four steps: (1) copper ions preconcentration at open circuit potential directly in the ethanol fuel sample; (2) exchange of the solution and immediate cathodic reduction of the absorbate at controlled potential; (3) differential pulse anodic stripping voltammetry; (4) electrochemical surface regeneration by applying a positive potential in acid media. Factors affecting the preconcentration, reduction and stripping steps were investigated and the optimum conditions were employed to develop the analytical procedure. Using a preconcentration time of 20min and reduction time of 120s at -0.3V versus Ag/AgCl(sat) a linear range from 7.5x10(-8) to 2.5x10(-6)mol L(-1) with detection limit of 3.1x10(-8)mol L(-1) was obtained. Interference studies have shown a decrease in the interference effect according to the sequence: Ni>Zn>Cd>Pb>Fe. However, the interference effects of these ions have not forbidden the application of the proposed method. Recovery values between 98.8 and 102.3% were obtained for synthetic samples spiked with known amounts of Cu(2+) and interfering metallic ions. The developed electrode was successfully applied to the determination of Cu(2+) in commercial ethanol fuel samples. The results were compared to those obtained by flame atomic absorption spectroscopy by using the F-test and t-test. Neither F-value nor t-value have exceeded the critical values at 95% confidence level, confirming that there are no significant differences between the results obtained by both methods.

Determinação de chumbo em álcool combustível por voltametria de redissolução anódica utilizando um eletrodo de pasta de carbono modificado com resina de troca iônica Amberlite IR 120

The use of pre-concentration and differential pulse anodic stripping voltammetry (DPASV) using a carbon paste electrode modified (CPEM) with ion-exchange resin (Amberlite® IR120) has been proposed for the determination of lead ions content in ethanol fuel. The lead oxidation peak was observed at -0.53 V (vs. Ag/AgCl) in HCl solution. The best DPASV response was reached for an electrode composition of 5% (m/m) Amberlite® IR120 in the paste, 0.1 mol L-1 HCl solution, scan rate of 10 mVs-1, pre-concentration time of 15 min and potential pulse amplitude of 100 mV. In these experimental conditions, the proposed methodology responds to lead ions in the concentration range of 9.9 x 10-9 to 1.2 x 10-6 mol L-1 with a detection limit of 7.2 x 10-9 mol L-1. Recoveries ranged from 96 to 102 % for Pb(II) spiked in an ethanol fuel sample at 10-7 mol L-1 level were achieved. Interferences were also evaluated.

Electrochemical Methods in Analysis of Biofuels

Energy is vital for modern society since commercial, agricultural and industrial activities are highly energy-consuming. Moreover, the marvels of the modern technology have enhanced the aspirations of people for an improved quality of life, which is more and more dependent on energy. It is a consensus that this very high energy demand can not be adequately supplied by traditional energy sources such as fossil-based fuels. In addition, there are many concerns about the usage of these fuels in future, mainly taking into account that they are obtained from a finite source and that their production and utilization are processes highly pollutant. These environmental concerns are widely justified taking into consideration, for example, that 56.6 % of anthropogenic green house gases (GHG) emissions of 2004 were associated to fossil fuel use [1]. Thus, for a sustainable future, it is essential to develop integrated energy models linking both traditional and modern renewable energy sources in order to decrease the global dependence on petroleum and to minimize environmental problems associated to its use as energy supply. A very complete discussion focused on the development and evaluation of integrated energy models can be found in the review written by Jebaraj and Iniyan [2]. Among renewable energy sources, biomass has the highest potential to adequately respond to energy requirements from modern society of both developed and developing countries [3]. Biomass is a term for all organic material originated from plants which convert sunlight into plant material through photosynthesis. The term includes all landand water-based vegetation, as well as all organic wastes. Therefore, biomass resource is the organic matter, in which the energy of sunlight is stored in chemical bonds [4]. Renewable energy is of growing importance in responding to concerns over the environment and the security of energy supplies. In this context, biomass provides the only renewable source of fixed carbon, which is an essential ingredient of our fuels [3]. Biomass can be converted in different forms of useful energy and the literature presents some works especially dedicated to the technologies of biomass conversion [5] including thermochemical conversion processes [6]. The conversion process and its efficiency are influenced by the composition of biomass. Therefore its knowledge is important to select the more appropriated conversion process. A complete discussion about biomass composition and its variability is presented by Vassilev et al [7].

Electrochemical reduction and voltammetric determination of diloxanide furoate in non-aqueous medium

The electrochemical reduction of diloxanide furoate (DF) in acetonitrile on glassy carbon electrode was studied in this work. It was observed that DF is reduced after a reversible one-electron transfer followed by an irreversible chemical reaction, diagnosed as C-Cl bond cleavage. Its reduction was followed by linear (LSV), differential pulse (DPV) and square wave voltammetry (SWV). Analytical curves were obtained for DF determination using all the investigated voltammetric techniques. For LSV was obtained a linear range (LR) from 5.0 x 10-4 to 1.0 x 10-2 mol L-1, with detection limit (DL) of 1.5 x 10-4 mol L-1 and sensitivity (S) of 2.1 x 104 µA mol-1 L. The analytical parameters obtained by DPV were: LR = 5.0 x 10-4 to 2.2 x 10-3 mol L-1, DL = 7.8 x 10-5 mol L-1, S = 3.7 x 104 µA mol-1 L. For SWV were obtained a LR = 7.5 x 10-6 to 1.2 x 10-3 mol L-1, DL = 5.5 x 10-6 mol L-1 and S = 2.8 x 105 µA mol-1 L. Thus, the SWV was the most sensible technique, which can be used for DF determination at low concentration levels. Statistics methods were used to evaluate the analytical procedure, where recovery around to 100% was obtained for all voltammetric techniques. Relative standard deviations were lower than 5.0% (N=5). The obtained t values evaluating all the three voltammetric methods were less than the tabulated ones, indicating that there are no evidences of systematic error.

A simple way to obtain glassy polymeric carbon and its use as electrode for analytical purposes

The present work describes the production the production of glassy polymeric carbon from phenol formaldehyde resins by using different heating rates for each step of pyrolysis. The final product is a suitable material that can be used as electrode for monitoring charge transfer reactions or as supporting electrode for electroactive polymers. The obtained material was characterized by powder X-ray diffraction, infrared spectroscopy and the electrochemical properties were investigated by cyclic voltammetric technique.