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The possibility of using thiocyanate to determine iron(II) and/or iron(III) in water-acetone mixture has been re-examined as part of a systematic and comparative study involving metallic complexes of pseudohalide ligands. Some parameters that affect the complete oxidation of the ferrous cations, their subsequent complexation and the system stability have been studied to optimize the experimental conditions. Our results show the viability and potentiality of this simply methodology as an alternative analytical procedure to determine iron cations with high sensitivity, precision and accuracy. Studies on the calibration, stability, precision, and effect of various different ions have been carried out by using absorbance values measured at 480 nm. The analytical curve for the total iron determination obeys Beer's law (r = 0.9993), showing a higher sensitivity (molar absorptivity of 2.10x10(4) L cm-1 mol-1) when compared with other traditional systems (ligands) or even with the "similar" azide ion [1.53x10(4) L cm-1 mol-1, for iron-III/azide complexes, in 70% (v/v) tetrahydrofuran/water, at 396 nm]. Under such optimized experimental conditions, it is possible to determine iron in the concentration range from 0.5 to 2 ppm (15-65% T for older equipments, quartz cells of 1.00 cm). Analytical applications have been tested for some different materials (iron ores), also including pharmaceutical products for anemia, and results were compared with atomic absorption determinations. Very good agreement was obtained with these two different techniques, showing the potential of the present experimental conditions for the total iron spectrophotometric determinations (errors < 5%). The possibility of iron speciation was made evident by using another specific and auxiliary method for iron(II) or (III).

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After several petroleum crises in the 70's and 80's successive increases in petroleum barrel prices stimulated worldwide research and development of alternative fuels relative to fossil fuels. In this context, fuel ethanol obtained from sugarcane has been successfully applied as an important automotive fuel used in large scale in Brazil, in the pure form or as an addictive for gasoline, thus contributing economic and environmental advantages.Due to its use as an alternative automotive fuel or raw material for the alcohol-chemical industry, the identification and control of organic and inorganic contaminants is necessary because of the possibility of engine corrosion effects or passivation of industrial catalyzers [1]. Several organic[1-4] and inorganic[5-13] contaminants have been studied in fuel ethanol by different instrumental methods. In this case, different sources of contamination can be cited, such as during the production stage (micronutrients and fermentation processes), and during transport and storage of this fuel in metallic containers. The analysis of metallic species in this kind of matrix is of special interest due to its potential for environmental pollution when emitted during the combustion process and its contribution to corrosion effects in motors. Nowadays, Brazilian official methods for analysis of metallic species consist of dosage of sodium[14], copper[15], and iron[16] by using photometric and spectrometric methodologies proposed by the Brazilian Association of Technical Norms (ABNT).According to the literature, the determination of metallic species in fuel ethanol has been carried out using mainly spectrometric [5-11] and voltammetric[12-13] methods.When trace analysis is necessary, the preconcentration of analyte species before the determination step is often required. Voltammetric methods employing an anodic stripping step and using mercury electrodes (film or drop) can provide a preconcentration of analyte at the surface electrode by application of a specific potential during a selected preconcentration time, allowing to detection limits from 10 -9 until 10 -8 mol L -1 for several metallic species [12,13]. However, this technique is limited to a small number of metallic species for direct analysis. Iron and Nickel, for example, require addition of specific chemical reagents and need other technique (adsorptive stripping) or specific chemically modified working electrodes. ____________________________________________________________________________________________________