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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. ____________________________________________________________________________________________________

IntroductionThe principle of detection by piezoelectric crystal is based on the fact that the vibration frequency of an oscillating sensor decreases in the presence of an amount of material added to its surface. As shown by Sauerbrey [1], the vibrating frequency of a quartz crystal, to a first approximation, changes proportionally to the mass deposited onto or removed from one or both their faces. Because the TSMR (thickness shear mode resonator) fundamental frequency is rather low (10 MHz), it is assumed that viscosity effects contribute to the frequency changes due to gas sorption to a negligible extent only [2]. Gaseous pollutants can be selectively sorbed by the detector if an ideal coating is deposited on the metallic electrodes. The observed frequency change is a measure of the amount of sorbed gas. The type of interaction taking place between the analyte molecules and the active coating determines the sorption and desorption characteristics. Lowenergy, perfectly reversible interactions such as physisorption generally lack a high degree of selectivity. On the other hand, the formation of chemical bonds or the chemisorption process tend to be less reversible. One approach to overcoming these problems includes the application of sensor arrays to compensate the low selectivity [3,4]. Another solution to this problem includes searching for "intermediate interactions", i.e., interactions that are weaker than chemisorption (≈300 kJ mol -1 ) but stronger than physisorption (0-40 kJ mol -1 ), such as coordination [5]. A central metal atom surrounded by neutral or charged (often organic) ligands is responsible for these interactions, where one or more donor atoms on these ligands interact with the metal ion. So, selectivity can be influenced by the choice of both the metal ion and the ligand, from both an eletronic and steric point of view [5]. When gaseous molecules interact with metal complexes in this way, they themselves become coligands either by occupying free coordination sites or by displacing other ligands. Earlier work in the field of coordination chemistry on mass-sensitive Abstract: Carbon monoxide was detected and determined by a piezoelectric quartz crystal sensor coated with nickel(II)-phthalocyanine 50 % (v/v) solution in glycerine. Studies on the effect of temperature, flow rate, and some possible interferents were carried out. Calibration curves, sensor stability (lifetime) and the precision of measurements were also verified. The resulting selectivity is probably due to the coordinative binding between the electronically unsatured metal complexes and the analyte. The analytical curve is linear in the concentration range 0.10 to 1.0 % (v/v).

Neste estudo, procurou-se desenvolver e avaliar filmes para a captação e quantificação de vapores de amônia no ar, através de um sensor piezelétrico de quartzo. Muitas substâncias e suas misturas, em diferentes proporções, foram investigadas como possíveis filmes captores. Em etapas seguintes, verificaram-se alguns parâmetros importantes como o efeito da vazão do poluente, da temperatura de trabalho e da massa da película, otimizando-se as condições experimentais para a montagem do método. Concluiu-se que o filme mais promissor, sob as condições ajustadas, seria uma mistura 2:1 (v/v) de solução comercial (A) de ácido glicólico (70 % m/m) em água, adicionada a tetrakis(hidroxietil)etilenodiamina - THEED (3:4 v/v), com uma solução saturada (B) de ácido tânico em acetona. Estudos de repetibilidade, do tempo de contacto (poluente-sensor) e de alguns possíveis interferentes completaram os trabalhos. As curvas analíticas resultantes mostraram faixas lineares na região de trabalho (2,0 a 11 ppm v ou 1,4 a 7,7 mg/m³ de NH3), para quatro diferentes tempos de exposição (0,5; 1; 2 e 3 min), com coeficientes de correlação (r) variando entre 0,9994 e 0,9980, e respectivas sensibilidades de 13,5 a 42,0 Hz / ppm v.

Neste estudo, procurou-se desenvolver e avaliar filmes para a captação e quantificação de vapores de amônia no ar, através de um sensor piezelétrico de quartzo. Muitas substâncias e suas misturas, em diferentes proporções, foram investigadas como possíveis filmes captores. Em etapas seguintes, verificaram-se alguns parâmetros importantes como o efeito da vazão do poluente, da temperatura de trabalho e da massa da película, otimizando-se as condições experimentais para a montagem do método. Concluiu-se que o filme mais promissor, sob as condições ajustadas, seria uma mistura 2:1 (v/v) de solução comercial (A) de ácido glicólico (70% m/m) em água, adicionada a tetrakis(hidroxietil)etilenodiamina - THEED (3:4 v/v), com uma solução saturada (B) de ácido tânico em acetona. Estudos de repetibilidade, do tempo de contacto (poluente-sensor) e de alguns possíveis interferentes completaram os trabalhos. As curvas analíticas resultantes mostraram faixas lineares na região de trabalho (2,0 a 11 ppmv ou 1,4 a 7,7 mg/m3 de NH3), para quatro diferentes tempos deexposição (0,5; 1; 2 e 3 min), com coeficientes de correlação (r) variando entre 0,9994 e 0,9980, e respectivas sensibilidades de 13,5 a 42,0 Hz/ppmv.