Determinação espectrofotometrica automatizada de cobre e zinco em amostras de interesse agroindustrial por metodos cineticos

AUTOR(ES)
DATA DE PUBLICAÇÃO

1993

RESUMO

Determination of copper and zinc in soiIs, fertilizers, plant materiaIs, aIloys, etc, are of great concern since they are included among the most required metaIs by man in modern life and are essential for animal and vegetal growth. In the present paper, kinetic methods for determination of these metaIs exploiting the flow injection principIe were proposed. In this way, serious drawbacks mostly those related to interferent removal were overcome. For copper, two catalytic methods were considered. The first was based on the catalytic effect of Cu in the resorcinol oxidation by hydrogen peroxide. The optimized reagents concentration were; 1 moI L NaOH, 0.2 moI L H2O2 and 0.65 moI L resorcinol. Analytical signals were recorded at room temperature after 34 s of reaction time. Effects of temperature, ionic strength and sample acidity were studied. Serious interferences from Fe, Mn and Ni ions, and to a lesser extent from Al, Cd, Cr, Pb, Sn and Zn, were found. Fe interference may sometimes impair copper determination in important samples such as fertilizers and plants. The proposed method was successfully applied to copper determination in distilled alcoholic beverages, brass, and bronze yielding precise results in agreement with those obtained by atomic absorption spectrometry. Cu also exerts a catalytic effect on the oxidation of thiosulfate by ferric ions. In this reaction, the reddish color of the intermediate complex ion [Fe(S2O3)2] fades quicker the greater the cupric ion concentration is. The effect was considered in designing a FIA system for copper determination where the reagent concentrations were defined as 0.25 mol L Fe and 0.1 moI L S2O3. The analytical procedure invoIved recording of the absorbance decrease produced by passage of the processed sample through the detector. The optimum concentration range was 0.1 to 3.0 mg L Cu. The high selectivity provided by the method has permitted analysis of complex samples with high levels of iron, zine, manganese, and other metaIs. Results for fertilizer, plant material and soil samples obtained by the proposed method were not significantIy different from those determined by atomic absorption spectrometry. Concerning zinc determination, sample solution with zinc and interferent was first mixed in the FIA system with 2 % (w/v) ascorbic acid solution for reduction of Fe, Cu, etc, which were then complexed by 1 % (w/v) KCN solution. Zinc cyanide complex was the first to be destroyed by action of formaldehyde and thus liberated to react with Zincon. The optimum concentration range was 1 to 5 mg L Zn. The proposed method was checked in the analysis of NPK fertilizer mixtures containing Zn, Fe, Cu, Mn, added at levels normally expected in commercial products. Results were similar to those obtained by atomic absorption spectrometry.

ASSUNTO(S)

agroindustria cinetica quimica espectrofotometria solos - teor de zinco cobre - analise

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