Otimização da metodologia de determinação de molibdênio em solos e plantas por voltametria de onda quadrada com redissolução catódica adsortiva / Optimization of the molybdenum determination in soils and plant methodology by cathodic adsorptive stripping square- wave voltammetry

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

Molybdenum is an element essential to life that can be found practically in the whole biosphere. It is present in the nitrase enzyme cofactor and is responsible for the reduction of the N2 molecule and its fixation in the terrestrial crust. In other enzymes, molybdenum acts in reactions of transference of atoms of oxygen to a lot of sub-stratums. Molybdenum also participates in the composition of metallic alloys and lubricating, and it is considered essential to the nutrition of animals and plants. However, the concentration levels found in these matrices are very low, what makes difficult for monitoring and detection and is possible when sensitive analytical methods associated to procedures that involve a great number of stages are used. The sensibility of the employed technique justifies the use of reagents with analytical grade and procedures against possible contaminations during the determinations. This present work describes the development of a method used for the direct determination of Mo in soils and plant using the cathodic adsorptive stripping square-wave voltammetry technique. The multivariate optimization of the parameters involved in the support electrolyte composition was used, and the use of KNO3 and acetic acid/sodium acetate buffer solution provided significant parameters through the Pareto graphics that were optimized in a factorial planning 2 (8-4)/IV. After the optimization of the eletroanalytical parameters, a complete factorial planning 23 was used, where the support electrolyte KNO3, the acetic acid/sodium acetate buffer solution and the complexant oxine concentrations with best results were obtained. The Mo(VI) reduction stream peak was optimized by experimentals planning. The parameters involved in the optimization of the square-wave voltammetry were: increment of voltage, amplitude, frequency and the deposition time and mercury drop size. A study of the factor levels was accomplished previously to the selection phase, and then a complete factorial planning 25 was used for the square-wave voltammetry. Three most significant factors were selected and the selected variations were studied with the central composite design (CCD) to obtain the response surface. The results obtained in the optimized conditions showed a linear response in the range of 1.0 to 6.0 μg L-1, the detection and quantification limit were 0.02 μg L-1 (20 ppt) and 0.08 μg L-1 (80 ppt), respectively. Recovery values within 96% and 102% were obtained to Mo enriched samples of soil and plant. The optimum conditions found to achive the best voltammetric answer were 1.0 mVs-1, 110 mV and 100 Hz, to increment of voltage, amplitude and frequency, respectively. The Mo(VI) cathodic stripping voltammetry of in the complexant 8- hydroquinoline (oxine) in acetic acid/sodium acetate buffer solution with pH 4.65 and KNO3 2.2 mol L-1 as support electrolyte made possible the molybdenum determination in soils and plants without any previous treatment of the samples. The optimization of voltammetric parameters for the determination of trace metals in soils and plants by stripping techniques was planned in this present work and factors as equipments cost, sensibility, selectivity and low detection limits were considered.

ASSUNTO(S)

quimiometria molibdênio chemometrics quimica analitica voltametria voltammetry molybdenum

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