Otimização de metodo catalitico para a determinação de Cr (VI) com o-Dianisidina, usando planejamento combinado de misturas e variaveis de processo

AUTOR(ES)
DATA DE PUBLICAÇÃO

1996

RESUMO

The kinetic catalitic procedure for the determination of Cr(Vl) based on the the o-dianisidine oxidation reaction with hydrogen peroxide in a weakly acid medium was optimized with respect to both the reactants [HCI, o-dianisidine and H2O2] and the solvent composition [ water, acetone and N,N-dimethylformamide] in order to achieve higher sensitivities. This was acomplished using a combined design approach involving mixture experiments having process variables, as well as surface response methodology. The experiments, carried out within the experimental limits, resulted in optimized mixture variables [70% m/m of water, 30% m/m of acetone and 0% m/m of N,N-dimethylformamide] and process variables [HCI: 6.0x10 mol/L, o-dianisidine: 1.9x10 mol/L and H2O2: 0,79 mol/L]. Under these experimental conditions, the method sensitivity was increased three times, with respect to the procedure described in the literature. Under the optimized conditions, the interfering species showed two distinct situations. Using the step by step approach, Cr(lII) and Cu(lI) presented serious interferences when present in the 2:1 proportion, while Fe(lII) began to interfere at a ratio of 15:1 and EDTA at 10:1. The other interfering species studied presented interference only when above the ratio of 100: 1 with respect to the concentration of Cr(VI). On the other hand, the multifactorial approach indicated that Cr(lII) did not have a significant principal effect and its interaction effects were all negative. In contrast, EDTA showed positive interaction effects but did not present significative principal effect. The overall interference effects indicate the need of a previous Cr(Vl) separation in real sample analysis. This was done in this work by extracting Cr(Vl) in methylisobutylketone in hydrochloric acid medium. The proposed procedure presented a relative standard deviation of ±0.5% at the level of 10 ng/mL of Cr(Vl) and limits of detection and quantification of 1,1 ng/mL and 3,2 ng/mL, respectively. Waste waters and plant samples were employed to test the procedure proposed for Cr(Vl) determination and the results presented relative standard deviations ranging from 1.2% to 3.0% with respect to he actual reference values for those samples.

ASSUNTO(S)

cromo otimização misturas espectrofotometria

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