Desenvolvimento de uma metodologia eletroquímica para determinação de Cr(III) e Cr(VI) utilizando sílica gel funcionalizada com 1,5-Difenilcarbazida

Abstract

A new method has been developed in this work for the determination and speciation of the Cr(III) and Cr(VI) based on the use of a modifíed carbon paste electrode with silica gel functionalized with 1,5-diphenylcarbazide. The first part of this work involved the modification of the silica gel surface with 3-aminopropyltriethoxysilane, followed by the coupling with glutaraldehyde and functionalization with 1,5-diphenylcarbazide (DPCI). This material was mixed with powder graphite and mineral oil for the preparation of the modifíed carbon paste electrode (MCPE). The Cr(VI) was preconcentrated on the MCPE by Cr(III)-diphenycarbazone complex, Cr(DFCO)3+, that is formed from the reaction of Cr (VI) with DFCI on the MCPE. The retention of Cr(DFCO)3+ on MCPE has been investigated by cyclic voltammetric technique in function to the accumulation time and Cr(VI) concentration in the aqueous solution, using 0.05 mol L’1 H2SO4 as supporting electrolyte. The cathodic peak current at potencial near to -0.45 V vs Ag/AgCl (KC1 3 mol L'1) was proporcional to the Cr(VI) concentration over 2x10'8 to 2x10'7 mol L’1 range. The relative standard desviation (10 replicates analysis) at the 3.2x10’ mol L’ of Cr(VI) was 6.7% and the detection limit of the MCPE to Cr(VI) was 1.7 xlO’9 mol L’1. The Cr(III) has been determined by diference between total Cr(VI) and Cr(VI), after oxidation to Cr(VI) by KMnO4, in acid media. The developed method has been applied to the determination of chromium speciation in tannery effluents by using the standard addition method and the Cr(VI) and Cr(III) concentrations obtained are in agreement with those obtained by standard spectrophotometric method.