Estudo crítico da geração de vapor de espécies de cromo empregando borohidreto como agente redutor

Abstract

The present work aims to give informations about the chemical vapor generation of inorganic species of chromium, trivalent and hexavalent, applying sodium tetrahydroborate as reductant agent in acid media. Chemical and physical ascpects are studied wich, according to cientific publications, can affect the performance of the mentioned reaction. The proposed system consists of a coupling between a batch module of vapor generation woth high-resolution continuum source atomic absorption spectrometry, using an externally heated quartz cell. The development of this word was made through quimiometric tools (full factorial designs and surface response methodology of Box-Behnken design) and univariate experiments. Two-level full factorial designs (2³) were applied to three variables screening: HCl concentration, NaBH4 concentration and NaBH4 flow rate. Considering the Cr(III) specie, HCl concentration had the greatest effect in CVG with negative value; for Cr(VI) specie, the interaction between the three variables had the greatest influence, with negative value. Subsequently, a Box-Behnken design was apllied for the three factor to each specie separately. Resulting mathematic models for both species did not presented lack of fit (p-value>0,05) and the response surfaces showed maximum of analytical response (absorption). Optimized experimental conditions were established: hydrochloridric acid concentration 0,076 and 0,070 mol L-1; NaBH4 concentration 0,79% and 0,85% (m/v); and NaBH4 flow rate 15,4 and 15,2 mL min-1 for Cr(III) and Cr(VI), respectively. Physical parameters were evaluated in one-at-a-time mode: the length of chemical vapor transferring tube to the quartz cell was not significative in chromium CVG, but the longer length was choosen due to higher absorption values of the analyte; and NaBH4 volume introduced in the solution was investigated for Cr(VI) and 2 mL of the reagent provided higher absorption signals. The study of the influence of auxiliary reagents for analytical signals of both species showed suppression of absorption in the presence of all considered reagents (thiourea, sulfanilamide, 8-hydroxiquinoline, tartaric acid, oxalic acid and cobalt), and no other reagent was added to the solutions. The investigation of possible interferents (Ni, Fe, Al, Cu, V, S and Mn) was performed with the hexavalent specie due to common use of oxidizing agents in sample treatment process; increments or suppression of absorptions higher than 5% were observed in the preence of all elements, except for Al when concentration was 100 mg L-1. Under optimized conditions for Cr(III) and Cr(VI), linear models adecquated the external calibration curves data and showed correlation coefficients r > 0,99. The detection and quantification limits obtained were 1,7 and 5,8 mg L-1 for Cr(III) and 5,8 and 19,5 mg L-1 for Cr(VI), respectively. The precision was less then 10% for both species. The proposed batch system for CVG of inorganic chromium species provided higher detection and quantification limits compared with registred ones in literature using continous flow.