2002 Volume 75 Issue 11 Pages 2415-2421
In order to explore the cause of a distinctive extraction behavior of Cr(VI) in the dithiocarbamate formation-solvent extraction systems, extracted chromium compounds were investigated and relationships between the formation of the compounds and the extraction behavior were studied. Chromatographic separation and spectroscopic analyses showed that two kinds of chromium(III) dithiocarbamates (i.e. tris(1-pyrrolidinecarbodithioato)chromium(III) and bis(1-pyrrolidinecarbodithioato)[1-pyrrolidinecarbothio(thioperoxoato)]chromium(III)) were formed and extracted in an ammonium 1-pyrrolidinecarbodithioate (APCD)/diisobutyl ketone (DIBK) solvent extraction system. The two products comprised most of the extracted chromium. Concentrations of the two dithiocarbamates in extracts were determined using double wavelengths analysis. The dependence of concentration ratio of the two complexes on pH was also investigated. The results of the investigation clarified that a major extract obtained under high pH condition was the thioperoxo complex and that the percentage of the tris complex increased with decreasing pH. Furthermore, it was clarified that the unusual extraction behavior of chromate was distinguished by the formation of thioperoxo complex. In order to elucidate the formation mechanism of the thioperoxo complex, the origin of an oxygen atom in the molecule of the thioperoxo complex was determined with an isotope labeling technique. The labeling experiments clarified that the source of the oxygen atom of the complex molecule was chromate ion and that the oxygen atom was fixed to chromium atom prior to the rate determination step of the formation of the thioperoxo complex.
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