Rate of Complex Formation and Solvent Extraction of Chromium(III) Produced by the Reduction of Chromium(VI) with Acetylacetone in Aqueous Solutions

Abstract

Abstract The rate and degree of solvent extraction of chromium(III), which was produced by the reduction of chromium(VI) with ascorbic acid in 0.1 mol dm-3 sodium nitrate solutions containing acetic acid/acetate ion as the buffer, were examined by using acetylacetone as the extractant and chloroform as the solvent. The rate of formation of an extractable acetylacetonato complex was greater than that when metal ion was added as hydrated Cr3+ until a certain amount of chromium(III) was extracted. Although the rate was first order with respect to the acetylacetone concentration and inverse first order with respect to the hydrogen-ion concentration, the extraction of chromium(III) formed in this way was dependent on the concentration of acetate ions in the aqueous solution. From these results, it was assumed that the chromium(III) produced from chromium(VI) formed a complex with an acetate ion just after the reduction occurred, and that the complex functions as a precursor to the extraction. The acetylacetone reacted with the precursor much more rapidly than the hydrated chromium(III) ion. From the change in the UV-vis spectrum of the complex and in the easiness of stripping of chromium(III) from chloroform with a nitric acid solution, the thus-formed extractable complex was assumed to be an intermediate which changed to a final complex similar to that obtained by the dissolution of crystals of tris(acetylacetonato)chromium(III) in chloroform.