Liquid-liquid extraction of chromium by diphenyl-2-pyridylmethane

Abstract

<p>The liquid-liquid extraction of chromium from aqueous solutions of hydrochloric acid by diphenyl-2-pyridylmethane (DPPM) dissolved in hydrogen-bonding organic diluents has made possible the separation, concentration and speciation of chromium at trace concentrations. In the extraction of Cr(VI) the most effective diluents employed were, in order: CH₂ClCH₂Cl, CH₂Cl₂ and CHCl₃. While Cr(VI) was extracted best from HCl solutions the actual order of extraction of the inorganic acids themselves, used in the protonation of the DPPM, was found to be: HClO₄ > HNO₃ > HBr > HCl > H₂SO₄, H₃PO₄. The extraction of Cr(VI) was followed by using commercial high specific activity ⁵¹Cr or enriched material prepared by a Szilard-Chalmers reaction. The distribution ratio (D) in all 0.10M DPPM/diluent extractions increased with an increase in the acid concentration up to about IM in HCl but decreased thereafter. To account for the extraction a model is proposed which suggests that the distribution of Cr(VI) depends mainly upon the availability of DPPMH⁺ in the aqueous phase and the form of the Cr(VI). Both the model and spectral evidence support the extraction of the CrO₃Cl⁻ species. With the large separation factors available, Cr(VI) may be separated from inorganic Cr(III), organic Cr(III) complexes, such as Cr(C₂O₄)₃³⁻, and many other metallic species. Total chromium in the aqueous sample may be determined by an analysis of the extract after a Ce(IV) oxidation. Further, with a distribution ratio as high as D≃700, it is possible to concentrate the chromium from an aqueous sample to facilitate the analysis in the simplified matrix of the organic phase. This extraction may overcome the problems associated with chromium analysis at trace concentrations, where samples are so subject to loss by adsorption or changes in oxidation state upon storage. While back extractions have been examined, the direct analysis of the chromium in the extract by atomic absorption (ng/mK concentrations) or spectrophotometric (μg/mL concentrations) methods is advocated.</p>