Critical evaluation of three analytical techniques for the determination of chromium(VI) in soil extracts

Abstract

Three different analytical techniques [1,5-diphenylcarbazide spectrophotometry, chelating ion-exchange chromatography (Chelex-100), and ion-pairing reversed-phase high-performance liquid chromatography (RP-HPLC) combined with electrothermal atomic absorption spectrometry (ETAAS)] were critically evaluated for the determination of Cr^VI in soil extracts. Spectrophotometry was not applicable to the analysis of most soil extract samples owing to its high limit of detection (LOD = 30 ng cm^–3), and the possibility of the instantaneous reduction of Cr^VI under the acidic conditions employed. A Chelex 100 column, although adequately sensitive (LOD = 1.5 ng cm^–3), is inclined to give higher results as inhert and moderately labile Cr^III complexes partially passed through the resin together with Cr^VI. In addition, very small particles (<0.45 µm) carrying chromium can produce severe positive systematic errors. In order to avoid this, filtration employing a 0.1 µm filter is recommended. Ion-pairing RP-HPLC was found to be the most sensitive technique (LOD = 0.3 ng cm^–3). It might also give high chromate results if negatively charged Cr^III complexes form ion pairs with tetrabutylammonium phosphate and their elution partially coincided with that of Cr^VI. Fulvate ligands showed this type of interference. Reversed-phase HPLC is not suitable for analysis of extracts obtained from soils with freshly added tannery waste owing to the effects of the undestroyed tannery waste matrix. This study showed that each method investigated was vulnerable to some type of interference.