| Résumé | Six sample preparation procedures were evaluated for selective extraction of Cr(VI) from commercial samples of chromium oxide green (Cr₂O₃) pigments prior to formation of its diphenylcarbazone complex [CrDPCO]⁻ for determination by visible spectrophotometry: (I) water-soluble chromium; (II) EPA method 3060A without Mg²⁺; (III) EPA method 3060A with Mg²⁺; (IV) Na₃PO₄ based extraction; (V) method IRSA16 based on acidic extraction and; (VI) Na₂CO₃ based extraction. Evaluation of the influence of concomitant Cr(III) ions, time and stability of the [CrDPCO]- complex was investigated. Recoveries of soluble and insoluble Cr(VI) species were 86% and 80%, respectively, using procedure (VI). Direct calibration against aqueous standards prepared in the extraction medium was successful for Cr(VI) in the concentration range 0.05-1.50 μg L⁻¹ . Limits of detection and quantitation were 0.3 µg g⁻¹ and 1.0 µg g⁻¹ , respectively, for 250 mg subsamples/25 mL. Procedure (VI) was applied to the analysis of four commercial samples of Cr₂O₃ pigments, three determined to have Cr(VI) within compliance limits below 1.0 µg g⁻¹ , but one at 16.6 ± 0.6 µg g⁻¹ , prohibiting use of this pigment in cosmetic formulations. This sample was conveniently employed to evaluate the accuracy of the method. The recommended procedure is simple and accurate and has been adopted by Tecpar’s laboratory of Parana Institute of Technology (Curitiba, Brazil). |
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