On-line speciation of Cr(III) and Cr(VI) by flow injection analysis with spectrophotometric detection and chemometrics

A flow injection system has been developed, for on-line speciation of Cr(III) and Cr(VI) by the Diphenylcarbazide (DPC) method with H ₂O₂ oxidation followed by spectrophotometric detection at the 550 nm wavelength. The data thus obtained were subjected to a chemometric analysis (PLS), which showed that it was possible to obtain reliable predictions of Cr(III) and Cr(VI) species. The method was validated on samples of stainless steel. In order to facilitate analysis of stainless-steel samples, it was essential to perform a step of evaporation to dryness followed by re-dissolution. The analysis showed that species of Cr(III) and Cr(VI) coexisted in the samples digested by this method. By spiking the samples with iron and nickel species, it was found that, although the total amount of chromium remained constant, the equilibrium of the two chromium species was displaced towards Cr(III). A detection limit of 10 μg/L and a maximum sampling frequency of 120 injections per hour were obtained, excluding calculations and sample pre-treatments.