Flow injection direct spectrophotometric assay for the speciation of trace chromium(III) and chromium(VI) using chromotropic acid as chromogenic reagent.

A new rapid and sensitive FI assay is reported for the simultaneous direct spectrophotometric determination of trace Cr(VI) and Cr(III) in real samples. The method is based upon the reaction of Cr(VI) with chromotropic acid (CA) in highly acidic medium to form a water-soluble complex (lambda(max)=370nm). Cr(III) reacts with CA only after its on-line oxidation to Cr(VI) by alkaline KIO(4). The determination of each chromium species in the sample was achieved by absorbance differences. The calibration curves were linear over the range 3-4000mugl(-1) and 30-1200mugl(-1) for Cr(VI) and Cr(III), respectively, while the precision close to the quantitation limit was satisfactory in both cases (s(r)=3.0% for Cr(VI) and 4.0% for Cr(III) (n=10) at 10 and 50mugl(-1) level, respectively). The method developed proved to be adequately selective and sensitive (c(L)=1 and 10mugl(-1) for Cr(VI) and Cr(III), respectively). The application of the method to the analysis of water samples (tap and mineral water) gave accurate results based on recovery studies (93-106%). Analytical results of real sample analysis were in good agreement with certified values.