Novel approach to two-component speciation analysis. Spectrophotometric flow-based determinations of Fe(II)/Fe(III) and Cr(III)/Cr(VI).

The proposed approach to two-component speciation analysis relies on simultaneous application of two calibration methods to determination of two different forms of an analyte. One form is determined in extrapolative way, whereas the second form is determined in interpolative way, with the use of the same calibration graph, after appropriate chemical treatment, e.g. oxidation or reduction. The applicability of the approach has been verified on the examples of spectrophotometric determinations of Fe(II)/Fe(III) and Cr(VI)/Cr(III) using 1,10-phenanthroline and 1,5-diphenylcarbazide methods, respectively. In the above methods, ascorbic acid and Ce(IV) have been used to reduce Fe(III) to Fe(II) and to oxidize Cr(III) to Cr(VI), respectively. Lab-In-Syringe and SIA systems (for determination of iron and chromium species, respectively) have been applied to make the implementation of the proposed approach more convenient. The approach was verified on the example of determination of the analytes in synthetic and certified reference materials of ground and waste water samples. Using the developed methods, Fe(II)/Fe(III) and Cr(VI)/Cr(III) were determined within the concentration ranges of 0.06-4.0/0.06-3.0 and 0.03-0.5/0.05-7.0mgL-1, with precision (RSD, %) less than 3.8/2.0 and 2.2/6.0, and accuracy (RE, %) better than 2.9/4.3 and 6.8/5.2, respectively. The detection limits (mgL-1) are 0.02/0.02 and 0.01/0.02 for determination of both forms of iron and chromium, respectively. The applicability of the approach has been checked by analysis of artesian water (Fe(II)/Fe(III)) and post-production waste samples (Cr(III)/Cr(VI)).