Preconcentration and speciation of chromium by the determination of total chromium and chromium(III) in natural waters by flame atomic absorption spectrometry with a chelating ion-exchange flow injection system

Abstract

A simple method for the determination of chromium(III) and total chromium in a flow injection system comprising chelating ion-exchange and flame atomic absorption spectrometry (FAAS) was described. A minicolumn (85 mm × 1.6 mm i. d.) filled with a poly(aminophosphonic acid) chelating resin (PAPhA, 20–30 mesh) was used for speciation and preconcentration of chromium because this resin is selective for Cr(III) ions. The flow injection manifold employed allows a high rate of introduction of the sample and retention of Cr(III) on the minicolumn. Total chromium was obtained by an efficient reduction of Cr(VI) to Cr(III) by ascorbic acid as reducing reagent. Elution was performed by using a small volume of 0.5 M hydrochloric acid. For 6.6ml samples, the detection limit for the total chromium and Cr(III) was 0.2 μg 1 −1. The precision (coefficient of variation) obtained for different amounts of Cr(III) and total Cr were 1.2–5.9% and 1.2–5.7%, respectively. Chemicals and flow variables, interferences and other figures of merit of the proposed methodology were studied. The method has been applied to the determination of total Cr, Cr(III) and by difference between these two species the Cr(VI) concentration in natural water samples: tap, mineral and river waters.