Determination of the speciation of chromium with an automated two-column ion-exchange system

An automated two-column ion-exchange system was developed to determine the speciation of chromium in water samples. The system is based on retention of cationic Cr(III) species by Chelex-100 resin and anionic Cr(VI) species by an anion resin. The chromium species retained by the resins are eluted sequentially with stripping reagents and are detected by an on-line flame atomic absorption spectrophotometer. Operation of valves and pumps, data acquisition, and data manipulation are controlled by a microcomputer. The detection limit with a 2 ml sample loop is 2 ng ml−1 for Cr(III) and Cr(VI). The fraction of Cr(III) retained by the Chelex-100 varies with the pH of the solution. In the presence of humic acid, Cr(III) forms negatively charged complexes, some of which are retained by the anion resin.