DETERMINATION OF DISSOLVED CHROMIUM(III) AND CHROMIUM(VI) IN SEA-WATER BY ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY

A method for the determination of dissolved Cr(III) and Cr(VI) in sea-water is reported based on preconcentration by coprecipitation and electrothermal atomic absorption spectrometry. Coprecipitation with Ga(OH)3 was carried out to preconcentrate dissolved Cr under optimum conditions. The average recovery was almost-equal-to 98% for Cr(III) and <1% for Cr(VI). After reduction of Cr(VI) to Cr(III) with hydroxylammonium chloride (OHNH3Cl) total Cr was determined in sea-water, and optimum conditions for this reaction was evaluated. The differential determination of Cr(III) and Cr(VI) in sea-water was demonstrated by supplementing 'clean' sea-water (i.e., [Cr]total being below the limit of detection) with known amounts of Cr(III) and Cr(VI). Recoveries for Cr(III) were 95-101% (+/- 3%) and for Cr(VI) were 94-99% (+/- 3%). The detection limit for Cr in the preconcentrated sample was almost-equal-to 2 mug l-1, which corresponds to almost-equal-to 0.02 mug l-1 in the initial sea-water sample. The proposed method was applied successfully to the analysis of sea-water samples collected at various stations along the French coasts (in Northern France) and in the English Channel transect. It is worth noting that sea-water samples were preconcentrated in the boat immediately after sampling. The findings revealed higher concentrations of dissolved Cr in coastal sea-waters as a result of anthropogenic activities. Conversely, in the French offshore coasts, the concentration of total Cr decreased significantly. Chromium(vi) predominates in oxic open sea-waters with a Cr(VI) to Cr(III) concentration ratio of 4.