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 ele ctrothermal atomic absorption spectrometry. Coprecipitation with Ga(OH )3 was carried out to preconcentrate dissolved Cr under optimum condit ions. 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 hydroxylammoniu m chloride (OHNH3Cl) total Cr was determined in sea-water, and optimum conditions for this reaction was evaluated. The differential determin ation of Cr(III) and Cr(VI) in sea-water was demonstrated by supplemen ting 'clean' sea-water (i.e., [Cr]total being below the limit of detec tion) 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 detect ion limit for Cr in the preconcentrated sample was almost-equal-to 2 m ug l-1, which corresponds to almost-equal-to 0.02 mug l-1 in the initi al sea-water sample. The proposed method was applied successfully to t he analysis of sea-water samples collected at various stations along t he French coasts (in Northern France) and in the English Channel trans ect. It is worth noting that sea-water samples were preconcentrated in the boat immediately after sampling. The findings revealed higher con centrations of dissolved Cr in coastal sea-waters as a result of anthr opogenic activities. Conversely, in the French offshore coasts, the co ncentration of total Cr decreased significantly. Chromium(vi) predomin ates in oxic open sea-waters with a Cr(VI) to Cr(III) concentration ra tio of 4.