Determination of dissolved chromium(III) and chromium(VI) in sea-water by electrothermal atomic absorption spectrometry

Abstract

A method for the determination of dissolved CrIII and CrVI 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 ≈ 98% for CrIII and <1% for CrVI. After reduction of CrVI to CrIII with hydroxylammonium chloride (OHNH3Cl) total Cr was determined in sea-water, and optimum conditions for this reaction were evaluated. The differential determination of CrIII and CrVI in sea-water was demonstrated by supplementing ‘clean’ sea-water (i.e., [Cr]total being below the limit of detection) with known amounts of CrIII and CrVI. Recoveries for CrIII were 95–101%(±3%) and for CrVI were 94–99%(±3%). The detection limit for Cr in the preconcentrated sample was ≈ 2 µg l–1, which corresponds to ≈ 0.02 µg 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 CrVI to CrIII concentration ratio of 4.