PRECONCENTRATION OF CHROMIUM(III) FROM SEA-WATER BY ADSORPTION ON SILICA AND VOLTAMMETRIC DETERMINATION

Cr-III generally forms only a minor part of total dissolved chromium in natural waters. However, it is interesting to determine the concentration of Cr-III as its ratio with Cr-VI can be used as an indicator of the redox potential of natural waters and disequilibria thereof as a result of photochemical and biological processes. Cr-VI and total dissolved chromiun in natural waters, including sea-water, can be determined accurately and sensitively by cathodic Stripping voltammetry, but the concentration of Cr-III is close to the Limit of detection. A method is presented to preconcentrate Cr-III from sea-water by selective adsorption on silica particles. The adsorbed Cr-III is recovered by re-oxidation to Cr-VI by UV digestion of the re-suspended silica in water in the presence of excess of dissolved oxygen. The concentration of the desorbed Cr-VI is determined using cathodic stripping voltammetry. A very low analytical limit of detection (approximately 1 pmol l(-1)) is obtained by this method when the Cr-III is preconcentrated from a 100 ml sample of sea-water; however, the limit of determination was higher at 0.03 nmol l(-1) Cr-III owing to a reagent blank. The silica preconcentration method was used successfully to determine Cr-III in sea-water samples from the NE Atlantic.