Preconcentration of chromium(III) from sea-water by adsorption on silica and voltammetric determination

Abstract

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 chromium 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.