The determination of the chromium speciation in sea water using catalytic cathodic stripping voltammetry

Abstract

A voltammetric procedure to determine chromium in aqueous solution using cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complex species with diethylenetriaminepentaacetic acid (DTPA) on a hanging mercury drop electrode is optimized for sea water. The optimized conditions include a pH of 5.2 and a DTPA concentration of 2.5 mM. Comparison of the chemical speciation of DTPA in sea water and distilled water with the CSV sensitivity indicated that the adsorptive species is a complex of chromium(III_with H 2DTPA. The sensitivity of the CSV procedure for chromium(VI) in sea water is a decade lower than in distilled water due to major cation competition (of calcium and magnesium) for DTPA in the sea water. The limit of detection for chromium(VI) in sea water is 0.1 nM at a deposition time of 2 min. Chromium(VI) produces a stable peak using the optimized CSV procedures, whereas the peak due to chromium(III) is unstable due to probable conversion of the chromium(III) complex to an electrochemically inert complex over a period of ≈ 30 min. The different behaviours of chromium(VI) and –(III) were utilized to devise a procedure to determine reactive chromium(III) and chromium(VI) in sea water samples. Experiments with an added chelator indicated that the reactive chromium(III) concentration does not include complexes with organic material. The total dissolved chromium concentration was determined after UV-irradiation of the sample at its original (neutral) pH. Storage experiments indicated that neither the redox speciation of chromium nor its total dissolved concentration in sea water could be stored. The CSV method was applied succesfully to certified sea water samples and to the ship-board determination of chromium(VI) and –(III) in the water column of the N.W. Mediterranean.