The chemical speciation of chromium in seawater by cathodic stripping voltammetry

Ramjuttun, Ravindlall (1997) The chemical speciation of chromium in seawater by cathodic stripping voltammetry. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Chromium has an interesting ecotoxic role in aquatic environments, where it occurs in two different oxidation states, Cr(lll) and Cr(VI). While Cr(lll) is considered an essential nutrient, Cr(VI) is toxic and carcinogenic. Hence it is important to monitor the concentrations of the individual Cr species in natural waters. -- This thesis describes the development, optimization and application of a new voltammetric method for the determination of Cr species in aqueous environmental samples, with particular reference to seawater. This method is based on the Square Wave Cathodic Stripping Voltammetry (CSV) of a Cr-Xylenol Orange (XO) complex preceeded by its adsorptive preconcentration on a hanging mercury drop electrode. The catalytic reduction of nitrate ions in the vicinity of the electrode is utilized to enhance the sensitivity of the method. Stable reduction peaks are obtained for both Cr(VI) and Cr(lll) species. The formation of the adsorbed analyte complexes derived from Cr(VI) and Cr(lll) is dependent on the deposition potential used for the preconcentration and this forms the basis for the speciation of Cr. Hence the need for lengthy sample pretreatment steps is precluded from the analytical procedure. -- Under optimized conditions, sensitivities greater than 1.8 µA (µg/L)⁻¹ min⁻¹ (90 nA nM⁻¹ min⁻¹) can be achieved, making the method adequate for the determination of Cr at the concentration levels prevalent in all types of natural waters. For 60 s preconcentration times, detection limits for Cr(VI) and Cr(lll) are respectively 1.2 and 1.8 ng/L in distilled, deionized water. The method is virtually free from interferences and is applicable to Cr analysis in seawater as well as in surface waters and has been applied to the determination of Cr speciation in the N.W. Atlantic Ocean. -- Results of experiments aimed at elucidating the electrode processes involved in the deposition and reduction of the analyte complex suggest that the adsorbed complex is Cr¹¹¹-XO. This complex is formed on the surface of the electrode by the reaction between nascent Cr species and adsorbed XO. The analytical signal is the current obtained when this complex is reduced to Cr¹¹-XO.

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