Search

A new procedure for the direct determination of picomolar levels of cobalt in seawater is presented. Cathodic stripping voltammetry is preceded by adsorptive accumulation of the cobalt-nioxime (cyclohexane-1,2-dione dioxime) complex from seawater containing 6 [[micro]meter] nioxime and 80 mM ammonia at pH 9.1, onto a hanging mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is catalytically enhanced by the presence of 0.5 M nitrite. Optimized conditions for cobalt include a 30 s adsorption period at -0.7 V and a voltammetric scan using differential pulse modulation. According to the proposed reaction mechanism, dissolved Co(II) is oxidized to Co(III) upon addition of nioxime and high concentrations of ammonia and nitrite; a mixed Co(III)-ammonia-nitrite complex is adsorbed on the electrode surface; the Co(III) is reduced to Co(II) (complexed by nioxime) during the voltammetric scan, followed by its chemical reoxidation by the nitrite, initiating a catalytically enhanced current. A detection limit of 3 pM cobalt (at an adsorption period of 60 s) enables the detection of this metal in uncontaminated seawater using a very short adsorption time. UV digestion of seawater is essential, as part of the cobalt may occur strongly complexed by organic matter and rendered nonlabile. The method was applied successfully to the determination of the distribution of cobalt in the water column of the Mediterranean.

An automated method for on-site monitoring of uranium(VI) in raffinate streams originating from nuclear fuel reprocessing plants is described. An in-line stripping procedure (based on liquid/liquid extraction) was developed to extract U(VI) from this stream, a solvent mixture of 20% tributyl phosphate and nitric acid in kerosene, into an aqueous sodium sulfate solution. Degradation products in the solvent mixture, especially dibutyl phosphate, give rise to very strong complexes and are responsible for moderate but constant U(VI) recoveries ([approximately]50%). Optimal conditions for in-line stripping comprise a mixing ratio of extractant (0.5 M sodium sulfate in water)/solvent mixture of [approximately]3 and a pumping rate of [approximately]0.4 mL [min.sup.-1] of the solvent mixture. The determination of U(VI) was by on-line cathodic stripping voltammmetry (CSV), preceded by adsorptive collection of the U(VI) as an oxine complex onto a hanging mercury drop electrode. Quantities of 1-2 mL of the aqueous extract were pumped into the voltammetric cell and diluted (1/5 to 1/10) with a background electrolyte containing 0.1 M PIPES buffer, 2 x [10.sup.-4] M oxine, [10.sup.-4] M EDTA, and 0.2 M hydrazine hydrate (pH 9.0). The CSV peak for U(VI) was obtained at -0.68 V with a detection limit of 20 nM in the raffinate stream using an adsorption time of 120 s. Both the in-line stripping procedure and the on-line measurement were fully automated, with a relative standard deviation in the measurements of

A procedure is presented for the determination of ammonia in seawater using adsorptive cathodic stripping voltammetry (CSV). The ammonia is reacted with formaldehyde (13% w/v) at pH 3.8 to form an imino compound (methylenimine) which is adsorbed on the hanging mercury drop electrode (HMDE). Subsequently, a differential pulse scan is made toward more negative potentials and the reduction current is measured. Optimized conditions include an adsorption potential of -0.85 V and an adsorption time of 60 s; the voltammetric scan was initiated from -0.6 V. The peak potential was at -0.91 V. The CSV response was linearly related to the ammonia concentration between 10 and 3000 nM. The limit of detection for ammonia was 4 nM. Comparative measurements of the concentration of ammonia in samples from the estuary of the River Mersey and in lake waters using the CSV method and conventional spectrophotometric technique showed good agreement between the two methods. The method was applied successfully to the determination of low ammonia levels in seawater from the water column of the northwest Mediterranean.