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Procedures for trace cobalt determinations by adsorptive stripping voltammetry at in situ and ex situ plated bismuth film electrodes are presented. These exploit the enhancement of the cobalt peak obtained by using the Co(II)-dimethylglyoxime-cetyltrimethylammonium bromide-piperazine-N,N'-bis(2-ethanesulfonic acid) system. The calibration graph for an accumulation time of 120 s was linear from 2 x 10(-10) to 2 x 10(-8) mol L(-1). The relative standard deviation from five determinations of cobalt at a concentration of 5 x 10(-9) mol L(-1) was 5.2%. The detection limit for an accumulation time of 300 s was 1.8 x 10(-11) mol L(-1). The proposed procedure was applied to cobalt determination in certified reference materials and in tap and river water samples.

A procedure for the extraction of Cr(VI) from solid soil-like samples was presented in which the complexing properties of diethylenetriaminepentaacetic acid (DTPA) were exploited to extract insoluble compounds of Cr(VI). A concentration of DTPA in an ammonium sulphate/ammonium hydroxide buffer equal to 0.02 mol l(-1) was chosen. The conditions of extraction of insoluble Cr(VI) from solid samples were optimised using soil certified reference material spiked with known concentration of insoluble Cr(VI) added as PbCrO(4). The extracts were analysed by adsorptive stripping voltammetry. Validation of the proposed procedure of extraction was carried out by analysis of certified reference material (CRM) 545 and comparison of the results obtained using the proposed and other methods of extraction in the course of analysis of natural soil samples.

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The voltammetric procedure for determination of traces of Cr(VI) [ Anal. Chim. Acta (1992) 262:103] was modified by changing the temperature of the measurements. It was found that at the temperature of 40 degrees C the time of decrease of the Cr(III) signal was shortened from 30 to 5 min. As a result the total analysis time was drastically shortened. The modified procedure does not show any disadvantage as compared to the original method. The results of Cr(VI) determination by the modified procedure are less affected by Cr(III) as compared to the original method. The detection limit of the method was 2.5 x 10(-11) mol L(-1) (1.2 ng L(-1)). The validation of the modified procedure was performed by comparison of the results of analyses of tap and river water samples with those obtained using original procedure.

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