Economic bismuth-film microsensor for anodic stripping analysis of trace heavy metals using differential pulse voltammetry.
Résumé
Stripping analysis has been widely recognised as a powerful tool in trace metal analysis. Its remarkable sensitivity is attributed to the combination of a preconcentration step coupled with pulse measurements that generate an extremely high signal-to-background ratio. Mercury-based electrodes have traditionally been used to achieve high reproducibility and sensitivity in the stripping technique. Because of the toxicity of mercury, however, new alternative electrode materials are highly desired, particularly for on-site monitoring. Use of thin films of bismuth deposited on platinum or glassy-carbon substrates has recently been proposed as a possible alternative to mercury-bismuth is "environmentally friendly", of low toxicity, and is in widespread pharmaceutical use. In this paper the preparation of economic bismuth-film microelectrodes by electrodeposition on a copper substrate and their application to heavy metal analysis are described. Bismuth-film electrodes were prepared by potentiostatic electrodeposition. Optimum conditions for chemical and electrochemical deposition to obtain an adherent, reproducible, and robust deposit were determined. The suitability of such microelectrodes for analysis of heavy metals was evaluated by anodic stripping voltammetry of cadmium. The analytical performance of bismuth-film electrodes for anodic stripping voltammetry of heavy metals was evaluated for non-deaerated solutions containing Cd-2, Pb-2, and Zn-2 ions. Well-defined peaks with low background current were obtained by use of differential pulse voltammetry. Linear calibration plots were obtained for Cd-2 in acidified tap water at concentrations ranging from 2x10(-8) to 1x10(-7) mol L-1 and from 1x10(-7) to 1x10(-6) mol L-1 with relative standard deviations of 5% (n=15) at the 1x10(-7) mol L-1 level. The method was then successfully used to monitor the Cd-2 content of plant extracts and validated by polarographic and ICP-MS measurements. These results open the possibility of using bismuth-coated copper electrodes as an alternative to mercury-based electrodes for analysis of heavy metals. The main problem remaining, which prevents on-site monitoring of heavy metals, is the need to use slightly acidic media, because formation of bismuth hydroxide on the film surface above pH 4.3 leads to non-reproducible measurements. Further experiments will be performed to discover whether electrode conditioning can be used to enable reproducible measurement in on-site monitoring of cadmium in natural waters. Moreover, further study should be conducted to evaluate the potential of BiFE for analysis of several pollutants of interest that are usually determined electrochemically by using mercury-based electrodes.