Abstract
The widely spread use of the hanging mercury drop electrode (HMDE) for multi-ion analysis is primarily ascribed to the following reasons: (i) excellent reproducibility owing to the easy renewal of the electrode surface avoiding any hysteresis effect (i.e., a new identical drop is generated for each measurement to be accomplished); (ii) a wide cathodic potential window originating from the passive hydrogen evolution and solvent electrolysis; (iii) the ability to form amalgams with many redox-active metal ions; and (iv) the achievement of (sub)nanomolar limits of detection. On the other hand, the main controversy of the HMDE usage is the high toxicity level of mercury, which has motivated the scientific community to question whether the HMDE deserves to continue being used despite its unique capability for multi-metal detection. In this work, the simultaneous determination of Zn2+, Cd2+, Pb2+, and Cu2+ using the HMDE is investigated as a model system to evaluate the main features of the technique. The analytical benefits of the HMDE in terms of linear range of response, reproducibility, limit of detection, proximity to ideal redox behavior of metal ions and analysis time are herein demonstrated and compared to other electrodes proposed in the literature as less-toxic alternatives to the HMDE. The results have revealed that the HMDE is largely superior to other reported methods in several aspects and, moreover, it displays excellent accuracy when simultaneously analyzing Zn2+, Cd2+, Pb2+, and Cu2+ in such a complex matrix as digested soils. Yet, more efforts are required towards the definitive replacement of the HMDE in the electroanalysis field, despite the elegant approaches already reported in the literature.
Highlights
Anodic stripping voltammetry (ASV) is an electrochemical technique traditionally employed for the detection of metal ions at trace levels
Metal ions are first preconcentrated onto the working electrode surface, which advantageously results in significantly lower limits of detection (LOD) as compared to direct voltammetry measurements (i.e.,nanomolar versus micromolar levels) [1]
In classical ASV measurements performed with the hanging mercury drop electrode (HMDE), metal ions are first electrodeposited on the working electrode during the so-called ‘deposition step.’
Summary
Anodic stripping voltammetry (ASV) is an electrochemical technique traditionally employed for the detection of metal ions at trace levels. Despite the excellent performance of the HMDE and MTFE, the high toxicity of mercury has encouraged the development of alternative electrodes for metal ion determination, in which the use of mercury in the preparation/measurement approach is totally avoided [11]. Despite all the great efforts reported in the literature at the time of writing, the HMDE is still today the recommended analytical approach in reference laboratory analysis [18] This fact highlights the difficulty in finding a new electrode that features the performance of the HMDE for trace metal detection and being proposed as the definitive replacement. Both experimental conditions and analytical characteristics are widely discussed, concluding that, even though the HMDE has been criticized as an ‘outdated’ and ‘toxic’ approach to accomplish the task for metal detection in real samples, its accuracy and multi-analyte feature are still superior to any other electrochemical method considering solid electrode materials
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