Nature and Stability of Mercury Thin Films on Glassy Carbon Electrodes under Fast-Scan Anodic Stripping Voltammetry

Abstract

The surface structure of mercury thin films (MTFs) on a glassy carbon electrode (GCE) and the changes of MTFs associated with anodic stripping voltammetry (ASV) were studied using ex situ optical microscopy and linear scan voltammetry. The results of microscopic examinations showed that, in contrary to tbe early observations of large scattered microdroplets on GCE surfaces, the original state of MTFs was close to a thin film, The surface coverage of the mercury microdroplets was continuous microscopically, and the size of the droplets ranged from less than 0.1 pm (unidentifiable under 400X magnification) to about 1 pm, depending on the total amount of mercury on the GCE surface. Under the condition of ASV scanning (2 V/s) with in situ MTFs, the stability of a mercury coating was shown to depend on the total amount of mercury on GCE. It was also shown that the severe noise spikes and the erratic current behavior were associated with the surface structural changes from a state of fine and smooth microdroplets to a state of large scattered microdroplets. The transformation from one state to another is believed to be driven by the surface perturbation of rapid potential changes under the ASV condition.We report here the surface conditions of mercury coatings on a glassy carbon electrode (GCE) and the surface changes associated with anodic stripping voltammetry (ASV). ASV is a sensitive tool for the analyses of trace amounts of heavy metals.'-' Perhaps the most widely used electrode form is the mercury thin film (MTF) on a glassy carbon substrate because of the versatility, the ease of preparation, and the wide potential window for a n a l y s e~.~-~l~ In particular, ASV with in situ formed MTF provides high sensitivity, low detection limit, and high resolution of neighboring peaks, as demonstrated by F l~r e n c e .~ Since their initial usage, the nature of mercury coatings on GCE has been the subject of many s t~d i e s~-~,~,~J~ because of the importance of a stable and reproducible mercury coating to ASV and the interesting properties of mercury thin films as a form of electrode. The model of homogeneous thin film has been used in theoretical treatments9-13 of metal diffusion (1) Vydra, F.; Stulik, K.; Julakova, E. in and out of the mercury coating and in experimental applications in ASV,435 with the film thickness ranging from 1 to 10 nm for in situ formed MTFs4 and from 0.1 to 1 pm for ex situ formed MTFs.~ On the other hand, mercury coatings on the GC surface have been shown, by various optical microscopic and SEM i m g a g e~,~J~J~ to be inhomogeneous with scattered microdroplets. Despite these reports, the nature of mercury coatings is still not at all certain because of the dynamics involved in stripping processes and the inconsistent behavior of ASV with various MTFs. Thus, to what degree these mercury coatings can be considered as thin films remains unanswered.The objectives of the present investigation of mercury coatings on GCE have been to understand the nature of mercury coatings under various conditio...

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