Mercury deposition, glassy carbon electrode

Film Electrodes and Related Hg Electrodes Mercury films were prepared on reticulated vitreous carbon flowthrough electrodes by Hg deposition from Hg + solutions in acetic buffer [31]. Such an electrode was designed for the purposes of trace metal analysis. Mercury film de-position/oxidation on reticulated vitreous carbon and glassy carbon electrodes were compared. 

Fig. 6.3. Stripping voltammograms for 50ppb Zn, Cd and Pb at glassy-carbon electrodes coated with bismuth (a) and mercury (b) films. Two-min deposition at -1.4 V. (Based on Ref. [12], with permission obtained from The American Chemical Society).

Braun and Metzger [52] showed that trace amounts of nickel obtained from natural environmental samples could be determined voltammetrically as nickel dimethyl glyoximate following adsorptive enrichment onto a rotating glassy carbon electrode, on which a thin mercury film has been deposited electrolytically. See Sect. 7.34.1. 

Lead and mercury are deposited as micron-sized clusters, predominantly at intercrystallite boundaries [105] so does lithium from the polyethylene oxide solid electrolyte. What is more, Li intercalates into the sp2-carbon [22, 138], Thus, observations on the Li intercalation and deintercalation enable one to detect non-diamond carbon on the diamond film surface. Copper is difficult to plate on diamond [139], There is indirect evidence that Cu electrodeposition, whose early stages proceed as underpotential deposition, also involves the intercrystallite boundaries [140], We note that diamond electrodes seem to be an appropriate tool for use in the well-known electroanalytical method of detection of traces of metal ions in solutions by their cathodic accumulation followed by anodic stripping. The same holds for anodic deposition, e.g. of, Pb as PbCh with subsequent cathodic reduction [141, 142], Figure 30 shows the voltammograms of anodic dissolution of Cd and Pb cathodically predeposited from their salt mixtures on diamond and glassy carbon electrodes. We see that the dissolution peaks are clearly resolved. The detection limit for Zn, Cd, and Pb is as low as a few ppb [143]. 

Bismuth is one of the elements most easily determined with anodic stripping voltammetry (Florence 1972) as it can be deposited onto electrodes at potentials at which most other elements are in solution. Glassy carbon electrodes coated with films of mercury, but also of gold, have been used (Florence 1974). In biological materials with low concentrations of Bi, interference from other metals is not to be expected because of the low levels of most metals in these materials. Best results are reached with solutions containing 0.1 M hydrochloric acid, with detection limits in the range of 1 (IfQand 1993). 

Solid electrodes are used for the analysis of metal ions that cannot be determined on mercury electrodes with reasonable efficiency, or not at all. Some examples are listed in Table II.7.3. They are representatives of two groups of ions. The stripping peak potentials of deposits of the first group are higher, or close to, the stripping potential of mercury. For instance, the concentration of bismuth ions cannot be measured on mercury electrodes below 5 x 10 mol/L, but on a glassy carbon electrode, these ions can be determined at the concentration level of 10 mol/L... 

During potentiometric stripping analysis (PSA), trace elements or ions are pre-concentrated by potentiostatic deposition on an electrode (e.g., mercury film on a glassy-carbon electrode). In contrast to DPASV, PSA is not subject to background... 

Owing to the toxicity of mercury and its disposal problem, solid electrodes are now very popular. In particular, electrodes made of carbon such as glassy carbon, graphite, carbon paste, and carbon fibers have gained popularity. Mercury, gold, bismuth, and other metals can be deposited as thin metal films on carbon and serves as thin metal film electrodes (TMFE) with excellent analytical advantages in trace metal analysis. The choice of working electrode is determined by the redox... 

If a stationary electrode is used, such as platinum, gold, or glassy carbon, the technique is called voltammetry. One useful voltammetric technique is called stripping voltammetry, in which the product of a reduction is deposited on the surface on purpose and then stripped off by an oxidizing potential— a potential at which the oxidation of the previously deposited material occurs. This technique can also use a mercury electrode, but one that is held stationary. 

Substrates DME = dropping mercury electrode FTO = fluorine-doped tin oxide G = graphite GC = glassy carbon GrC = graphic carbon ITO = indium tin oxide-coated glass SC = single crystals SS = stainless steel TCO = transparent conducting oxide VC = vitrious carbon. Miscellaneous ECALE = electrochemical atomic layer epitaxy ED = electrodeposition ML = monolayer RT = room temperature SMD = sequential monolayer deposition V = vacuum. 

Bismuth-film electrodes (BiFEs), consisting of a thin bismuth-film deposited on a suitable substrate, have been shown to offer comparable performance to MFEs in ASY heavy metals determination [17]. The remarkable stripping performance of BiFE can be due to the binary and multi-component fusing alloys formation of bismuth with metals like lead and cadmium [18]. Besides the attractive characteristics of BiFE, the low toxicity of bismuth makes it an alternative material to mercury in terms of trace-metal determination. Various substrates for bismuth-film formation are reported. Bismuth film was prepared by electrodeposition onto the micro disc by applying an in situ electroplating procedure [19]. Bismuth deposition onto gold [20], carbon paste [21], or glassy carbon [22-24] electrodes have been reported to display an... 

Instead of drops, liquid mercury can be used as thin films electrodepo-sited on some solid electrodes in order to increase the useable negative potential range of these electrodes. For example, on glassy carbon substrates the deposition can be done from a solution of 10-5 m Hg(II) in 0.1mHNO3, applying a convenient potential (—1.0 V vs. SCE) during a few minutes, and stirring the solution. Alternatively a mercury film can be formed on other substrates, such as copper, by simple immersion in liquid mercury. The special properties of these thin film electrodes are described in Section 9.10. 

Another electrode that is widely used is a mercury film electrode, where the film is deposited onto a glassy carbon or wax-impregnated disk. Such films are often less than 10 nm in thickness. Mercury film electrodes have smaller volumes than a conventional HMDE and thus permit higher sensitivity determinations. Solid electrodes have been used, but much less frequently than mercury electrodes. 

The mercury electrode used in stripping analysis is either a conventional HMDE or a mercury film electrode (MFE). In current practice, the MFE is typically deposited onto a rotating glassy carbon or wax-impregnated graphite disk. One usually adds mercuric ion... 

The same solution gives a peak current of 25 jjiA at a 100-A thick mercury film electrode on glassy carbon when the deposition time is 1 min, the electrode rotation rate is 2000 rpm, and the sweep rate is 50 mV/s. What currents would be observed for sweep rates of 25 and 100 mV/s under otherwise unchanged conditions Compare this situation to the one observed for a deposition time of 1 min, a sweep rate of 50 mV/s, and a rotation rate of 4000 rpm Suppose the film thickness were varied by the use of different concentrations of mercuric ion in the analyte. What effect would one see on the peak current under otherwise constant conditions ... 

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