Use of mercury electrodes

Despite the environmental issue concerning the use of mercury, small amount of mercury at the room temperature is actually innocuous. Only when mercury is heated above 100 °C to produce vapor or organomercury compounds, the human health will be affected. Mercury electrodes can be safe and promising tools for electroanalysis when operated with care. Different electrochemical techniques associated with the use of mercury electrodes for the improvement of measurement sensitivity are introduced in the following section. 

Rising concerns over the toxicity of mercury have made the use of mercury electrodes unfavorable, and hence, the application of metal ion-based ECIA less desirable. Yet this can change in the future as advances in both the design of mercury electrodes and the automation and miniaturization of systems using mercury electrodes are expected to make the use of mercury electrodes safe [14,15]. However, as discussed next, enzyme labels that followed metal labels in ECIA are often superior to metal labels. The niche for metal labels probably lies in immunoassays where using enzymes is difficult because of unfavorable sample conditions such as extreme temperatures to which metals are invulnerable. 

Due to environmental and toxicological issues, great effort has been made to decrease the use of mercury electrodes, and, since its first publication [41], the bismuth film electrode has appeared to be the best successor for such purpose. 

The majority of work on the electrified interface has been carried out using a mercury electrode, which has the advantage that it has a well-defined and reproducible surface and a highly polarisable interface when immersed in a solution. In the case of solid metals the concepts outlined are equally applicable, but modifications are necessary to allow for the following ... 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

For complexation titrations involving the use of EDTA, an indicator electrode can be set up by using a mercury electrode in the presence of mercury (II) EDT A complex (see Section 15.24). 

Of recent years the use of mercury film electrodes based on substrates other than platinum has been explored, and increased sensitivity is claimed for electrodes based on wax-impregnated graphite, on carbon paste and on vitreous carbon a technique of simultaneous deposition of mercury and of the metals to be determined has also been developed. 

There are several types of mercury electrodes. Of these, the dropping mercury electrode (DME), the hanging mercury drop electrode (HMDE), and mercury film electrode (MFE) are the most frequently used. 

The limited anodic potential range of mercury electrodes has precluded their utility for monitoring oxidizable compounds. Accordingly, solid electrodes with extended anodic potential windows have attracted considerable analytical interest. Of the many different solid materials that can be used as working electrodes, the most often used are carbon, platinum, and gold. Silver, nickel, and copper can also be used for specific applications. A monograph by Adams (17) is highly recommended for a detailed description of solid-electrode electrochemistry. 

The properties of anodic layers of HgS formed on mercury in sulfide solutions have been investigated in comparison with anodic sulfide layers of cadmium and bismuth. Also, the electrochemistry of mercury electrodes in aqueous selenite solutions has been studied (see Sect. 3.2.1). The problem with the presence of several cathodic stripping peaks for HgSe in acidic Se(IV) solutions has been addressed using various voltammetric techniques at a hanging-mercury-drop electrode [119]. 

Kunai and Ishikawa et al. have reported that electrolysis of monochloro-silanes in 1,2-dimethoxyethane using a platinum cathode and a mercury anode gives disilanes in high yield (Scheme 40) [84]. Silver can also be used as an excellent anode material in place of mercury. The electrolysis of a mixture of two different monochlorosilanes produces unsymmetrical disilanes. Trisilanes can also be synthesized by the electrolysis of a mixture of monochlorosilanes and dichlorosilanes. They also reported that the use of copper electrodes is effective for the synthesis of disilanes, trisilanes, tetrasilanes, and pentasilanes [85]. 

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. 

Indirect electroreduction of allyl alcohols leading to the corresponding unsaturated hydrocarbons is attained using a mercury electrode in a strongly acidic medium containing an iodide salt [554]. The reaction involves transformation of the alcohol into the iodide, the reaction of the iodide with mercury, the protonation of the... 

Ion-exchange reactions were used for the accumulation of europium(III) [158] and iron(III) [159] ions on the surface of GCE coated with Nafion , and chromium(VI) ions on the surface of GCE covered by a pyridine-functionalized sol-gel film [160], which were combined with the stripping SWV Furthermore, a cathodic stripping SWV was used for the determination of sulfide [161,162], thiols [163-166], selenium(lV) [167-170], halides [171-173] and arsenic [174] accumulated on the snrface of mercury electrode. 

Concerning more general application of mercury electrode in the studies on com-plexation equilibria, one should mention the paper by Jaworski et al. [59], who have investigated oxidation of mercury microelectrode in solutions with thiocyanates without any background electrolyte added. In the experiments, normal pulse voltammetry and staircase voltammetry were used. The authors have developed a general procedure for the determination of the stability constants, based on the data taken from the voltammograms. They have applied it to the analysis of Hg(II)-SCN complexes. 

Mercury is quickly limited at positive potentials (+0.25 V with respect to SCE). Beyond this potential, anodic dissolution of mercury occurs. However, mercury can be used at up to —1.8 or —2.3 V depending on whether the supporting electrolyte is acidic or alkaline. This range offers several possibilities, especially for the determination of heavy metals. The mercury used must be extremely pure (six-time distilled, under nitrogen). Unfortunately, the use of mercury as an electrode is a disadvantage because of its toxicity the mercury must be recycled after each use. 

The use of mercury is nearly ideal for working electrode construction for several reasons. Mercury has a large liquid range (-38.9 to 356.9°C at normal pressure), and therefore electrodes of various shapes may be easily prepared either in pure form or by deposition of mercury on the conducting support. The surface of such electrodes is highly uniform and reproducible if the mercury is clean. 

The following types of mercury electrodes have been widely used for voltammetry dropping mercury electrode (DME), hanging mercury drop electrode (HMDE), static mercury drop electrode (SMDE), streaming mercury electrode (SME), and mercury film electrode (MFE). We begin our discussion with a description of the construction and properties of the DME because this electrode has a long history and continues to be used for both analytical and fundamental studies. 

The rapid flow of mercury from a capillary will assume the potential of zero charge, which is then simply measured. This method was proposed at the end of the 19th century by Helmholtz, and the first experiments were carried out by Ostwald [34]. Later, a number of investigators adapted this approach [35-39]. This method is in wide use today in the determination of the zero charge potentials of mercury electrodes in different solutions (see for instance [40]). 

The potential danger associated with mercury has led to the development of other strategies that avoid the use of a mercury solution. These strategies use glassy carbon electrodes (GCE) coated with a mercury film modified with Nafion [3,4], cellulose acetate [5], naphthol derivative [6], etc., where mercury is generated in situ and this way avoiding the manipulation of mercury solutions as done previously. Composite electrode containing HgO as a built-in mercury precursor, which supply mercury-film formation, has even been reported to avoid the use of mercury solution [7]. 

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