Mercury amalgamated gold electrodes

The active part of the working electrode normally consists of a small disk or sphere of platinum (depending on whether one wishes to work under conditions of planar or radial diffusion). Alternatively, one can use gold, mercury (as a gold-mercury amalgam), or carbon. 

Mercury is the electrode material of choice for many electrochemical reductions and some unique oxidations (see Chap. 14). We have explored the use of both small mercury pools and amalgamated gold disks in thin-layer amperometry. Other workers have used pools in a capillary tube [7] and amalgamated platinum wire [8]. In 1979, Princeton Applied Research introduced a unique approach based on their model 303 static mercury drop electrode (see Sec. II.F). Our laboratories and MacCrehan et al. [9] have focused on the use of amalgamated gold disks. This approach results in an inexpensive, easily prepared, and mechanically rigid electrode that can be used in conventional thin-layer cells (Sec. II.C) of the type manufactured by Bioanalytical Systems. 

The amalgamated gold electrode should be used for calibration of the mercury sensors within 10 min or stored at —20°C up to 4 days. The longer storage time may be possible but was not tested. 

Figure 55. Cyclic voltammogram of COT in HMPA on a hanging mercury drop electrode (uppermost curve). Cyclic voltammogram of COT on a 15-cm amalgamated gold electrode in an ESR cell (central curve). ESR-potential curve recorded simultaneously with the central curve (lowermost curve). (From Ref. 378.)...

The absence of a chromophore for sensitive detection is a problem which is met with HPLC determination of these thiol analytes. However, two approaches are possible to resolve this point 1) direct detection using electrochemistry, either amperometry on gold/mercury amalgamated electrodes or coulometry on porous graphite electrodes ... 

There have been few direct electrochemical studies of peroxidase and catalase due to the highly irreversible nature of these electrode reactions. Horseradish peroxidase was found to be electroinactive at the dropping mercury electrode. Tarasevich and co-workers observed a cyclic voltam-metric response for the electron transfer of horseradish peroxidase at an amalgamated gold electrode. However, this response was ascribed to the disulfide bonds of the protein at neutral pH and not to the heme group. No response was detected at pyrolytic graphite electrodes. ... 

This illustrates that the deposited metal dissolves in the mercury electrode. Other elements, such as selenium, arsenic, mercury, silver, gold, or tellurium, which either do not form amalgam or oxidize at potentials anodic to that of mercury can be measured by ASV using a variety of solid electrodes. The deposition step is usually carried out under conditions of forced convection to facilitate the transport of the metal ions to the electrode surface. The convective transport is achieved mainly by electrode rotation, solution stirring, and flow. The deposition potential (Ed in Figure 1) should be 0.3-0.4V more negative than the reduction potential of the metal ion to ensure an efficient deposition process. The duration of the preconcentration step is selected according to the... 

In the second and more common approach, cysteine is detected at a mercury or amalgamated gold electrode. The reaction has been studied frequently and different reactions have been suggested. These are summarized in the literature [50]. Today cysteine is considered to undergo the following reaction at a mercury electrode [50,66-68] ... 

Gold/mercury amalgam electrodes have been successfully employed for the detection of thiols by CEEC. In this case, it is the catalytic oxidation of Hg in the presence of thiols that provides the response. 

The electrochemical characteristics of a solid electrode can be chemically, electrochem-ically, as well as physically modified in order to increase its stability, sensitivity, or most commonly selectivity. An important example of selectivity modification is the formation of a mercury amalgam on gold electrodes to specifically detect thiols. Other examples are the adsorption of metals, polymers, self-assembled monolayers (SAMs), or enzymes but their applications to HPLC are yet scarce. 

Similarly, there are no reliable data on the standard reduction potential for the gold sulfite complex, for which values in the range 0-0.4 were reported [25], However, assuming for the stability constant the previously quoted value of 26.8 [23], the standard reduction potential of the sulfite complex can be calculated as 0.11 V, which is in excellent agreement with the formal potential of the sulfite complex at a mercury amalgam electrode, namely, 0.116 V, as measured by Baltrunas et al. [26]. 

About 20 amalgam-forming metals, including Pb, Sn, Cu, Zn, Cd, Bi, Sb, Tl, Ga, In and Mn, are easily measurable by stripping strategies (ASV and PSA) based on cathodic deposition onto mercury electrodes. Additional metals, such as Se, Hg, Ag, Te and As are measurable at bare solid electrodes such as carbon or gold. 

Potentiometric EDTA titrations are best carried out with a mercury pool electrode (Figure 5.6) or a gold amalgam electrode. When this electrode dips into a solution containing the analyte together with a small amount of added Hg-EDTA complex, three interdependent reactions occur. For example, at pH = 8 the half cell reaction (a) which determines the electrode potential is related to the solution equilibrium by (b) and (c). 

One can see immediately that the easy oxidation of mercury renders it of little use for anodic scans. Note that to construct a solid mercury electrode one can simply immerse a gold electrode in mercury for a few seconds. The amalgam that forms produces a mercury electrode much more manageable than the dropping electrode used in polarography. 

Electrodes of this sort have many different chemical properties from pure mercury electrodes, because of the formation of a gold or platinum amalgam [18]. Normally, a drop is suspended just prior to an experiment, so this problem will be of no serious consequence. Nevertheless, since the solubility of these noble metals in mercury is about 0.05 M at room temperature [19], the concentration of gold or platinum in mercury may be quite significant on a longer time scale. In such cases, gold or platinum may form intermetallic compounds with several metals that are electrodeposited into the mercury [18]. 

J. Tafel found that while nitric acid is reduced only to hydroxylamine q.v.) by mercury or well-amalgamated electrodes, a copper cathode reduces it to ammonia and at the same time has no action on hydroxylamine. A. Brochet and J. Petit studied the electro-reduction of nitric acid by an alternating current. T. H. Jeffery described the electrolysis of nitric acid with a gold anode, and obtained from the anode liquor crystals of aurinitric acid, HAu(N03)4.3H20. R. Ihle s observations on the oxidation-potential of nitric acid have been discussed in connection with nitrous acid (q.v.). He found that if the cone, of the nitric acid be expressed by... 

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