Amalgamated gold electrode

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. 

As a first example, curves in Fig. 6-5 reproduce RDE and HMRDE experiments for nitrobenzene on an amalgamated gold electrode at the same nitrobenzene concentrations. 

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.)...

Figure 5. Potentiodynamic curves (2) on the amalgamated gold electrode in hemin solution (10 M, pH = 7.2). (1) Background curve.

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. ... 

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] ... 

An alternative method was described by Pinzauti et al (40) for determination of several antithyroid drugs potentimetrically with 0.01 M mercuric acetate with use of a mercuric sulfate reference electrode and an amalgamated gold or a silver indicator-electrode. The method is rapid and the results are reproducible the errors are all within + 0.38%. 

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. 

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. 

Fig. 10.9. Controlled potential cathodic scans of Tl+ at an amalgamated gold disk electrode. All A scans are RDE curves, all B scans are in-phase HMRDE curves. (A, B) 0.01 mol dm" HC104 (A2, B2) 2.0 x 10-7 mol dm- TT in 0.01 mol dm- HCI04. The current sensitivities are indicated by the markers, zero current in all cases is the dashed line. For all curves ft = 3600 rpm, for HMRDE (B) Aft = 6 rpm, frequency = 3 Hz, averaging time constant is 3 s and the scan rate is 2 mV s"1 (after Reference [27]).

FIG. 22 Ejection of cadmium ions during oxidation of a polypyrrole film at +0.5 V vs. SCE. Monitored by an amalgamated 50 gm gold electrode biased at —0.8 V. Electrolyte 100 mM KC1. (From Ref. 59. Copyright 1999 American Chemical Society.)... 

The electrode elements of an electrochemical sensor are often metallic, semiconductive, or inert. For metallic electrode elements, noble metals such as gold, platinum, and silver are often used in conventional electrochemical sensors. Mercury or amalgam as electrode materials for microfabricated electrochemical sensors are seldom used due to the difficulty involved. Because mercury has a relatively high vapor pressure, it does not lend itself well in any fabrication process using a vacuum or low-pressure environment. The formation of amalgam requires the use of mercuric ion containing reagent. This step can be elaborate and complicated. 

Similar data were obtained on the amalgamated gold and pyrographite electrodes. The half-wave potentials are equal to E /2 = -0.03 V and E /2 = -0.37 V and are practically independent of the nature of the electrode. The anode-cathode polarization curves obtained in the presence of a mixture of the oxidized and reduced forms of MV are given in Figure 14. An analysis of the kinetics of mediator oxidation and reduction at the electrode reveals that the process proceeds on the carbon electrode under close to reversible conditions and is controlled by concentration polarization. Thus, MV fully satisfies the above-formulated requirements of mediators for electron transport in electrochemical systems with the participation of enzymes. 

Fadrna etal. [70] has proved that a polished silver solid amalgam electrode, free from liquid mercury, is a suitable substitute for the HMDE in CSV analyses of purine bases and of acid-treated ODNs. The analysis was done at nanomol level in alkaline medium in the presence of Cu(ll). Similarly, the application of gold amalgam-alloy electrode for a sensitive voltammetric detection of ODNs containing the purine units within the ODN chains in the presence of Cu(ll)... 

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