Mercury oxide electrode

Fig. 23. Current-potential (MOE = mercury oxide electrode) curves for hydrogen evolution in 1 M NaOH on an electrolytically prepared NiSx electrode. (1) Initial behaviour (2) After stabilization. After ref. 442, by permission of Pergamon Press.

Fig. 29. Synergetic effects with intermetallic compounds. Hydrogen evolution in 1 M KOH at 30 °C on (1) Ni, (2) La, and (3) LaNis. (MOE = mercury oxide electrode). Adapted from ref. 226, by permission of Elsevier Sequoia.

A similar pH dependence is operative when the metal is covered by a hydroxide layer, or an oxide hydrate layer, provided that the appropriate equilibrium constants are considered. Usually, these electrodes need calibration (the mercury-mercury oxide electrode is an exception and it is a common reference electrode in strongly alkaline systems). Metal-metal oxide electrodes are applicable in a pH range that is limited to low pH values due to the oxide (or hydroxide) dissolution. 

Fig. 9-2 Cyclic voltammetry of two electrocatalysts a) Pt in acidic electrolyte solution, b) Au in alkaline electrolyte solution. The potential scan rate 1 = 50 mV/s two different reference electrodes were used, RHE reversible hydrogen electrode, Hg/HgO mercury/mercury oxide electrode. (Courtesy V. M. Schmidt, Mannheim, Germany)...

Under experimental conditions the SHE is rarely used. Reference electrodes of a second kind are used instead, which are simpler to handle and are commercially available. The Ag/AgCl electrode was already mentioned. Other examples are the calomel electrode based on Hg/Hg2Cl2/KCl (for instance, as saturated calomel electrode (SCE)), the mercury sulfate electrode Hg/Hg2S04/H2S04 (0.5 mol 1 ), and the mercury oxide electrode Hg/HgO/ NaOH (Imol 1 ). Potentials of some reference electrodes versus the SHE are shown in Table 3.2. 

The formal potential of this electrode, Ef" (Hg,HgO) is 0.9258 V [2], Because of its solubility properties, the use of the mercury/mercury oxide electrode is confined to strong alkaline solutions. According to Ives and Janz [2], the mercuric oxide is best prepared by gentle ignition of carefully crystallised mercuric nitrate. The construction is similar to the calomel electrode with an alkaline solution [e.g. saturated Ca(OH)2] instead of the potassium chloride as the electrolyte solution. 

In general, metal/metal oxide electrodes are used in systems of high alkalinity. Ideally, these electrodes respond to the pH of the electrolyte solution in a similar way as the hydrogen electrode. They may be regarded as a special sort of electrodes of the second kind, because the oxide ions are in equilibrium with hydroxyl ions of the solvent. In the case of mercury/mercury oxide electrodes we can formulate the following equilibria ... 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

After tq is passed, the second step starts by scanning the potential from Ed to a potential when all the deposited metals are re-oxidized (the reverse of reaction 25). The oxidation current recorded as a function of potential is the anodic stripping voltammogram (ASV). A typical ASY of three metals (Cd, Pb, and Cu) deposited on a mercury film electrode is shown in Fig. 18b.12b. The sensitivity of ASY can be improved by increasing the deposition time and by using the pulse technique to record the oxidation current. ASV in Fig. 18b. 12b was obtained by using the square wave voltammetry. In most cases a simple linear or step ramp is sufficient to measure sub-ppm level of metals in aqueous solution. The peak current of a linear scan ASV performed on a thin mercury film electrode is given by... 

To learn that one of the most important areas of coulometry for the electroanalyst is stripping , in which analyte is allowed to accumulate on the surface of, e.g. a hanging mercury-drop electrode ( pre-concentration ), and then electro-oxidized ( stripped ). 

Few oxidative electrode reactions are possible at liquid mercury surfaces (see below), so only negative potentials can be employed. The earliest polarographers found it convenient to scan toward more negative potentials during a run... 

Self-assembled surface layers of 6-thioguanme (6TG) on mercury electrode have been described and electrochemicaUy characterized by Arias etal. [108, 109]. Several condensed phases of chemically adsorbed 6TG have been described. It has been found that under conditions of complete coverage, the films of chemisorbed molecules significantly inhibit mercury oxide formation at the electrode [108]. 

By media variables we mean the solvent, electrolyte, and electrodes employed in electrochemical generation of excited states. The roles which these play in the emissive process have not been sufficiently investigated. The combination of A vV-dimethylformamide, or acetonitrile, tetra-n-butylammonium perchlorate and platinum have been most commonly reported because they have been found empirically to function well. Despite various inadequacies of these systems, however, relatively little has been done to find and develop improved conditions under which emission could be seen and studied. Electrochemiluminescence emission has also been observed in dimethyl sulfite, propylene carbonate, 1,2-dimethoxyethane, trimethylacetonitrile, and benzonitrile.17 Recently the last of these has proven very useful for stabilizing the rubrene cation radical.65,66 Other electrolytes that have been tried are tetraethylam-monium bromide and perchlorate1 and tetra-n-butylammonium bromide and iodide.5 Emission has also been observed with gold,4 mercury,5 and transparent tin oxide electrodes,9 but few studies have yet been made1 as to the effects of electrode construction and orientation on the emission character. 

Voltammetry is a collection of methods in which the dependence of current on the applied potential of the working electrode is observed. Polarography is voltammetry with a dropping-mercury working electrode. This electrode gives reproducible results because fresh surface is always exposed. Hg is useful for reductions because the high overpotential for H+ reduction on Hg prevents interference by H+ reduction. Oxidations are usually studied with other electrodes because Hg is readily oxidized. For quantitative analysis, the diffusion current is proportional to analyte concentration if there is a sufficient concentration of supporting electrolyte. The half-wave potential is characteristic of a particular analyte in a particular medium. 

In the last two decades, mercury film electrodes (MFEs) have been used frequently in electroanalytical practice. Using such electrodes, metal ions present in the solution in trace amounts may be determined with satisfactory accuracy by their reduction on the surface of MFEs, formation of relatively concentrated amalgam, and anodic oxidation from MFEs of the preconcentrated metal in a final step (see Chap. 24). 

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. 

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