Electrode mercury/mercuric oxide

Similar designs are used for other REs on the basis of poorly soluble mercury compounds (1) the mercury-mercurous sulfate RE with H2SO4 or K2SO4 solutions saturated with Hg2S04, for which = 0.6151V and (2) the mercury-mercuric oxide RE, for measuring electrode potentials in alkaline solutions, with KOH solution saturated with HgO, for which = 0.098 V and E = 0.920 V. 

In selecting reference electrodes for practical use, one should apply two criteria that of reducing the diffusion potentials and that of a lack of interference of RE components with the system being studied. Thus, mercury-containing REs (calomel or mercury-mercuric oxide) are inappropriate for measurements in conjunction with platinum electrodes, since the mercury ions readily poison platinum surfaces. Calomel REs are also inappropriate for systems sensitive to chloride ions. 

The mercury-mercuric oxide electrode. The mercury-mercuric oxide electrode is uniquely well behaved among metal-metal oxide electrodes.32 The potential of the cell... 

If chloride ions must be avoided, a mercury mercurous sulfate electrode [Hg/ Hg2S04(s), K2S04(s) E = 0.621 V versus NHE] may be employed. In alkaline solution a mercury mercuric oxide electrode (E = 0.098 versus NHE) may be useful. 

The Mercury-Mercuric Oxide Electrode. A pool of mercury covered with a paste of mercuric oxide and a solution of a base. 

Mercury/mercuric oxide electrode 0.098 Aqueous, mixed... 

Mercury-mercuric oxide. This electrode is easy to prepare and is recommended for use in alkaline solutions. 

The most efficient dry cell is the mercury cell, which has an excellent stabilized voltage. Developed in 1942 by Ruben, it consists of zinc, which dissolves and becomes the negative electrode and mercuric oxide, which is reduced to mercury at the positive electrode. The overall cell, which has no salt bridge, can be represented as follows ... 

In battery practice, hydrogen reference electrodes are not used. They are not only difficult to handle, but include in addition the risk of contamination of the battery s electrodes by noble metals like platinum or palladium (4). Instead, a number of reference electrodes are used, e.g. the mercury/mercurous sulfate reference electrode (Hg/Hg2S04) in lead-acid batteries, and the mercury/mercuric oxide reference electrode (Hg/HgO) in alkaline solutions (e.g. Ref. 5). In lithium ion batteries with organic electrolyte the electrode potential is mostly referred to that of the lithium electrode (cf. Chapter 18). 

Coulometric Titrations. An aqueous iodide sample may be titrated with mercurous ion by anodizing a mercury pool electrode. When metallic mercury is oxidized to mercurous ion by a current passing through the system, the mercurous ion reacts directly with the iodide ion to precipitate yellow Hg2l2 ... 

On the positive electrode, discharge results in direct reduction of mercuric oxide to metallic mercury (without formation of intermediate products or... 

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. 

To simulate corrosion in lead-acid battery environments, Dacres et al. [118] and others [119,120] anoically polarized test materials at 1.226 V (versus mercury/mercurous sulfate reference electrode) in sulfuric acid solutions (of 1.285 specific gravity) at 50, 60, and/or 70°C. At 1.226 V, lead and water are oxidized to lead dioxide (Pb02) and molecular oxygen (O2), respectively [122,123]. About one third of the total anodic current is consumed in the oxidation of lead under these conditions [120]. 

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