Mercury/mercuric-oxide, potentials

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... 

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). 

It has been repeatedly observed that dilute solutions of mercury are not stable unless the oxidation potential and the acidity of the solution is raised 6, 10, 43). For example, equilibration of about 10 grams of A1 and Fe hydroxides with a 0.1% solution of mercuric acetate, then rinsing with 10% acetic acid until mercury is absent from the washings, and drying at 50°C for 4 hours, yields A1 and Fe hydroxides retaining about 25 /xg/gram mercury (24). 

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. 

The mercury(II) oxide (mercuric oxide) RE is limited to use in highly alkaline solutions due to the stability of the oxide. While mercury(II) oxide is the only stable oxide, it can exist as either red or yellow powders that differ in solubility (37). The potential of the mer-cury(II) oxide RE is independent of the electrolyte, but depends on the activities of both OH and H2O (36). The potential can be calculated as follows (36) ... 

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