Mercury-mercurous sulfate

Fig. 5.6 (Left) Comparison of band energy levels for different II-VI compounds. Note the high-energy levels of ZnSe. Representation is made here for electrodes in contact with 1 M HQO4. The reference is a saturated mercury-mercurous sulfate electrode, denoted as esm (0 V/esm = +0.65 V vs. SHE). (Right) Anodic and cathodic decomposition reactions for ZnSe at their respective potentials (fidp, Fdn) and water redox levels in the electrolytic medium of pH 0. (Adapted from [121])...

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. 

Next we discuss four types of reference electrodes hydrogen, calomel, silver-silver chloric, and mercury-mercurous sulfate electrodes. 

Mercury-Mercurous Sulfate Electrode. In this reference electrode the metal is mercury, the sparingly soluble compound is mercurous sulfate (Hg2S04), and the source of S04 anions is sulfuric acid or potassium sulfate. The electrode is made in the same way as a calomel electrode, and it is represented as... 

MetaUInsoluble Salt/Ion Electrodes. Electrode potentials are usually reported relative to normal hydrogen electrode (NHE a(H+) = 1, p(H2) = 1), but they are actually measured with respect to a secondary reference electrode. Frequently used secondary reference electrodes are calomel, silver-silver chloride, and mercury-mercurous sulfate electrodes. These secondary reference electrodes consist of a metal M covered by a layer of its sparingly soluble salt MA immersed in a solution having the same anion Az as the sparingly soluble MA. The generalized reference electrode of this type may be represented as M MA AZ and may be considered to be composed of two interfaces one between the metal electrode M and the metal ions Mz+ in the salt MA... 

The mercury-mercurous sulfate electrode. Several commercial suppliers offer the mercury-mercurous sulfate electrode with a saturated potassium sulfate electrolyte. The potential (E° + E ) of this electrode system is 0.658 V on the hydrogen scale at 22°C.34 The electrode constitutes one-half of the Weston standard cell,35 an international secondary voltage standard, and is outstanding in reproducibility,36 in spite of the slight tendency of mercurous sulfate to hydrolyze and its rather high solubility. 

A mercury/mercurous sulfate (Hg/Hg2S04) reference electrode A voltmeter... 

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 peak around —0.38 V versus mercury/mercurous sulfate reference electrode (MSE), during the negative sweep, is associated to gold species whereas the one at —0.8 V versus MSE corresponds to platinum species. For pure catalysts, with an upper potential limit of +0.25 V versus MSE, the charge values of 493 and 543 pC cm-2 were obtained for platinum and gold, respectively (Figure 21.7).43,44 The atomic content of the Pt-Au nanoparticles can be deduced as follows ... 

The Mercury-Mercurous Sulfate Electrode. A pool of mercury covered with a paste of mercurous sulfate and a solution containing sulfate. 

When leakage of chloride ion through the reference electrode into the test solution is not permissible (as in titrations involving Ag+), a mercury/mercurous-sulfate reference electrode may be used instead of calomel or silver-chloride electrodes. This consists of a mercury electrode in contact with a sulfate electrolyte saturated with excess mercurous sulfate ... 

The potentials of the two electrodes of the lead—acid cell are measured vs. a reference electrode. Thus, the lead—acid cell turns into a three-electrode cell. During measuring the potential of the two electrodes of the LA cell, the reference electrode should not be polarized, i.e. its potential should remain constant. The most common reference electrodes are hydrogen, cadmium, mercury-mercurous sulfate and silver-silver sulfate electrodes. Cadmium sticks are widely used in industrial quality control laboratories to measure the electrode potentials of the manufactured batteries. Cadmium does not form poorly soluble cadmium sulfate, which is the reason why during the measurement the electrolyte in the cell absorbs a few Cd ion impurities that do not affect the performance of the battery, however. 

Table 15.5 Potentials of the mercury-mercurous sulfate reference electrode vs. H2 H electrode at different sulfuric acid concentrations and temperatures.

Mercury/mercurous sulfate electrode 0.6151(5) Ag/Ag2S04, Pb/Pb2S04 Aqueous, mixed... 

Another commonly used electrode is the mercury-mercurous sulfate electrode, represented by Eq. (10) ... 

In this paper Clark described the battery composed of mercury-mercurous sulfate, zinc and saturated zinc sulfate (known as the Clark cell). 

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]. 

SSE mercury/mercurous sulfate/ saturated potassium sulfate reference electrode (Esse = +0.65 V/NHE). 

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). 

Potentiodynamic polarization curves of ED Ni-SiC nanostructured composite coating in 0.5 M KjSO recorded by direct potential scan at 0.1 V/min under the same test conditions as Fig. 8.22 Curve (1), no friction applied Curve (2), continuous friction at 10 N 120 rpm Curve (3), continuous friction at 15 N, 120 rpm. (Potentials are in volts vs. mercury-mercuric sulfate electrode (SSE)) (Benea et at., 2009). 

---