Mercury/mercurous-sulfate, potentials

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. 

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. 

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 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.

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). 

Ideally a standard cell is constmcted simply and is characterized by a high constancy of emf, a low temperature coefficient of emf, and an emf close to one volt. The Weston cell, which uses a standard cadmium sulfate electrolyte and electrodes of cadmium amalgam and a paste of mercury and mercurous sulfate, essentially meets these conditions. The voltage of the cell is 1.0183 V at 20°C. The a-c Josephson effect, which relates the frequency of a superconducting oscillator to the potential difference between two superconducting components, is used by NIST to maintain the unit of emf. The definition of the volt, however, remains as the Q/A derivation described. 

The mercurous sulfate [7783-36-OJ, Hg2S04, mercury reference electrode, (Pt)H2 H2S04(y ) Hg2S04(Hg), is used to accurately measure the half-ceU potentials of the lead—acid battery. The standard potential of the mercury reference electrode is 0.6125 V (14). The potentials of the lead dioxide, lead sulfate, and mercurous sulfate, mercury electrodes versus a hydrogen electrode have been measured (24,25). These data may be used to calculate accurate half-ceU potentials for the lead dioxide, lead sulfate positive electrode from temperatures of 0 to 55°C and acid concentrations of from 0.1 to Sm. 

Hg UPD on Au(lll) electrodes in the presence of bisulfate anions has been studied by Abruna et al. [27] in order to illustrate the effects of the partial charge, retained by the metal, on the interactions between the adsorbed metal and the anion. In order to obtain structural information on the adsorbed species, the authors have carried out grazing incident X-ray diffraction measurements at several potentials. Three ordered structures were observed depending on the applied potentials, which were adjusted from cyclic voltammograms. At the early stages of Hg UPD, when mercury was still partially charged, an ordered mercurous-sulfate bilayer structure was formed at the electrode... 

The reference electrode most commonly used is a saturated calomel, a mercurous sulfate, or silver/silver-chloride electrode. When accurate measurement of the potential is not required, a mercury pool or a platinum wire, foil, or gauze can be used. In nonaqueous solutions, various other reference electrodes may be more suitable. 

Mercurylmercury(I) sulfate also mercurous sulfate), abbreviated as Mercury Sulfate Reference Electrode (MSRE), is the second (26) most used mercury RE after SCE. The construction of the MSRE is similar to that of the SCE, although it is less sensitive to the purity of the mercury. The reproducibility (31) of the MSRE is second only to the NHE, and given the absence of chloride ion in the construction of the MSRE, it has found use in systems with sensitivity to chloride content. The reference potential of the MSRE is calculated as follows ... 

Mercuric and mercurous salts — Salts of Hg(II) and Hg(I), respectively. Soluble mercuric and mercurous salts such as acetates and nitrates are used for the deposition of mercury films on conducting substrates (see -> anodic stripping voltammetry). Insoluble salts, e.g., chloride and sulfate of Hg(I) in chloride and sulfate medium, respectively, can be used to prepare reference electrodes (see -> calomel electrode). The formation of insoluble salts of mercury on - mercury electrodes determines, among others, the positive limit of their voltam-metric potential window. 

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