Mercury Sulfate Reference Electrode

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 ... 

The mercury/mercury(I) sulfate electrode, abbreviated as mercury sulfate reference electrode (MSRE), is a second-kind electrode which consists of metallic mercury coated with slurry of mercury(I) sulfate (Hg2S04) [100, 133] (Fig. 5.3.4). The properties of this electrode have been reviewed in [101]. 

Fig. 5.3.4 Schematic view of mercury sulfate reference electrode [99]. Reprinted with permission from Elsevier...

Table 5.3.2 Standard potentials for mercray/ mercury sulfate reference electrode [101, 142]...

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 ... 

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 ... 

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

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]. 

NHE Normal Hydrogen Electrode (a + =1) SHE Standard Hydrogen Electrode (< + =1 ) SCE Saturated Calomel Electrode (saturated KCl) SSCE Saturated Salt Calomel Electrode (saturated NaCl) NCE Normal Calomel Electrode (1 M KCl) MSRE Mercury(l) Sulfate Reference Electrode... 

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). 

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. 

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... 

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

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. 

A mercury-sulfate electrode served as a reference electrode. All electrode potentials are referred to the potential of the reversible hydrogen electrode in the same electrolyte and at the same temperature as the test electrode. Adsorption measurements were performed in the 0.5 M H2SO4 solution prepared using special purity B-5 sulfuric acid and water doubly-distilled. To remove oxygen dissolved in the electrolyte, pure helium or argon was bubbled through acid solution. 

Reagent grade chemicals were utilized. Electrodeposition was carried out in a conventional three-electrode cell under potentiostatic control with a mercury sulfate electrode as reference (in the following, all potentials are quoted versus this reference) and a platinum wire as counter electrode. Solutions were stirred and deoxygenated by bubbling nitrogen. Backside ohmic contacts of Si samples were achieved with an InGa eutectic. The i-U characteristics of the solid state junctions were measured in air. 

With the liquid level above the analyte solution, some contamination i>f the sample is inevitable. In most instances, the amount of contamination is too slight to be of concern. In determining ions such as chloride, potassium, silver, and mercury, however, precaution must often be taken to avoid this source of error. A common w-ay is to interpose a second salt bridge between the analyte and the reference electrode this bridge should contain a noninierfering electrolyte, such as potassium nitrate or sodiujn sulfate. Double-junction electrodes based on this design are offered by several manufacturers. 

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. 

Infrared absorption spectroscopy Isophthalic acid Low energy electron diffraction Lowest imoccupied molecular orbital Mechanically controlled break-junction Mercury-sulfate electrode Potential of zero charge q = 0 Quasireference electrode Real hydrogen electrode Reference electrode Alkanedithiols HS(CH2)nSH Self-assembled monolayer(s)... 

The book edited by Ives and Janz [1 ] and more recently that by Bard, Parsons, and Jordan [2] contain both theoretical and practical aspects related to reference electrodes. Preparation, application and limitations of various types of reference electrodes such as the hydrogen electrode, the calomel and other mercury-mercurous salt electrodes, the silver-silver halide electrodes, and sulfide and sulfate electrodes are covered and general reference to these excellent critical reviews is recommended. 

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. 

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. 

Concentric cylindrical gauze electrodes ensure uniform potential and current distributions. When a reference electrode is employed - historically calomel or mercury/mercury sulfate, more recently (for occupational health reasons) using silver-based couples and occasionally a wire of the metal to be plated - placement of the tip of the salt bridge close to the working electrode minimizes the ohmic potential drop. 

These electrodes have nitrate-sensitive ion-exchange material incorporated into poly(vinyl chloride)-based membrane electrodes. Care is necessary to avoid contamination by the chloride from the saturated calomel reference electrode and a mercury/me-rcurous sulfate electrode is preferable as a reference electrode. Industrial monitors using nitrate ion-selective electrodes are commercially available. 

Most commonly used reference electrodes are based on mercury (calomel electrode) and silver, in equilibrium with the saturated solution of the corresponding chlorides. Their potential will be constant if the chloride ion concentration around the electrode is constant. The mercury-mercury(I) sulfate electrode is used instead of the calomel electrode when the presence of chloride is undesirable. 

Closed electrodes have their own internal electrolyte of a strictly determined composition they usually have a complex structure but ensure a higher potential measurement (control) accuracy. Their indications to a smaller degree depend on the composition and temperature changes of the environment. The following are the most frequently used electrodes of this type calomel, silver chloride, mercury sulfate, and mercury oxide. A list of reference electrodes used in anodic protection with reference to published papers is given in Novak (1987). 

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