Mercuric electrode

We will discuss four types of reference electrodes hydrogen, calomel, silver-silver chloride, and mercury-mercurous electrodes. 

The Standard Potential of the Mercurous-Mercuric Electrode. A method for obtaining standard potentials of oxidation-reduction electrodes which utilizes the best procedure so far developed in this field is the one that was used by Fopoff and associates. The method may be illustrated by the determination of the standard potential of the mercurous-mercuric electrode. The type of cell used by Popoff, Riddick, Worth and Ough1 was... 

Table II. Variation op Eq" for the Mercurous-Mercuric Electrode with the Molality, hi, of Perchloric Acid...

The electrode potential of this type of electrode is defined by the concentration (activity) of the anions. They are used in corrosion engineering as standard half-cells or reference electrodes. In corrosion engineering practice they are called secondary reference electrodes to differentiate them from the hydrogen electrode, which is a primary reference electrode. The following electrodes of the second kind are of interest in electrochemical and corrosion studies calomel electrode, silver-silver chloride electrode, and mercury-mercurous electrode. 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

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. 

Hydrogen SulBde. Sulfide ion from 10 to 1 Af can be measured potentiometricaHy with an ion-selective electrode. Mercuric ion interferes at concentrations >10 M. The concentration of hydrogen sulfide can be calculated knowing the sample pH and the piC for H2S. 

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. 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

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. 

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. 

It is also important to understand how the potential gradient between an electrode and the bulk solution is established and controlled. Because the potential difference between the electrode and the bulk solution is not measurable, a second electrode must be employed. Although in general the potential difference between an electrode and solution cannot be determined, the potential difference between two electrodes in that solution can be determined. If the solution electrode potential difference of one of the electrodes is held constant by maintaining a rapid redox couple such as silver-silver chloride or mercury-mercurous chloride (calomel), then the potential... 

The described electrodes, and especially the silver chloride, calomel and mercurous sulphate electrodes are used as reference electrodes combined with a suitable indicator electrode. The calomel electrode is used most frequently, as it has a constant, well-reproducible potential. It is employed in variously shaped vessels and with various KC1 concentrations. Mostly a concentration of KC1 of 0.1 mol dm-3, 1 mol dm-3 or a saturated solution is used (in the latter case, a salt bridge need not be employed) sometimes 3.5 mol dm-3 KC1 is also employed. The potentials of these calomel electrodes at 25°C are as follows (according to B. E. Conway) ... 

The mostly used Weston cell consists of mercurous sulphate and cadmium amalgam electrodes ... 

In contrast to the equilibrium electrode potential, the mixed potential is given by a non-equilibrium state of two different electrode processes and is accompanied by a spontaneous change in the system. Besides an electrode reaction, the rate-controlling step of one of these processes can be a transport process. For example, in the dissolution of mercury in nitric acid, the cathodic process is the reduction of nitric acid to nitrous acid and the anodic process is the ionization of mercury. The anodic process is controlled by the transport of mercuric ions from the electrode this process is accelerated, for example, by stirring (see Fig. 5.54B), resulting in a shift of the mixed potential to a more negative value, E mix. 

The following are important legal terminologies for this section. The term mercuric-oxide battery means a battery that uses a mercuric-oxide electrode. 

The methods most commonly used to detect hydrogen sulfide in environmental samples include GC/FPD, gas chromatography with electrochemical detection (GC/ECD), iodometric methods, the methylene blue colorimetric or spectrophotometric method, the spot method using paper or tiles impregnated with lead acetate or mercuric chloride, ion chromatography with conductivity, and potentiometric titration with a sulfide ion-selective electrode. Details of commonly used analytical methods for several types of environmental samples are presented in Table 6-2. 

An alternative method was described by Pinzauti et al (40) for determination of several antithyroid drugs potentimetrically with 0.01 M mercuric acetate with use of a mercuric sulfate reference electrode and an amalgamated gold or a silver indicator-electrode. The method is rapid and the results are reproducible the errors are all within + 0.38%. 

Figure 4.4 The saturated calomel electrode. A platinum wire makes electrical contact with an electrode which is composed of a paste of metallic mercury, mercuric chloride (calomel) and potassium chloride. A saturated solution of potassium chloride completes the half-cell and provides electrical contact through a porous plug.

The following data refer to the reduction of a mercurous complex at a DME. Draw a Heyrovsky-llkovic plot to determine both E1/2 and the number of electrons transferred in the electrode reaction. 

Basically, the calomel electrode consists of mercury, mercurous chloride (calomel), and chloride ion. The concentration of potassium chloride is 0.1 M in an aqueous-organic solvent (50 50) of the same nature as that contained in the solution to be investigated. The junction with the test solution is realized either with a capillary or a porous stone. When the capillary is used, a small hydrostatic pressure is maintained inside it in order to avoid any electrode contamination by the test solution. In the main part of our investigation, the porous stone junction was used. Moreover, the calomel electrode is thermostatted at 20°C, and temperature variations of this electrode giving appreciable emf variations involve uncertainty on the pon determination on the order of 0.2-0.3 poH unit/ 10°C. 

L. Bailey, J. Wilson, S. Kaipel and M. Riley, Application of chloride electrodes based on mercurous chloride/mercuric sulphide, ia Ion-Selective Electrodes (ed. E. Pungor and L Buz5s), Conference 1977, Akad6miai Kiad6, Budapest (1978), p. 201. 

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