Calomel electrode, 6.30

The calomel electrode is very similar in both construction and theory of operation to the silver/silver chloride electrode described below. The metal is mercury, the electrical connection being made by an inert metal wire and the salt is mercurous chloride. The equilibrium electrode potential is a function of the chloride concentration of the electrolyte. When the electrolyte is saturated potassium chloride, it is known as a saturated calomel electrode (SCE) producing an electrode potential of -1-0.224 V vs SHE. Potassium chloride is used because the ionic mobility 

Silver wire coated with siiver chloride 

Electrode type Potassium chloride concentration (mol L ) Potential (V SHE) 

This is another source of variability in the operating potentials reported in the literature and it is not always possible to ascertain the precise nature of the silver/ silver chloride reference electrode being employed. Depletion of potassium chloride will lead to an increase in the equilibrium electrode potential. 

The direct reading type of instrument, although possibly less accurate than potentiometric is also used exclusively in modem soil laboratories. The e.m.f of the glass electrode-calomel electrode cell is applied across a resistance, and the resulting current after amplification is passed through an ammeter causing deflection of the pointer across a scale marked in pH rmits. These instruments are available to operate on mains A.C. current. In most pH meters temperature control knob is provided to adjust at temperature of the test solution. 

4 Electrode Potential Determination Illustration with Calomel Electrode Hydrogen 

If 2n denote the electrode potentials of zinc in a solution of Zn++ ions of activity 

activity of pure zinc metal, U2 = 1, we have 

In order to assign numerical values to the electrode potential it is necessary to choose a standard electrode and assign an arbitrary value to the potential of the same. For this purpose the reference electrode is the normal hydrogen electrode, (Pt) V2 Hg (1 atm) (gas) H+ (a = 1) (electrode process V2 Hg = + e ) in which pure hydrogen gas at unit pressure is kept in 

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Example 13 The following data were recorded for the potential E of an electrode, measured against the saturated calomel electrode, as a function of concentration C (moles liter ). 

Table 8.31 Half-Wave Potentials (vs. Saturated Calomel Electrode) of Organic...

Calomel Electrodes Calomel reference electrodes are based on the redox couple between Hg2Cl2 and Hg (calomel is a common name for Hg2Cl2). 

The potential of a calomel electrode, therefore, is determined by the concentration of Ch. 

The saturated calomel electrode (SCE), which is constructed using an aqueous solution saturated with KCl, has a potential at 25 °C of -hO.2444 V. A typical SCE is shown in Eigure 11.8 and consists of an inner tube, packed with a paste of Hg, HgiCli, and saturated KCl, situated within a second tube filled with a saturated solution of KCl. A small hole connects the two tubes, and an asbestos fiber serves as a salt bridge to the solution in which the SCE is immersed. The stopper in the outer tube may be removed when additional saturated KCl is needed. The shorthand notation for this cell is... 

The following data were collected for the analysis of fluoride in tap water and in toothpaste, (a) For the analysis of tap water, three 25.0-mL samples were each mixed with 25.0 mL of TISAB, and the potential was measured with an F ISE relative to a saturated calomel electrode. Five 1.00-mL additions of a standard solution of 100.0-ppm F were added to each, measuring the potential following each addition. 

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. 

The titanium oxide film consists of mtile or anatase (31) and is typically 250-A thick. It is insoluble, repairable, and nonporous in many chemical media and provides excellent corrosion resistance. The oxide is fully stable in aqueous environments over a range of pH, from highly oxidizing to mildly reducing. However, when this oxide film is broken, the corrosion rate is very rapid. Usually the presence of a small amount of water is sufficient to repair the damaged oxide film. In a seawater solution, this film is maintained in the passive region from ca 0.2 to 10 V versus the saturated calomel electrode (32,33). 

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

On the other hand, CU-CUSO4 should not be used as built-in electrodes for potential test probes (see Section 3.3.3.2) because there is a danger of copper precipitating on the steel electrode. Calomel electrodes with saturated KCl solution are preferred in this case and present no problems. 

The electrolysis is carried out at a reference potential of -2.4 volts vs a standard calomel electrode. An initial current density of 0.0403 amp/cm is obtained which drops to 0.0195 amp/cm at the end of the reduction, which is carried on over a period of 1,682 minutes at 15° to 20°C. The catholyte is filtered, the solid material is washed with water and dried. 430 g of the 2,3-bis-(3-pyridyl)-butane-2,3-diol is recrystallized from water, MP 244° to 245°C. 

Finally, calomel electrodes (and more especially hydrogen electrodes) are not suitable for field measurements because they are not sufficiently robust. The calomel electrodes are however essential for calibrating the field reference electrodes. Saturated KCI calomel electrodes are the most suitable because there is then no doubt about the reference potential of the calibrating electrode. Lack of adequate calibration is a common cause of cathodic protection system mismanagement. 

Fig. 19.7 Reference electrodes and capillaries, (a) Reference electrode, salt bridge and Luggin capillary (A) calomel electrode (c) frontal types of capillaries and positions (r/) rearside capillaries (after von Fraunhofer and Banks )...

Various types of reference electrodes have been considered in Section 20.3, and the potentials of these electrodes and their variation with the activity of the electrolyte are listed in Table 21.7, Chapter 21. It is appropriate, however to point out here that the saturated calomel electrode (S.C.E.), the silver-silver chloride electrode and the copper-copper sulphate electrode are the most widely used in corrosion testing and monitoring. 

The mechanism of the action of metallic copper was investigated by Streicher who determined the potential of a Type 314 stainless steel, the redox potential of the solution (as indicated by a platinised-Pt electrode) and the potential of the copper. The actual measurements were made with a saturated calomel electrode, but the results reported below are with reference to S.H.E. In the absence of copper the corrosion potential of the stainless steel was 0-58 V, whereas the potential of the Pt electrode was... 

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