Potentials of calomel electrodes

According to Nemst equation the potential of calomel electrode is dependent on chloride ions activity (concentration). In practice, three cases are encountered normal calomel electrode (with KCl solution of concentration 1 mol dm ), 0.1 normal calomel electrode (with KCl solution concentration 0.1 mol dm ), and most often used saturated calomel electrode (with saturated KCl solution and the presence of KCl crystals), abbreviated as the saturated calomel electrode , SCE. A version with saturated NaCl solution ( sodium chloride saturated calomel electrode , SSCE) is also widely used in solution, where risk of a potassium salt precipitation occurs, e.g., in perchlorate ions containing solutions. 

The potential of calomel electrode is very reproducible if the electrode is produced and operated carefully. 

Covington et al. [127], taking into account the disproportionation reaction resulting in formation of mercuric-chloride complexes, reevaluated the standard potential of calomel electrode to = 0.261%N. 

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

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. 

Potential control with zinc reference electrodes presented a problem because deposits of corrosion products are formed on zinc in hot water. This caused changes in the potential of the electrode which could not be tolerated. Other reference electrodes (e.g., calomel and Ag-AgCl reference electrodes) were not yet available for this application. Since then, Ag-AgCl electrodes have been developed which successfully operate at temperatures up to 100°C. The solution in the previous case was the imposition of a fixed current level after reaching stationary operating conditions [27]. 

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. 

This electrode is perhaps next in importance to the calomel electrode as a reference electrode. It consists of a silver wire or a silver-plated platinum wire, coated electrolytically with a thin layer of silver chloride, dipping into a potassium chloride solution of known concentration which is saturated with silver chloride this is achieved by the addition of two or three drops of 0.1M silver nitrate solution. Saturated potassium chloride solution is most commonly employed in the electrode, but 1M or 0.1 M solutions can equally well be used as explained in Section 15.1, the potential of the electrode is governed by the activity of the chloride ions in the potassium chloride solution. 

The potential of an electrode of the second kind is determined by the activity (concentration) of anions, or more correctly, by the mean ionic activity of the corresponding electrolyte [see Eq. (3.50)]. The most conunon among electrodes of this type are the calomel REs. In them, a volume of mercury is in contact with KCl solution which has a well-defined concentration and is saturated with calomel Hg2Cl2, a poorly soluble mercury salt. The value of such an electrode is 0.2676 V (aU numerical values refer to 25°C, and potentials are reported on the SHE scale). Three types of calomel electrode are in practical use they differ in KCl concentration and, accordingly, in the values of ionic activity and potential ... 

Because of solubility changes, the saturated calomel RE has a large temperature coefficient (0.65 mV/K). Its main advantages are ease of preparation (an excess of KCl is added to the solution) and low values of diffusion potential at interfaces with other solutions (see Section 5.2). The potentials of calomel REs can be reproduced to 0.1 mV. These electrodes are very convenient for measurements in neutral solutions (particularly chloride solutions). 

I 2 Standard, Formal, and Other Characteristic Potentials of Selected Electrode Reactions SCE (saturated calomel electrode)... 

There are other secondaiy standard electrodes, and one that is often used as a secondaiy standard is the Ag-AgQ electrode. It consists of an Ag wire that has been made an anode in a chloride-containing solution. The resulting evolution of Cl2 forms a thin porous film of AgCl on the silver wire. The wire and its layer are immersed in a standard KC1 solution and the electrode is reversible with respect to Cl- on the grounds of reasoning similar to that presented for calomel. The potential of this electrode is 0.222 V versus the standard H2 electrode. 

The solubility product of the Hg2Cl2 (calomel) is very low (Ksp = 1.3 x 10 l7). The potential of this electrode is again determined by the concentration of the chloride ion in the inner compartment. When a saturated solution of KC1 is used its potential against the SHE is n = +241 mV. Use of a saturated KC1 solution hides a certain danger the higher temperature sensitivity, which is due to the temperature effect on solubility. 

In principle, we can measure the potential of an electrode with a hydrogen reference electrode. We can also calculate the reversible potential of the cell composed of the electrode of interest and the hydrogen reference electrode. In practice, a hydrogen electrode is difficult to operate properly and is rarely used in engineering measurements. Instead, commercially available reference electrodes (e.g., calomel, Ag/AgCl, and Hg/HgO) are used. 

As already stated, the standard oxidation potential of the calomel electrode related to unity activity of the chloride ion at 25 °C is ea. 11 01101- — —0.268 V. Usually, however, potentials valid directly for various types of calomel electrodes are given (see Table 10). 

The electrolyzer consisting of the investigated electrode A of a known surface area and the auxiliary electrode B is connected to the direct current source 0. The potential of the electrode under observation A is measured against a suitable reference electrode (e. g. a calomel one), whioh is unpolarizable both... 

The measured electrode potential with respect to calomel electrode is added to the value of calomel electrode to obtain the value of the potential with respect to the standard hydrogen electrode. 

TABLE 1 Half-cell potentials of calomel reference electrodes... 

A less accurate method for the determination of solubility products, but which is of wider applicability, is the following. If MA is the sparingly soluble salt, and NaA is a soluble salt of the same anion, then the potential of the electrode M, MA(s), NaA aq. may be obtained by combining it with a reference electrode, e.g., a calomel electrode, thus... 

E.M.F. across the cell. The latter includes the IR drop (cf. p. 440), due to the resistance of the electrolyte, which varies with the current strength as a general rule, however, the variation is small, but allowance for it can be made if necessary. In order to eliminate the possibility of polarization at the anode, some workers employ a separate non-polarizable anode consisting of a large calomel electrode the potential of this electrode has a known constant value, and so the evaluation of the cathode potential is facilitated. 

A similar approach can be used with other electrodes. In the following example, the titration of a chloride sample with standard silver nitrate, the potential of a silver electrode in combination with a saturated calomel reference electrode was used to follow the course of the titration. The potential of the electrode pair is a direct measure of the free chloride ion concentration as the chloride ion concentration decreases, the potential increases. The titration results are shown in Figure 20-3. 

Relative electrode potential The potential of an electrode with respect to another (ordinarily the standard hydrogen electrode or saturated calomel electrode). 

The calomel electrode is based on reaction (9.2.42). Insoluble mercury(I) chloride (calomel) lies on top of the liquid mercury phase and ensures that the solution is saturated with Hg2. The potential of the electrode depends on the chloride ion activity as follows ... 

This is the nearest approach we can make to a physical interpretation of P, le P is a quantity whose logarithm is proportional to the electrolytic potential of the electrode in question The following table gives the electrolytic potentials of a few electrodes, the values m the first column being referred to the normal calomel electrode as a zero electrode, the values of the second column referring to the normal hydrogen as a zero electrode... 

In which 22 is the standard potential of the electrode. However iu a saturated calomel solution the relation ... 

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