The calomel electrode

As in the hydrogen half-cell, there are a number of designs for the calomel half-cell, which uses mercury and calomel (mercurous chloride) as 

As with all reference half-cells, there are special considerations in fabrication to insure stability and reproducibility of the end product. The 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is 

One form of calomel electrode is shown in Fig. 15.1(a). It consists of a stoppered glass vessel provided with a bent side tube fitted with a three-way tap which carries a short upper and a long lower tube the latter is drawn out to a constriction at the bottom end. A short platinum wire is fused into the bottom of the vessel so that it protrudes into the interior, and a narrow glass tube sealed to the bottom of the vessel is bent round parallel to the vessel. A little mercury placed in the bottom of this tube provides electrical connection with the interior of the vessel through the sealed-in platinum wire. Mercury and mercury compounds must be handled with care (see Section 16.8). 

To set up a saturated calomel electrode, a saturated solution of potassium chloride is first prepared from pure potassium chloride and de-ionised water, and this is then shaken for some hours with analytical grade mercury(I) chloride so that the solution is also saturated with this substance. Pure mercury is placed in the electrode vessel to a depth of about 1 cm the platinum contact must be 

Compact, ready-prepared calomel electrodes are available commercially and find wide application especially in conjunction with pH meters and ion-selective meters. A typical electrode is shown in Fig. 15.1(h). With time, the porous contact disc at the base of the electrode may become clogged, thus giving rise to a very high resistance. In some forms of the electrode the sintered disc may be removed and a new porous plate inserted, and in some modern electrodes an ion exchange membrane is incorporated in the lower part of the electrode which prevents any migration of mercury(I) ions to the sintered disc and thus 

Some commercial electrodes are supplied with a double junction. In such arrangements, the electrode depicted in Fig. 15.1(h) is mounted in a wider vessel of similar shape which also carries a porous disc at the lower end. This outer vessel may be filled with the same solution (e.g. saturated potassium chloride solution) as is contained in the electrode vessel in this case the main function of the double junction is to prevent the ingress of ions from the test solution which may interfere with the electrode. Alternatively, the outer vessel may contain a different solution from that involved in the electrode (e.g. 3M potassium nitrate or 3M ammonium nitrate solution), thus preventing chloride ions from the electrode entering the test solution. This last arrangement has the disadvantage that a second liquid junction potential is introduced into the system, and on the whole it is preferable wherever possible to choose a reference electrode which will not introduce interferences. 

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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. 

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. 

The calomel electrode Hg/HgjClj, KCl approximates to an ideal non-polarisable electrode, whilst the Hg/aqueous electrolyte solution electrode approximates to an ideal polarisable electrode. The electrical behaviour of a metal/solution interface may be regarded as a capacitor and resistor in parallel (Fig. 20.23), and on the basis of this analogy it is possible to distinguish between a completely polarisable and completely non-polarisable... 

Reference Electrode an equilibrium (reversible) electrochemical half-cell of reproducible potential against which an unknown electrode potential can be measured. Examples of those commonly used in corrosion are the Pt, H /H (the hydrogen electrode), Hg/Hg Clj/Cl" (the calomel electrode), Cu/CuS04/Cu, Ag/AgCl/Cl", all with fixed activities of the dissolved ions. 

If it is desired to use the biamperometric method for detecting the end point, then the calomel electrode and also the silver rod (if used) must be removed and replaced by two platinum plates 1.25 cm x 1.25 cm. The potentiometer (or pH meter) used to measure the e.m.f. must also be removed, and one of the indicator electrodes is then joined to a sensitive galvanometer fitted with a variable shunt. The indicator circuit is completed through a potential divider... 

For some purposes, modifications of the calomel electrode may be preferred. Thus, if it is necessary to avoid the presence of potassium ions, the electrode may be prepared with sodium chloride solution replacing the potassium chloride. In some cases the presence of chloride ions may be inimical and a mercury(I) sulphate electrode may then be used this is prepared in similar manner to a calomel electrode using mercury)I) sulphate and potassium sulphate or sodium sulphate solution. 

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. 

Commercial forms of the electrode are available and in general are similar to the calomel electrode depicted in Fig. 15.1(h) with the replacement of the mercury by a silver electrode, and calomel by silver chloride. The remarks concerning clogging of the sintered disc, and the use of ion exchange membranes and double junctions to reduce this are equally applicable to the silver-silver chloride electrode. 

One combination of electrodes that could be used to determine pH is a hydrogen electrode connected through a salt bridge to a calomel electrode. The reduction half-reaction for the calomel electrode is... 

The standard Volta potential of the calomel electroded (Hg.Cl ) was determined for the first time by Klein and Lange, but the accepted and usually used value of this potential (equal to 0.207 V) was found by Randles.48 Later measurements by Parrel and McTigue et al.21 and Antropov et al. gave 0.212 V and 0.220 V, respectively. In the methanol a value equal to 0.25 has been found. 

Expression (8) can be used to calculate the real energy of ions in any solvent, provided the standard potential of the calomel electrode, and the standard potentials of the elements and the A ° P° (Hg, Cl") under study... 

In the calculation of H the standard Volta potential of the calomel electrode found by Randles has been used (see Section V). The other estimators of (abs), for instance, -4.73 V by Corner andTryson seem to be doubtful. A detailed discussion of this topic may be found in the papers by Trasatti. ... 

Metal insoluble-salt These consist of a metal in contact with one of its slightly soluble salts this salt in turn is in contact with a solution containing the anion of the salt. An example is represented as Ag AgCl Or (c). The electrode process at such an electrode as AgCl (s) Ag + Cl" Ag + e- —> Ag (s) or overall, AgCl (s) + e- Ag (s) + Cl". The electrode reaction involves only the concentration of Cl" as a variable, in contrast with the Ag Ag electrode, which has the Ag concentration as a variable. The most frequently electrode of this type is the calomel electrode (see text for description). 

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) ... 

Like the calomel electrode, the saturated KC1 version of this electrode is the most convenient to prepare. 

E = Faraday constant). The equilibrium potential E is dependent on the temperature and on the concentrations (activities) of the oxidized and reduced species of the reactants according to the Nemst equation (see Chapter 1). In practice, electroorganic conversions mostly are not simple reversible reactions. Often, they will include, for example, energy-rich intermediates, complicated reaction mechanisms, and irreversible steps. In this case, it is difficult to define E and it has only poor practical relevance. Then, a suitable value of the redox potential is used as a base for the design of an electroorganic synthesis. It can be estimated from measurements of the peak potential in cyclovoltammetry or of the half-wave potential in polarography (see Chapter 1). Usually, a common RE such as the calomel electrode is applied (see Sect. 2.5.1.6.1). Numerous literature data are available, for example, in [5b, 8, 9]. 

Similarly, the calomel electrode, reversible with respect to Cl, has a potential... 

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. 

Calomel Electrode. The calomel electrode consists of mercury covered with mercurous chloride (calomel) in contact with a solution of KCl ... 

The most-used secondary standard is the calomel electrode, shown in Fig. 7.40. It consists basically of a pool of mercuiy on top of which is spread a thin layer of Hg2Cl2 (calomel), a substance only slightly soluble in water. A KC1 solution (either at the unit activity with respect to Cl- or saturated) is in contact with the calomel Hg system and a Pt wire connects the electrode to the rest of the circuit. 

Thus the calomel electrode is reversible with respect to... 

Figure 15-6 Use of Ag and calomel electrodes to measure [Ag ]. The calomel electrode has a double junction, like that in Figure 15-4. The outer compartment of the electrode is filled with KN03, so there is no direct contact between Cl in the inner compartment and Ag1 in the beaker.

Predict the value of E for a calomel electrode saturated with KC1, given that E° for the calomel electrode is 0.268 V. (Your answer will not be exactly the value 0.241 used in this book.)... 

A cell was prepared by dipping a Cu wire and a saturated calomel electrode into 0.10 M CuS04 solution. The Cu wire was attached to the positive terminal of a potentiometer and the calomel electrode was attached to the negative terminal. 

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