Commercial Calomel Reference Electrode

The calomel reference electrode is a metal/salt-type electrode, so electrochemistry of the Hg/Hg2Cl2 and Ag/AgCl electrodes are similar. The electrochemical halfreaction of the Hg/Hg2Cl2 reference electrode is 

Similar to the Ag/ACl reference electrode, the electrode potential of the calomel electrode at a concentration of KCl(aq) equal (1) 0.1 mol (2) 1.0 mol L , (3) saturated with respect to KCl(s) can be calculated and compared with values given in [Chapter 10, Table 10.13]. The calomel and silver/silver chloride electrodes have similar precision and stability. A disadvantage of the calomel reference electrode is the use of Hg, which is a particularly hazardous chemical. 

FIGURE 5.1 Schematic of calomel reference electrode (1) platinum wire, (2) filling hole, (3) KCl(aq) solution (usually saturated), (4) liquid mercury, (5) mercury-calomel (Hg/Hg2Cl2) paste, (6) glass frit, and (7) ceramic, or quartz, or glass porous plug. 

---

If a commercial polarograph which includes a potentiostat is employed, then the three-electrode procedure (Sections 16.7 and 16.8) is conveniently used with the controlled potential supplied by the potentiostat applied between the dropping electrode and the calomel reference electrode, while the electrolysis current flows between the working (mercury) electrode and the auxiliary... 

The saturated calomel reference electrode is an example of a constant-potential electrode. A drawing and a photograph of a typical SCE available commercially are shown in Figure 14.4. It consists of two concentric glasses or tubes, each isolated from the other except for a small opening for electrical contact. 

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. 

Figure 5.13 Calomel reference electrodes (a, b) commercial designs with cracked-glass and fiber junctions, respectively (c) home-made design with cracked-glass junction on the bottom of left tube.

Figure 15.7 (a) A schematic glass electrode for pH measurement, (b) A complete pH measurement cell, with the glass indicator electrode and an external saturated calomel reference electrode, (c) A commercial combination pH electrode, with built-in internal Ag/AgCl reference electrode. 

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. 

The commercial SCE depicted in Fig. 18a.4 is generally an H-cell. One arm contains mercury covered by a layer of mercury(II) chloride (calomel). This is in contact with a saturated solution of potassium chloride a porous frit is used for the junction between the reference electrode solution and the sample solution at the end of the other arm. Similar to the silver/silver chloride reference system, a calomel electrode also warrants precautionary measures to maintain the chloride concentration in the reference electrode. 

Several types of reference electrodes are convenient for use in analytical electrochemistry. The use of high-input-impedance operational amplifiers in the reference electrode inputs of potentiostats ensures that very low levels of current are drawn from the reference electrode (see Chap. 6). This permits the use of reference electrodes that do not have to contain a large number of redox equivalents in order to ensure a constant reference potential and are therefore very small. Three reference-electrode designs that are convenient for use in analytical electrochemistry are shown in Figure 9.4. Saturated calomel and silver-silver chloride (of various concentrations of chloride) are among the most common commercially available or conveniently fabricated reference electrodes. 

Potentiometric measurements are simple the redox potential is measured compared to a reference electrode. For pH measurements, commercially available electrodes, comprising the working (glass electrode) and reference electrodes, can be used. For redox potential measurements, the working electrode is usually an inert (e.g., platinum) electrode, and the reference electrode can be a hydrogen electrode, calomel, or other electrodes. Ion-selective electrodes are also based on potential measurements. 

There are several issues related to the reference and counterelectrodes. Reliable reference electrodes are commercially available. The saturated calomel electrode (SCE) is extremely robust and is commonly used for studies in chloride solutions. For studies in which chloride is to be avoided, the mercurous sulfate electrode (MSB) is suitable. The location of the reference electrode is critical in cells in which large ohmic potential drops exist. In these cases, a Luggin capillary should be used to bring the sensing location of the reference electrode close to the working electrode... 

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. 

This electrode of the second kind is the most frequently used reference electrode in practical measurements, because the construction is very simple, the potential is very well reproducible, and last, but not least, this electrode is free of mercury Normally, a silver wire is covered with silver chloride, which can be achieved electrochemically or thermally [1]. Electrochemically produced films are thinner than thermally produced films. The construction of a commercially available sil-ver/silver chloride electrode is similar to the calomel electrode (see Fig. III.2.3). A very simple method for preparing a silver/silver chloride electrode has been described by Thomas [4]. Because reference systems based on silver/silver chloride can be produced in a very small size, they are often used in microsystems [5-9]. The electrolyte solution in these reference systems is normally a potassium chloride solution (mostly saturated or 3 M), and only seldom sodium or lithium chloride. The electrode net reaction is... 

Mercury-mercurous chloride. This is probably the most widely used reference electrode. It is reversible to chloride ion and is usually made up in saturated aqueous potassium chloride solution, although Irnoldm " and O.lmoldm solutions are also common. In commercial electrodes, the solution is often retained with a porous plug or ceramic frit saturated aqueous KCl, being very dense, easily leaks out. A separate compartment will therefore be necessary for the reference electrode if chloride ions must be kept out of the working solution. Calomel electrodes can easily be prepared by shaking clean dry mercury with the powdered mercurous chloride which forms a skin around the mercury. The chloride ion solution is then carefully poured on top to complete the electrode. Home-made calomel electrodes can have a very low resistance and high performance. 

In practice, the so-called reference electrodes of the second kind, e.g., the Ag/AgCl, CPelectrode or the Hg/Hg2Cl2, Cl (calomel) electrode, are preferably applied because they are more convenient to use. Their potential is stable and well defined in relation to the potential of the SHE. Such electrodes are commercially available in a variety of chemical systems and embodiments for different applications [3]. A substantial drawback of conventional reference electrodes is their liquid electrolyte filling. The performance of such electrodes is position... 

All of the catalysts synthesized are listed in Table 2, and the performance in rotating disk electrode (RDE) in acid conditions are depicted in Fig. 6 [65, 67, 70-72]. In RDE tests (RRDE-3A ALS coupled with a Bio-Logic SP-150 multi-potentiostat), a conventional three-electrode electrochemical cell configuration, equipped with a glassy carbon disk working electrode (0.1256 cm geometric area), a Pt helical wire counter electrode, and a saturated calomel (SCE) reference electrode, was used. The activity of a commercial 40 % Pt/C (Hyspec 6000 from Johnson Matthey) catalyst was assessed as well, as reference catalyst toward ORR. 

Electrodes of the secrnid kind such as Ag AgCl and the calomel electrode (Hg HgaCb) are the most frequently used reference electrodes. In solutions containing sulfuric acid Hg Hg2S04 are applied, while in alkaline media Hg HgO are also applied. The potential of these electrodes depends oti the concentration (activity) of the electrolyte used. In the commercial calomel electrodes usually KCl... 

EFC power sources do not need a reference electrode to operate, but reference electrodes are important for the design and development of EFCs as a means to identify the performance and lifetime limiting electrode. Integration of EFCs with a reference electrode allows for measurement of the individual electrodes voltage at open circuit and under electric loading this allows for identihcation whether the anode or cathode is power (and/or stability) limiting. As EFCs operate in aqueous, pH-neutral, buffered electrolyte, Ag/AgCl electrodes or saturated calomel electrodes (SCEs) are the typical reference electrodes of choice and are commercially available in the form of microelectrodes with diameters as low as 0.5 mm. 

An electrode sensitive to both cationic and anionic surfactants can be made from a glass tube sealed at the bottom with a membrane of poly(vinylchloride) containing a high percentage of plasticizer, 40% or more. Dissolved in the plasticized PVC is a surfactant ion pair, such as hexadecyltrimethylammonium dodecylsulfate. The tube is filled with a dilute solution of anionic surfactant with a small amount of a chloride salt, into which a silver/ silver chloride electrode is inserted. For potential measurements in solutions, the circuit is completed with a reference electrode such as saturated calomel. Commercial surfactant-selective electrodes based on PVC membranes have been available since approximately 1990. 

---