Mercury/mercurous-chloride half cells

The internal element appears as a grayish-white cylindrical pack with shiny mercury at the top of the element, if it is a calomel internal. This mercury-mercurous chloride half cell provides a potential of 244 mV versus the normal hydrogen electrode at 25°C if it is surrounded by saturated potassium chloride filling solution. It is important that this element be kept wet and uncontaminated in order to provide a stable and reproducible potential. With use, the pack may show some separation within the element tube, but this usually does not cause error or deviation of its potential. 

The accepted primary reference electrode is the hydrogen half cell described in association with Fig.2.1 (Ref 5). It consists of platinum (which serves as an inert conductor) in contact with a solution at 25 °C, saturated with hydrogen gas at one atmosphere pressure, and containing hydrogen ions at pH = 0 (aH+ = 1). In practice, the major use of the standard hydrogen electrode (SHE) is for calibration of secondary reference electrodes, which are more convenient to use. Two common reference electrodes are the calomel or mercury/saturated-mercurous-chloride half cell with a potential of +241 mV relative to the SHE and the sil-ver/saturated-silver-chloride half cell with a relative potential of+196 mV. Both of these electrodes are saturated with potassium chloride to maintain a constant chloride and hence metal-ion concentration. 

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.

At the heart of the SCE is a paste of liquid mercury and mercurous chloride (Hg2Cl2), which has the old-fashioned name calomel . Figure 7.13 depicts a simple representation of the SCE. The half-cell reaction in the SCE is... 

The outer tube has a porous fiber tip, which acts as the salt bridge to the analyte solution and the other half-cell. A saturated solution of potassium chloride is in the outer tube. The saturation is evidenced by the fact that there is some undissolved KC1 present. Within the inner tube is mercury metal and a paste-like material known as calomel. Calomel is made by thoroughly mixing mercury metal (Hg) with mercurous chloride (Hg2Cl2), a white solid. When in use, the following half-cell reaction occurs ... 

The hydrogen half-cell is not very convenient for routine laboratory usage—indeed, 1 m H+ (corresponding to a pH of 0 ) and 1 atm H2 (explosive) are dangerous. Hence, secondary standards are used, e.g., mercury/mercurous (calomel) or silver/silver chloride electrodes, which have midpoint redox potentials of 0.244 V and 0.222 V, respectively. 

The saturated calomel electrode (SCE) is composed of metalhc mercury in contact with a saturated solution of mercurous chloride, or calomel (Hg2Cl2). A Pt wire in contact with the metallic Hg conducts electrons to the external circuit. The mercurous ion concentration of the solution is controlled through the solubility product by placing the calomel in contact with a saturated potassium chloride solution. It is the saturated KCl solution that gives this electrode the saturated name there are other calomel reference electrodes used that differ in the concentration of KCl solution, but all contain saturated mercurous chloride solution. A typical calomel electrode is shown in Fig. 15.5. The half-cell... 

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. 

The calomel reference electrode has long been a standard reference electrode used in the laboratory. It consists of mercury in equilibrium with Hg2", the activity of which is determined by the solubility of Hg2Cl2 (mercurous chloride, or calomel). The half-cell reaction is... 

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... 

It is the most commonly used reference electrode. It has a constant and reproducible potential. The electrode basically consists of a platinum wire dipped into pure mercury which rests in a paste of mercurous chloride and mercury. The paste is in contact with a solution of potassium chloride which acts as a salt bridge to the other half of the cell (Fig. 2.13). 

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