Mercury-mercurous chloride electrode

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. 

It is also important to understand how the potential gradient between an electrode and the bulk solution is established and controlled. Because the potential difference between the electrode and the bulk solution is not measurable, a second electrode must be employed. Although in general the potential difference between an electrode and solution cannot be determined, the potential difference between two electrodes in that solution can be determined. If the solution electrode potential difference of one of the electrodes is held constant by maintaining a rapid redox couple such as silver-silver chloride or mercury-mercurous chloride (calomel), then the potential... 

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.

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. 

The mercury-mercurous chloride (calomel) electrode. The calomel electrode was used extensively as a chloride electrode, but it has been all but abandoned for this purpose in favor of the silver chloride electrode. The fixed-potential saturated or 3.5 M KC1 calomel electrode always has been popular for use with glass electrodes in pH measurements and in polarographic work most of the vast compilations of aqueous polarographic half-wave potentials were referred to the aqueous saturated calomel electrode (SCE). 

Perhaps the most widely used reference electrode for electrochemical measurements is one form or other of the calomel electrode. This electrode consists of mercury, mercurous chloride (calomel), and a chloride-ion solution ... 

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 modem measuring systems the current carrying role of the counter electrode is separated from its potential control role by introducing the auxiliary electrode (AE) as a third electrode of the cell. The addition of the auxiliary electrode means that the counter electrode is now used only to control the potential of the working electrode and so becomes a true reference electrode (RE). Two electrodes which are commonly used as reference electrodes for the precise control of the working electrode potential in aqueous media are the silver-silver chloride electrode in a solution of fixed chloride concentration and the saturated calomel electrode or SCE (a mercury - mercurous chloride electrode in a saturated KCl solution). These electrodes are robust, easily constructed and maintain a constant potential. The three electrode system will be discussed in more detail in Chapter 25.3. 

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. 

A reference electrode composed of mercury, mercurous chloride (calomel), and a saturated aqueous chloride solution. 

A number of the most common potentiometric electrode systems and their applications are summarized in Table I. One of the most important and extensively used indicator electrode systems is the glass-membrane electrode that is used to monitor hydronium-ion activity. Although developed in 1909, it did not become popular until reliable electrometer amplifiers were developed in the 1930s (modern pH meters use high-input-impedance digital voltmeters). Figure 1 gives a schematic representation of this electrode and indicates that the primary electrode system is a silver/silver-chloride (or mercury/mercurous-chloride) electrode in contact with a known and fixed concentration of hydrochloric acid (usually about 0.1 M). When... 

In selecting reference electrodes for practical use, one should apply two criteria that of reducing the diffusion potentials and that of a lack of interference of RE components with the system being studied. Thus, mercury-containing REs (calomel or mercury-mercuric oxide) are inappropriate for measurements in conjunction with platinum electrodes, since the mercury ions readily poison platinum surfaces. Calomel REs are also inappropriate for systems sensitive to chloride ions. 

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

A reference electrode composed of metallic mercury and solid mercurous chloride. The redox half-reaction is ... 

ELECTRODE. Either ul two substances having different electromotive activity that enables an electric current to flow in the presence of an electrolyte. See also Electrolyte. Electrodes are sometimes called plates or terminal. Commercial electrodes are made uf a number of materials that vary widely in electrical conductivity, i.e.. lead, lead dioxide, zinc, aluminum, copper, iron, manganese dioxide, nickel, cadmium, mercury, titanium, and graphite research electrodes may be calomel mercurous chloride), platinum, glass or hydrogen. 

We will discuss four types of reference electrodes hydrogen, calomel, silver-silver chloride, and mercury-mercurous electrodes. 

MetaUInsoluble Salt/Ion Electrodes. Electrode potentials are usually reported relative to normal hydrogen electrode (NHE a(H+) = 1, p(H2) = 1), but they are actually measured with respect to a secondary reference electrode. Frequently used secondary reference electrodes are calomel, silver-silver chloride, and mercury-mercurous sulfate electrodes. These secondary reference electrodes consist of a metal M covered by a layer of its sparingly soluble salt MA immersed in a solution having the same anion Az as the sparingly soluble MA. The generalized reference electrode of this type may be represented as M MA AZ and may be considered to be composed of two interfaces one between the metal electrode M and the metal ions Mz+ in the salt MA... 

Mercury represents a serious environmental risk, and the study of removal of mercury from wastewater has received considerable attention in recent years. Mercury concentration was usually reduced by deposition on a cathode with high surface area. Removal of mercury is studied using extended surface electrolysis which reduces the level of mercury to below acceptable concentrations of 0.01 ppm in wastes by employing a Swiss roll cell with a cadmium-coated, stainless-steel cathode. An industrial cell with a fluidized bed electrode has also been studied. Graphite, as an efficient porous electrode, has been used to remove traces of mercuric ions form aqueous electrolyte solutions. In order to apply the electrochemical method for some effluents, it is necessary to use sodium hypochlorite to convert elemental mercury and less soluble mercury compounds to water-soluble mercuric-chloride complex ions. 

Note that the metal does not dissolve in the solution. An electrode of this type, which is often used because of its stability, is the calomel electrode shown in Fig. 4. This consists of a platinum wire inserted into liquid mercury, which is coated with mercurous chloride. The electrode reaction is... 

Another important example is the oxidation of Cl- at a mercury [Hg2(l)] electrode to form calomel [mercurous chloride, Hg2Cl2(s), Cl—Hgn— Hgn-Cl(s)] ... 

A small quantity of pure mercury is placed at the bottom of the vessel and covered with a layer of a paste consisting of mercurous chloride, mercury and potassium chloride solution of an appropriate concentration. Above the paste the vessel is filled with a potassium chloride solution saturated with mercurous chloride. According to the concentration of the potassium chloride solution (0,1 N — KOI, 1 A — KC1 or saturated solution of KC1) the decinormal, normal or saturated calomel electrodes are distinguished. 

Calomel electrode — is an - electrode of the second kind. It was introduced in 1890 by Ostwald, F.W. Asa- reference electrode of fixed, well-known, and very reproducible -+potential, it is still a commonly used reference electrode in electrochemistry [i—iii]. It consists of mercury, sparingly soluble mercurous chloride (calomel), and a chloride-containing solution. The electrode net reaction can be formulated in the following way ... 

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. 

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