Mercury/mercurous-chloride calomel

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 ... 

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

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. 

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

Corrosive Sublimate and Calomel. A method for preparing a rather pure mercurous chloride (calomel) was known to Parisian physicians before 1608 (83). Oswald Croll prepared it by a secret process, and Jean Beguin in his Tyrocinium Chymicum, which was published in 1608, described the process. This mild sublimate was made by rubbing corrosive sublimate with as much mercury as could be "killed or made to combine with it (240, 241, 242). Calomel, corrosive sublimate, and vermilion have been manufactured for centuries at Hankow, China (243). 

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 ... 

Hydrargyri Chloridum Corrosivum (corrosive sublimate) Like all the mercurials, mercuric chloride (HgCl2) was regarded as an alterative. A biliary stimulant and cathartic used in a wide range of ailments (see Chapter 7). It was also supplied as yellow iodide of mercury, red oxide of mercury, and in pill and ointment. Not quite the same as calomel (Hydrargyi Chloridum Mite or mercurous chloride [HgCI]). Because it was much harsher than calomel, corrosive sublimate was used far more judiciously by medical staff. 

Garza-Ocanas L,Torres-Alanis O, Pineyro-Lopez A. Urinary mercury in twelve cases of cutaneous mercurous chloride (calomel) exposure effect of sodium 2, 3-dimercaptopropane-l-sulfonate (DMPS) therapy. J Toxicol Clin Toxicol 1997 35(6) 653-5. 

Some mercury compounds are known to be poisonous. For example, mercuric chloride (corrosive sublimate) was often used to kill pests and, sometimes, people. However, some mercury compounds have been used as medicines. For instance, mercurous chloride (calomel) was long used as a cure for skin rashes. Since the 1960s, the dangers of mercury have become better known. As a result, mercury use is now being phased out. 

The bulk of the information regarding toxicity resulting from oral exposure to inorganic mercury comes from studies of mercuric chloride. However, a few studies are also available on the effects of oral exposure to mercuric acetate, mercurous chloride (calomel), and mercuric sulfide (cinnabar). Discussion of these compounds has not been separated in this section, but the specific inorganic compound responsible for any effect is noted both in the text and in Table 2-2 and Figure 2-2. 

Mercurous Chloride. Calomel mild mercury chloride mercury monochloride mercury protochloride mercu-... 

Another commonly used electrode is the calomel electrode, illustrated in Figure SAa In this, mercury is in contact with mercurous chloride (calomel, Hg2Cl2) immersed either in a 1 m solution of potassium chloride or in a saturated solution of potassium chloride. If the cell... 

CHEMICAL PROPERTIES mercury salts yield metallic mercury when heated with sodium earbonate mereury salts may be redueed to metal by hydrogen peroxide in presenee of alkali hydroxide soluble ionized mereurie salts give a yellow precipitate or mereuric oxide with sodium hydroxide and a red precipitate of mercury diiodide with alkali iodide mercurous salts give a black precipitate with alkali hydroxides and a white precipitate of mercurous chloride (calomel) with hydrogen ehloride or soluble chlorides decomposes slowly on exposure to sunlight. 

Calomel Electrode. The normal hydrogen electrode [a platinum wire in 1.288 N HCl solution ( H+ = 1) with H2pressure bubbling through it] was used to define the standard electrode potential scale (see Section 7.3.2). This electrode is not convenient to use on an everyday basis, so a series of secondary reference electrodes has been developed for this purpose. One of the most commonly used laboratory reference electrodes is the saturated calomel electrode. The electrode (Fig. 7-36) consists of a platinum wire set in a paste that is a mixture of mercury (Hg(fj), mercurous chloride (calomel, HgaClajsj), and potassium chloride (KCl). The paste is in contact with a solution that is saturated with KCl and Hg2Cl2(s), The electrode can be represented as... 

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. 

Kalotnel mercurous chloride, calomel, mercury subchloride Quecksilber-(II)..J zweiwertiges QuecksMber mercuric, mercury(ll)... QuecksMberH(ll)-chlorid/... 

Quecksilber-(II), zweiwertiges Q. mercuric chloride/sublimate/ mercury dichloride/ corrosive mercury chloride Quecksilber-(II)-chlorid, Sublimat mercurous /mercury(l)... Quecksilber-(I), einwertiges Q. mercurous chloride/ calomel/... 

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. 

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