Mercury mercurous chloride

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. 

Chemical Designations - Synonyms Albus Aminomercuric chloride Ammoniated mercury Mercuric chloride, ammoniated Mercury ammonium chloride Mercury (II) chloride ammonobasic Chemical Formula HgNHjCl. 

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... 

Property Elemental Mercury Mercurous Chloride Mercuric Chloride Methylmercury Chloride... 

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). 

The activity of the halogens is greatly influenced by the presence of traces of water. Thus, dry chlorine does not attack copper, and dry hydrogen chloride docs not unite with ammonia.8 Although dry chlorine attacks mercury, mercurous chloride does not dissociate upon heating, when thoroughly dried. The same is true for ammonium chloride 8 at 850° C. and phosphorus pentachloride. 

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. 

Carbon disulfide (bp 46°) is used as the solvent in many Friedel-Crafts reactions. It must be used with caution because of its very low flash point, which permits it to ignite on contact with heated surfaces. It should be distilled in such a manner that the vapors do not escape from the apparatus, and heat should be supplied by a steam bath and precautions taken to prevent overheating. The commercial material contains some odoriferous sulfur compounds which may be removed by successive treatment with mercury, mercuric chloride solution, and permanganate solution, and then washing with water, drying over phosphorus pentoxide, and distilling. ... 

EPA Cancer Classification elemental (metallic) mercury methyl mercury mercuric chloride Dh C C IRIS 1997... 

Chemical Name Elemental Mercury Mercuric Chloride Mercurous Chloride Methylmercuric Chloride ° Dimethylmercury... 

Mercurous ohloride (horn-mercury) Mercuric chloride... 

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 ... 

There are inadequate human cancer data available for all forms of mercury. Mercuric chloride has caused increases in several types of tumors in rats and mice, and methylmercuiy has caused kidney tumors in male mice. The HPA has determined that mercuric chloride and methylmercury are possible human carcinogens. 

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... 

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