Mercury/saturated-mercurous-chloride

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. 

Various types of vessels have been described for the purpose of setting up calomel electrodes the object of the special designs is generally to prevent diffusion of extraneous electrolytes into the potassium chloride solution. In order to obtain reproducible results the mercury and mercurous chloride should be pure the latter must be free from mercuric compounds and from bromides, and must not be too finely divided. A small quantity of mercury is placed at the bottom of the vessel it is then covered with a paste of pure mercurous chloride, mercury and potassium chloride solution. The vessel is then completely filled with the appropriate solution of potassium chloride which has been saturated... 

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... 

It is more usual to measure potentials in the laboratory against a saturated calomel electrode (mercury in saturated mercuric chloride). This cell is recommended for calibrating field reference electrodes. It is possible to adjust the chloride content of a silver/silver chloride cell so that it behaves like a calomel cell. A calomel cell is not usually used in the field because it contains mercury. 

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. 

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 ... 

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.

That sulphur dioxide can function either as an oxidising agent or as a reducing agent, according to the conditions, has been shown in the case of certain chlorides, sulphates and phosphates.7 The concentration of acid which is added is an important factor, for, by varying the amount present, the sulphur dioxide can be made either to oxidise or to reduce. The action of sulphur dioxide on the chlorides of mercury illustrates this diversity of action. Quantitative results are produced only under very specific conditions. A solution of mercuric chloride (1 80) saturated with sulphur dioxide at 70° to 80° C. and kept at... 

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). 

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. 

Mercury di-p-chlorophenyl —Five grams of p-chlorophenyl-mercuric chloride, 6-5 grams of copper and 35 c.c. of pyridine are alioAved to stand for a day, then the mixture is saturated with carbon dioxide. An 81 per cent, yield, or 2-5 grams, of the mercury com]>ound is obtained. It crystallises from hot acetone in w hite needles, M.pt. 242 to 243 C., readily soluble in pyridine, less soluble in Avarm chloroform or ether. 

Amalgamated tin. Schaefer prepared this reagent by shaking a mixture of 100 g. of 30 mesh tin, 15 g. of mercuric chloride, and 100 ml. of water in a stoppered flask for a few minutes until all the tin appeared to have a shiny coating of mercury. Amalgamated tin and hydrochloric acid reduces conjugated enediones smoothly and without side reactions to the saturated diketones. In contrast, reduction with zinc and acetic acid is often attended with serious side reactions, o o... 

The calomel electrode consists of mercury in contact with saturated solution of mercurous chloride and a lar excess of potassium chloride solution which may be either saturated solution or normal solution. 

The formation of vinyl chloride from acetylene and hydrogen chloride is catalyzed by activated carbon impregnated with mercuric and mercurous chlorides.54- 5S The catalyst is prepared by saturating activated carbon with mercury vapor at 200° C and then adding hydrogen chloride to form mercuric and mercurous chlorides. 

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... 

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... 

In the case of standard electrode potential, it is appropriate to have a standard electrode whose reversible potential is made arbitrarily zero and against which the potentials of other electrodes can be measured. The hydrogen electrode is an accepted standard. It is composed of a rod of platinum covered with platinum black saturated with hydrogen gas at atmospheric pressure. Electrode potential based on this zero are said to refer to the hydrogen scale. However, in experimental work, it is often more suitable to use another standard electrode. Calomel is a common example. It consists of a pool of mercury covered with calomel (mercurous chloride) and immersed in a solution of potassium chloride. 

The calomel electrode is very similar in both construction and theory of operation to the silver/silver chloride electrode described below. The metal is mercury, the electrical connection being made by an inert metal wire and the salt is mercurous chloride. The equilibrium electrode potential is a function of the chloride concentration of the electrolyte. When the electrolyte is saturated potassium chloride, it is known as a saturated calomel electrode (SCE) producing an electrode potential of -1-0.224 V vs SHE. Potassium chloride is used because the ionic mobility... 

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. 

Mercurylmercury(I) chloride also mercurous chloride or calomel), abbreviated as Saturated Calomel Electrode (SCE), is the most widely used mercury RE (26). The preparation of this electrode, especially the purity of the mercury and the method of addition of mercury(I) chloride to the mercury, strongly affects the performance and potential exhibited. This RE can be very reproducible if produced and handled carefully. The potential of... 

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