Calomel

Incidentally, a quantity called the rational potential is defined as E for the mercury-water interface (no added electrolyte) so, in general, = E + 0.480 V if a normal calomel reference electrode is used. 

For many purposes the hydrogen electrode is not convenient and it can be replaced by another cell of known standard electrode potential. A well-known example is the calomel cell shown in Figure 4.5. 

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

The product, commonly called calomel, is a white solid, insoluble in water in its reactions (as expected) it shows a tendency to produce mercury(II) and mercury. Thus under the action of light, the substance darkens because mercury is formed addition of aqueous ammonia produces the substance HjN—Hg—Hg—Cl, but this also darkens on standing, giving HjN—Hg—Cl and a black deposit of mercury. 

Example 13 The following data were recorded for the potential E of an electrode, measured against the saturated calomel electrode, as a function of concentration C (moles liter ). 

Table 8.31 Half-Wave Potentials (vs. Saturated Calomel Electrode) of Organic...

Temp., °C O.lMKCl Calomel l.OMKCl Calomel 3.5M KCl Calomel Satd. KCl Calomel l.OMKCl Ag/AgClt l.OMKBr Ag/AgBrf l.OMKI Ag/Agl ... 

Solvent, wt % Methanol, Ag/AgCl Ethanol, Ag/AgCl 2-Propanol, Ag/AgCl Acetone, Ag/AgCl Dioxane, Ag/AgCl Ethylene glycol, Ag/AgCl Methanol, calomel Dioxane, calomel... 

Calomel Electrodes Calomel reference electrodes are based on the redox couple between Hg2Cl2 and Hg (calomel is a common name for Hg2Cl2). 

The potential of a calomel electrode, therefore, is determined by the concentration of Ch. 

The saturated calomel electrode (SCE), which is constructed using an aqueous solution saturated with KCl, has a potential at 25 °C of -hO.2444 V. A typical SCE is shown in Eigure 11.8 and consists of an inner tube, packed with a paste of Hg, HgiCli, and saturated KCl, situated within a second tube filled with a saturated solution of KCl. A small hole connects the two tubes, and an asbestos fiber serves as a salt bridge to the solution in which the SCE is immersed. The stopper in the outer tube may be removed when additional saturated KCl is needed. The shorthand notation for this cell is... 

If the copper electrode is the indicator electrode in a potentiometric electrochemical cell that also includes a saturated calomel reference electrode... 

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

The following data were collected for the analysis of fluoride in tap water and in toothpaste, (a) For the analysis of tap water, three 25.0-mL samples were each mixed with 25.0 mL of TISAB, and the potential was measured with an F ISE relative to a saturated calomel electrode. Five 1.00-mL additions of a standard solution of 100.0-ppm F were added to each, measuring the potential following each addition. 

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