Calomel electrodes, reference electrode standard potential

The hydrogen ion activity of the equilibrated aqueous phase was determined with a Beckman 39301 glass pH electrode and a Beckman saturated calomel electrode. The electrode potential was measured by an Orion 801 Ionanalyzer. The reference voltage of the system was calculated through electrode measurement of standard buffer solutions. From the Nemst Equation the reference voltage becomes... 

It is sometimes necessary to convert potentials between different reference scales. If an electrode has a potential of —0.461 V with respect to a calomel electrode, what is the potential with respect to a silver-silver chloride electrode What would be the potential with respect to the standard hydrogen electrode ... 

Linear polarization techniques are often used to conduct an initial electrochemical characterization of a metal or alloy, prior to more complex investigations, torr is first determined relative to a reference eicctrxxle, usually the standard calomel electrode (SCE). A small potential is then applied and swept from about 20 mV below to 20 mV anodic to it. The current density is measured and is calculated. 

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 electrolysis is carried out at a reference potential of -2.4 volts vs a standard calomel electrode. An initial current density of 0.0403 amp/cm is obtained which drops to 0.0195 amp/cm at the end of the reduction, which is carried on over a period of 1,682 minutes at 15° to 20°C. The catholyte is filtered, the solid material is washed with water and dried. 430 g of the 2,3-bis-(3-pyridyl)-butane-2,3-diol is recrystallized from water, MP 244° to 245°C. 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

All potentials refer to a standard hydrogen electrode in the used solvent. Exemptions are stated explicitly. Potential reported versus a saturated calomel electrode, converted assuming iiscE=-0-241 V vs. SHE. 

Additionally, other reference electrodes are used which are easier to maintain at standard conditions. These include the silver/silver chloride electrode and the saturated calomel electrode (SCE). The voltage difference between the working electrode and the reference electrode is proportional to the electrochemical potential difference between them. This is written... 

This type of counter electrode is defined as a reference electrode. As we will see in Chapter 3, Section 1.2, at 25° C the saturated calomel electrode (SCE) has a potential of +0.2415 V with respect to the standard hydrogen electrode (NHE), which, although difficult to use, is the internationally accepted standard for the potential scale, having conventionally E° — 0.000 V. 

The cell emf comprises two half-cell potentials. One of these half cells will be a standard reference, such as a saturated calomel electrode (SCE), while the other will be an inert electrode such as platinum or gold. ... 

Potentials of some reference electrodes relative to either the standard hydrogen electrode or the saturated calomel electrode. Further data in ref. [17]. 

Fig. 4.1 Skeleton structure of a photoelectrochemical cell (PEC) comprised of a photoanode and cathode. Potentials of both are measured with reference to a third electrode, the standard calomel electrode.

All aqueous potential values are referenced to the standard hydrogen electrode. Nonaqueous potential values are referenced to ferrocene (Fc) if possible. Other references are indicated in parentheses where SCE represents the standard calomel electrode, A1 represents the Ag/Ag+ reference electrode ([Ag+] = 0.01 M unless otherwise indicated) and A2 represents the Ag/AgCl reference electrode. In acetonitrile, potential values referenced to SCE may be corrected to the ferrocene reference standard by subtracting 0.380 V, depending upon the anion present (a) Ref 11, (b) Ref 10c. c [Ag+] = 0.1 M. 

The quoted potentials are rarely expressed versus standard hydrogen electrode (SHE) but mainly versus an aqueous saturated calomel electrode (SCE) or versus Ag/Ag couple in ACN or in a solvent used in particular experiments. However, in some cases the Mg/Mg + couple in THE or other solvents is used as the reference electrode. 

Figure 6.24. (a) Cyclic voltammogram of electro-oxidation of formic acid on a Pt(100) single-crystal showing current spikes indicating sustained current oscillations and (b) current oscillations measured at various fixed potentials all potentials are with reference to the Standard Calomel Reference Electrode (SCE) (adapted from [140]). 

The value of the constant V, and hence the values of standard potentials, depend on the choice of the reference electrode and on the character of electrode reaction, which takes place on it With the reference electrode potential conventionally taken as zero, we can choose, for example, the normal hydrogen electrode (NHE), i.e., an electrode, for which the equilibrium at the interface is attained due to the reversible redox reaction H+ + e = H2, provided the activity of H+ ions in the solution is 1 mol/liter and the pressure of gaseous hydrogen above the solution is 1 atm. Many of the measured potentials are given below relative to the saturated calomel electrode (SCE) its potential relative to the NHE is 0.242 V. 

The primary reference electrode for aqueous solutions is the standard hydrogen electrode (SHE), expressed by H+(a=l) H2(p=105 Pa) Pt (see 11 in Section 4.1). Its potential is defined as zero at all temperatures. In practical measurements, however, other reference electrodes that are easier to handle are used [24]. Examples of such reference electrodes are shown in Table 5.4, with their potentials against the SHE. All of them are electrodes of the second kind. The saturated calomel electrode (SCE) used to be widely used, but today the saturated silver-silver chloride electrode is the most popular. 

In aqueous solutions, the method of measuring electrode potentials has been well established. The standard hydrogen electrode (SHE) is the primary reference electrode and its potential is defined as zero at all temperatures. Practical measurements employ reference electrodes that are easy to use, the most popular ones being a silver-silver chloride electrode and a saturated calomel electrode (Table 5.4). The magnitude of the liquid junction potential (LJP) between two aqueous electrolyte solutions can be estimated by the Henderson equation. However, it is usual to keep the LJP small either by adding the same indifferent electrolyte in the two solutions or by inserting an appropriate salt bridge between the two solutions. 

The most widely used working electrodes are vitreous carbon, platinum, gold and mercury (Fig. 19.1). These electrodes are flexible because they can be used between two potential values that depend on the support electrolyte, the pH and the nature of the reference electrode. For example, the limits for the Pt electrode are +0.65 V relative to the standard calomel reference electrode (SCE) (oxidation of water HzO —> 5O2 + 2H++ 2e ) and —0.45 V (reduction of water HzO + 2e H2 + 20H"). 

The most common electrode of this type is the saturated calomel electrode (SCE) which consists of mercury in contact with a layer of insoluble Hg2Cl2 immersed in a saturated aqueous solution of KC1. The SCE is used as a secondary standard reference electrode. At 25°C it has an electrode potential of + 0.2415 V. 

However, it is not an easy matter to relate these scales of E° to the standard electrode potentials in water. This is because of the unknown liquid junction potential that is inevitably introduced when one attempts to calibrate the potential of a reference electrode in a given non-aqueous solution against a common reference electrode, such as the standard hydrogen electrode or saturated calomel electrode in aqueous solution. 

In constructing an electric cell for potentiometric titrations it is necessary, of course, to use a second electrode to complete the circuit, in addition to the measuring electrodes (commonly called indicator electrodes ) described above. Ideally the second electrode would be a hydrogen electrode which (as explained in the entry on electrode potential) is the standard reference electrode for which die potential, in equilibrium with its ions, is defined as zero. Since it is awkward to use, other electrodes of known potential, such as the calomel electrode or the glass electrode, are commonly used as reference electrodes. The arrangement of the apparatus is as shown in Fig. 1. 

Suppose the reference electrode for Table 12.1 were the standard calomel electrode, Hg2Cl2/Hg,Cl ([Cl] = 1.00 mol L1), with ° set equal to 0. Under this system, what would be the potential for (a) the standard hydrogen electrode (b) the standard Cu2+/Cu redox couple ... 

Because the reference electrode is the standard calomel electrode, which has E = E° = 0.28 V (Appendix D), the half-cell potential for the hydrogen electrode is 0.27 V ... 

When other reference electrodes are used as, for example, the saturated Calomel electrode (SCE), a constant quantity needs to be added to the right-hand side of Nemst s equation which reflects the difference between the SCE and the SHE electrodes, i.e., in this case the standard potential is... 

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