Other reference electrodes

The commonest type of reference electrode is of the following form metal/saturated solution of sparingly soluble salt of metal + additional strongly ionized salt with a common anion. The calomel electrode is a case in point, represented by 

Other examples are the silver/silver chloride and mercury/mercurous sulphate electrodes, 

The potential adopted by each of these reference electrodes is controlled by the activity of the anion in solution. Thus there are three types of calomel electrode corresponding to 0-1 N,N and saturated KCl whose potentials, on the hydrogen scale, at 298 K are collected in Table 5.2. Two forms of saturated calomel electrode and their components are showm in Fig. 5.7. 

Consider a calomel electrode containing KCl, the activity of the chloride ions in the solution being aci-- The potential of the mercury depends upon the activity of the mercurous ions, Hg2, so that 

Since the bracketted terms form a constant, which we may denote by J cij Equation (5.22) may take the form 

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Potential control with zinc reference electrodes presented a problem because deposits of corrosion products are formed on zinc in hot water. This caused changes in the potential of the electrode which could not be tolerated. Other reference electrodes (e.g., calomel and Ag-AgCl reference electrodes) were not yet available for this application. Since then, Ag-AgCl electrodes have been developed which successfully operate at temperatures up to 100°C. The solution in the previous case was the imposition of a fixed current level after reaching stationary operating conditions [27]. 

It is apparent that since the electrode potential of a metal/solution interface can only be evaluated from the e.m.f. of a cell, the reference electrode used for that purpose must be specified precisely, e.g. the criterion for the cathodic protection of steel is —0-85 V (vs. Cu/CuSOg, sat.), but this can be expressed as a potential with respect to the standard hydrogen electrode (S.H.E.), i.e. -0-55 V (vs. S.H.E.) or with respect to any other reference electrode. Potentials of reference electrodes are given in Table 21.7. 

Values from different reference electrodes, can be converted to the NHE scale or any other scale by referring to their reduction potentials versus the NHE-electrode or other reference electrodes, and correlation of the activity of the electroactive cation. Reduction potentials are SCE 0.2412V Ag/AgCl - 0.197V Li/Li ... 

In these reactions, (2) is the process taking place at the reference electrode which therefore determines the potential scale. In practice other reference electrodes, such as the saturated calomel electrode are frequently used but the data are normally expressed on the hydrogen scale. 

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... 

Nevertheless, since in thermodynamics the values of the standard potentials are normally referred to the NHE electrode, when one wishes to obtain thermodynamic data from electrochemical experiments it is usual to transform the redox potentials obtained employing other reference electrodes into values with respect to the NHE, according to the scales given in Chapter 3, Section 1.2. 

To learn that the primary reference electrode is the standard hydrogen electrode (SHE), and that the potential of all other reference electrodes (so-called secondary references) are determined with respect to the SHE. 

Other reference electrodes are discussed in standard electrochemistry textbooks (see the Bibliography). 

Interference. Most common reference electrodes contain K+ and Cl- ions, which provide good ionic transport within the junctions. Other reference electrodes are also available for those samples that are sensitive to these ions (i.e., Hg2S04 with a K2S04 salt bridge, or reference electrodes with a double salt bridge construction). 

However, at least two other reference electrodes, calomel (Hg. 7.42) and silver silver chloride electrodes, are in common use as secondary reference electrodes (they are easier to set up than die hydrogen reference electrode). Potentials of electrodes measured using one of die secondary reference electrodes can be directly converted to values on die hydrogen scale, if die potential of die secondary reference electrode with respect to the hydrogen electrode is known (see also Section 7.5.73). 

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. 

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... 

Although it is a highly reproducible, very accurate and stable reference electrode, the SHE is impractical for many applications. Therefore, other reference electrodes have been developed. The most common ones are classified into three main categories [3] ... 

Other reference electrodes for use in polar aptotic solvents. Emphasis has been given to the use of the silver-silver ion reference electrode because it is almost universally applicable, and because standardization on the use of one reference electrode system simplifies the comparison of data between different workers. However, a number of other reference electrodes have been used (see Table 5.4), particularly those that have resulted from the vast amount of batteiy research. These include the Li/Li(solv)+ and other alkali metal electrodes that function reversibly in Me2SO, propylene carbonate, and hexa-methylphosphoramide. The thallium-thallous halide electrodes of the second kind also function reversibly in Me2SO and propylene carbonate. The cadmium amalgam-cadmium chloride reference electrode also functions reversibly in dimethylformamide and may be a useful substitute for the silver-silver ion reference electrode, which may be unstable in dimethyformamide.54... 

Although a number of other reference electrodes are capable of reproducible... 

Fuel cell researchers have also investigated other reference electrodes, such as a pseudo-reference electrode constructed by inserting a micro-sized carbon filament between two polymer electrolyte membranes [73], The main advantage of pseudoreference electrodes is their easy implementation, although one disadvantage is that their DC potential is unknown. However, this DC potential may not be that critical because EIS measurements mainly rely on the AC perturbation signal from which the impedance is calculated. 

As reference electrode any electrode whose potential is well defined and constant may be used by far the most widely used reference electrodes in aqueous and partly aqueous solution are the calomel (SCE, saturated calomel electrode) and the silver/silver chloride electrodes, both of which are electrodes of second kind. In non-aqueous solutions quite a few other reference electrodes have been used besides the calomel electrode. A discussion of reference electrodes is included in standard monographs on electroanalytical techniques, and comparisons between the diiferent types of electrode have been made.45-48... 

A reference electrode is needed to provide a potential scale for E° valnes as all voltages are relative. Any electrochemical reaction with a stable, well known potential can be nsed as a reference electrode. The NHE or standard hydrogen electrode (SHE) (Pt/H2,1.0 M H+) was the first well known reference electrode and is used as a reference in most tables of redox potentials. An NHE is difficult to construct and operate and therefore, is not typically used experimentally. Since the NHE is widely accepted, potentials are still often referenced to the NHE, converted from other reference electrodes. For aqueous solvents the SCE (Hg/Hg2Cl2 (KCl)) and the silver/silver chloride (Ag/AgCl) electrode are now commonly used as reference electrodes. To convert from the SCE to the NHE, E (vs. NHE) = E (vs. SCE) + 0.24 V. For nonaqueous solvents the silver/silver nitrate (Ag/AgNOs) reference electrode is often used. A pseudo-reference electrode can also serve as a reference point for aqueous or nonaqueous solutions. A silver or platinum wire can be used as a... 

The standard hydrogen electrode, which is the reference half-cell electrode, defined as 0.0 V at all temperatures, against which values of E" are expressed. H2 gas at 1 atmosphere pressure is bubbled over a platinum electrode immersed in an acid solution with an activity of unity. This electrode is rarely used for analytical work, since it is unstable and other reference electrodes are easier to construct and use. 

A reference electrode [183-185] is a half-cell that defines a potential to which all other measurements are referred. The primary standard electrode is the standard hydrogen electrode (SHE), but as this electrode is inconvenient for practical work, other reference electrodes are used. Such reference electrodes must have a potential that changes very little and is known to within 1 mV or so. In some cases of controlled potential electrolysis, it is sufficient to know the potential of the working electrode during the electrolysis within 10-20 mV, because the potential variations between different points of the electrode are of this magnitude (see earlier), and less precise electrodes may be termed comparison electrodes. In principle, any electrode at the surface of which an electrochemical reaction with a large exchange current can take place may be used as a reference electrode. 

Other reference electrodes have been proposed for use in the nonaqueous solvents that are widely used in coordination chemistry. Their main advantage is that they allow one to work with a single solvent. Among these electrodes, the Ag+/Ag electrode is reversible in many solvents.4 Ag+ ions are introduced as salts, such as AgCl or AgBF4. However, the inner solution has to be refreshed due to the reactivity of Ag+. Another class consists of redox electrodes in which the two components are in solution, such as ferrocenium ion/ferrocene Fc+/Fc.5 Since the potential is dependent on the concentration ratio of the redox couple, this ratio must be kept constant. An attractive solution to prevent the use of a junction lies in the preparation of a functionalized-polymer coated electrode such as poly(vinylferrocene).6 The polymer is deposited by electrooxidation in its oxidized form, polyFc+, and then partially reduced to yield poly Fc+/Fc. Their use is limited by their relative stability in the different solvents. 

Several other reference electrodes that are more convenient for routine measurements have been developed. Some of these are described in Section 2IB. 

See Table 2.2 for potentials of other reference electrodes vs NHE. Cationic vanadiimi species are present in the form of their aqua complexes. Nicotine adenine dinucleotide in its oxidised and reduced form. Glutathione in its oxidised and reduced form. 

The standard potential of am electrode is defined as the standard potential of a cell in which the other (reference) electrode is the arbitrary zero of potential (equation (2)) as described above. In this chapter the methods for obtaining standard potentials from emf measurements of cells without liquid junctions will be discussed, and the available data will be used for computing such potentials. The order adopted will be, more or less, that of the increasing complexity of the methods employed, Later chapters will deal with liquid junctions and the less accurate standard potentials that can be obtained from emf values of cells containing such junctions. 

Let us now consider the formation of the semiconductor/solution interface. The Fermi level in the solution phase, can be identified as Ji by (18.2.4) and is calculated in terms of values by the procedures described in Section 2.2. For most electrochemical purposes, it is convenient to refer values to the NHE (or other reference electrodes), but in this case it is more instructive to estimate them with respect to the vacuum level. This can be accomplished, as discussed in Section 2.2.5, by theoretical and experimental means with relaxation of thermodynamic rigor, so that one obtains an energy level value for the NHE at about —4.5 0.1 eV on the absolute scale (45) (Figure 18.2.5a). Consider the formation of the junction between an n-type semiconductor and a solution containing a redox couple 0/R, as shown in Figure 18.2.5. When the semiconductor and the solution are brought into contact, if electrostatic equilibrium is attained, in both phases must become equal (or equivalently the Fermi levels must become equal), and this can occur by... 

The reference electrode most commonly used is a saturated calomel, a mercurous sulfate, or silver/silver-chloride electrode. When accurate measurement of the potential is not required, a mercury pool or a platinum wire, foil, or gauze can be used. In nonaqueous solutions, various other reference electrodes may be more suitable. 

Thus, is a measure of the energy level of an electron in metal M with respect to that in M,. In other words, E represents the energy level of the redox couple with respect to a reference electrode since ji = jjfi, where /le is the electrochemical potential of electron in a solution S, and is called a redox potential. For the convention, E with respect to the hydrogen electrode is most often used. Once a value with respect to the hydrogen electrode or any other reference electrode is known, a value with respect to some other reference electrode is easily calculated by using... 

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