Secondary reference electrodes

Fig. 16. Small-scalo laboratory cell for preparative electrolysis. A, Pt gauze working electrode. B, Pt sheet secondary electrode. C, Reference electrode. D, Luggin capillary on a syringe barrel so that the position of the tip of the Luggin probe relative to the working electrode is readily adjustable. E, Glass sinter to separate anode and cathode compartments. F, Gas inlet to allow stirring with inert gas or the continuous introduction of reactant. G, Three-way tap where a boundary between the reference electrode and the working solutions may be formed.

As a secondary reference electrode, the Ag/AgCl electrode is the most common due to its simplicity, stability, and capability of miniaturization. A conventional Ag/AgCl reference electrode is a silver wire that is coated with a thin layer of silver chloride either by electroplating or by dipping the wire in molten silver chloride. The electrode reaction is as follows... 

By far the most common secondary reference electrode is the SCE Hg Hg2Cl2 KCl(sat d) ... 

The silver-silver chloride electrode (SSCE) is another secondary reference electrode. A schematic of its half-cell is ... 

Reference Electrodes By definition, the normal hydrogen electrode (N H E) is the reference for electrode potentials (see Sect. 2.3.2.1), but practically it is scarcely usable. A reference electrode (RE) has to provide a well-defined potential between the electrolyte and its electric connector, joined with the input of the measuring instrument. Usually, a metal and a slightly soluble salt of this metal is applied (secondary electrode) [76, 77]. The electrolyte in the RE is connected to the electrolyte in the electrochemical cell via a diaphragm, which has to separate both electrolytes, as far as possible without a potential difference (see below). 

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. 

We see that calculations with the SHE are so easy as to be almost trivial. In practice, however, the SHE is so difiicult to operate experimentally, and not particularly safe because it involves elemental hydrogen gas, that we avoid the SHE if it is at all possible. Instead, we employ a secondary reference electrode, where the word secondary implies a reference electrode other than the SHE, but for which the potential is known with respect to the SHE. 

The SHE is experimentally cumbersome, and is hazardous to use owing to the involvement of elemental hydrogen gas, while the values of h+,H2 can fluctuate quite badly during operation because of the cyclic nature of bubble formation. For these reasons, the SHE is avoided experimentally unless a secondary reference electrode requires calibration. We will not consider the SHE any further because it is so unlikely that an analyst would in fact wish to calibrate a new reference electrode. 

Scanning tunneling microscopy (STM), 787. 1157 bioelectrochemistry and, 1159 electrochemistry and. 1158 electrodeposition and. 1310 nanotechnology, 1345 piezoelectric crystal, 1158 tunneling current. 1157 underpotential deposition, 1313, 1315 Scavanger electrolysis, electrodeposition, 1343 Schlieren method, diffusion layer. 1235 Schmickler, 1495,1510 Schrodinger equation, 1456, 1490 Schultze 923,1497.1510 Screw dislocation, 1303, 1316, 1321, 1326 Secondary reference electrode, 815, 1109 Self-consumed electrode, 1040 Semiconductors... 

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 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. 

In situ eiectrolysis-EPR methods usually employ a wire or grid electrode contained in a conventional flat or tube EPR cell. The constraints on the geometric configuration are such that secondary and reference electrodes are usually remote from the generating electrode, which often leads to problems in the control of the potential nevertheless it is a valuable technique for recording spectra of EPR active intermediates. These and related spectroelectrochemical techniques have been reviewed by Robinson.5... 

The symbol for the electron in tables of values of E° in liquid ammonia is thus equivalent to NH3(1) + /iH2(g /= 1) NH4+(liq NH3 a = 1). As for aqueous solutions, several secondary reference electrodes have proved more convenient for the actual measurement of E° in liquid ammonia, e.g. silver/silver chloride. This procedure has been applied to other inorganic solvents and numerous organic solvents, and tables of values are readily available.32... 

MetaUInsoluble Salt/Ion Electrodes. Electrode potentials are usually reported relative to normal hydrogen electrode (NHE a(H+) = 1, p(H2) = 1), but they are actually measured with respect to a secondary reference electrode. Frequently used secondary reference electrodes are calomel, silver-silver chloride, and mercury-mercurous sulfate electrodes. These secondary reference electrodes consist of a metal M covered by a layer of its sparingly soluble salt MA immersed in a solution having the same anion Az as the sparingly soluble MA. The generalized reference electrode of this type may be represented as M MA AZ and may be considered to be composed of two interfaces one between the metal electrode M and the metal ions Mz+ in the salt MA... 

Its realization is shown in Fig. 6.9. This electrode is impractical and secondary reference electrodes such as those discussed in these sections are used instead. 

Fig. 6.12, in which potential Ex has been measured against a secondary reference electrode and it is necessary to convert it to the standard hydrogen electrode scale which defines the zero . Thus, if the potential measured against Ag/AgCl (sat.) KC1//... is —99 mV, it is —142 mV versus SCE, —451 mV versus Fc/Fc+ redox electrode, but +100 mV versus SF1E. 

The Ag/AgCl electrode is most commonly used as a secondary reference electrode. The hydrogen electrode, the primary reference electrode, is inconvenient to use in practice, since it requires H2(g) and... 

The chemical stability and electrochemical reversibility of PVF films makes them potentially useful in a variety of applications. These include electrocatalysis of organic reductions [20] and oxidations [21], sensors [22], secondary batteries [23], electrochemical diodes [24] and non-aqueous reference electrodes [25]. These same characteristics also make PVF attractive as a model system for mechanistic studies. Classical electrochemical methods, such as voltammetry [26-28] chronoamperometry [26], chronopotentiometry [27], and electrochemical impedance [29], and in situ methods, such as spectroelectrochemistry [30], the SECM [26] and the EQCM [31-38] have been employed to this end. Of particular relevance here are the insights they have provided on anion exchange [31, 32], permselectivity [32, 33] and the kinetics of ion and solvent transfer [34-... 

Measurements can be done using the technique of redox potentiometry. In experiments of this type, mitochondria are incubated anaerobically in the presence of a reference electrode [for example, a hydrogen electrode (Chap. 10)] and a platinum electrode and with secondary redox mediators. These mediators form redox pairs with Ea values intermediate between the reference electrode and the electron-transport-chain component of interest they permit rapid equilibration of electrons between the electrode and the electron-transport-chain component. The experimental system is allowed to reach equilibrium at a particular E value. This value can then be changed by addition of a reducing agent (such as reduced ascorbate or NADH), and the relationship between E and the levels of oxidized and reduced electron-transport-chain components is measured. The 0 values can then be calculated using the Nernst equation (Chap. 10) ... 

The single cell design employs an outer compartment which contains the working and reference electrodes and an inner compartment which contains the auxiliary electrode. The secondary compartment hangs into the working compartment and makes solution contact through a porous glass frit. 

Figure 1 Schematic of the experimental UHV/electrochemical transfer system used for studies on modified platinum single-crystal surfaces. (From Ref. 26.) The UHV system has facilities for X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (LEISS), low-energy electron diffraction (FEED), and temperature-programmed desorption (TPS). The electrochemical chamber allows the electrochemical cell, 0 with integral counter, reference, and secondary working electrode, to be brought to the surface allowing contact of the electrolyte with the transferred surface.

One way whereby the difficulties are overcome is to use a reference electrode, of a form more convenient than the hydrogen electrode, as a secondary standard. One such reference electrode is the silver-silver chloride electrode. As already seen, the standard e.k.f. of the cell formed by combining a hydrogen electrode with this electrode is 0.2224 volt at 25 C ( 39h, 45d) thus, the e.h.f. of the cell... 

The determination of the standard potential of the silver-silver chloride electrode will be dealt with in detail because it has been carefully studied by various authors and because such electrodes have been much used, as secondary reference electrodes, in recent researches. We have already considered the galvanic cell... 

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. 

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