Electrode secondary standard electrodes

The standard hydrogen electrode is not the most convenient to use because it requires highly flammable hydrogen gas to be bubbled over the platinum electrode. Other electrodes can be used as secondary standard electrodes, such as the... 

As a result of a variable liquid-junction potential, the measured pH may be expected to differ seriously from the determined from cells without a liquid junction in solutions of high acidity or high alkalinity. Merely to affirm the proper functioning of the glass electrode at the extreme ends of the pH scale, two secondary standards are included in Table 8.14. In addition, values for a 0.1 m solution of HCl are given to extend the pH scale up to 275°C [see R. S. Greeley, Anal. Chem. 32 1717 (I960)] ... 

The most-used secondary standard is the calomel electrode, shown in Fig. 7.40. It consists basically of a pool of mercuiy on top of which is spread a thin layer of Hg2Cl2 (calomel), a substance only slightly soluble in water. A KC1 solution (either at the unit activity with respect to Cl- or saturated) is in contact with the calomel Hg system and a Pt wire connects the electrode to the rest of the circuit. 

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. 

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. 

For a measurement of pH with cell (I) to be traceable to the SI, an uncertainty for the Bates-Guggenheim convention must be estimated. One possibility is to estimate a reasonable uncertainty contribution due to a variation of the ion size parameter. An uncertainty contribution of 0.01 in pH should cover the entire variation. When this contribution is included in the uncertainty budget, the uncertainty at the top of the traceability chain is too high to derive secondary standards as used to calibrate pH meter-electrode assemblies. 

The hydrogen half-cell is not very convenient for routine laboratory usage—indeed, 1 m H+ (corresponding to a pH of 0 ) and 1 atm H2 (explosive) are dangerous. Hence, secondary standards are used, e.g., mercury/mercurous (calomel) or silver/silver chloride electrodes, which have midpoint redox potentials of 0.244 V and 0.222 V, respectively. 

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. 

The standard hydrogen electrode is not the most convenient of electrodes to set up, because of the necessity of bubbling hydrogen over the platinum electrode. Several other electrodes are commonly used as secondary standard electrodes. One of these is the standard silver-silver chloride electrode, in which a silver electrode is in contact with solid silver chloride, which is a highly insoluble salt. The whole is immersed in potassium chloride solution in which the chloride ion concentration is 1 m. This electrode can be represented as... 

I Another electrode comnnonly used as a secondary standard is the glass electrode,... 

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... 

The hydrogen electrode is the ultimate standard electrode not only for the determination of (relative) potentials, but also for the determination of pH values. Owing to the experimental difficulties associated with it, however, it is seldom used for routine measurements, but rather for the evaluation of secondary reference and pH electrodes such as the calomel reference electrode and the glass pH electrode. 

Secondary Standards. In addition to the six primary standards, the NBS has designated two secondary standards, one on the acidic and one on the basic end of the intermediate pH region, in order to affirm the proper functioning of the glass electrode (Table 2.3). Furthermore, a great number and variety of secondary pH standard solutions have been collected in various treatises and compendia [4,6,7]. A secondary standard may often prove convenient because it may be more easily prepared or more stable than the primary standards, or matches more closely the composition and pH of a group of unknowns on which many measurements are to be made over a period of time. Particularly in process-control applications with, for example, very concentrated electrolyte solutions, reproducibility may be the most important concern. 

Like most of the older standard methods, BS2067 employs dual electrode systems intimate electrodes in contact with the specimen, and secondary rigid electrodes carrying the connections to the measuring device, Such methods suffer from the dangers of poor contact and or contamination through the use of adhesive materials. 

The electrode potential of this type of electrode is defined by the concentration (activity) of the anions. They are used in corrosion engineering as standard half-cells or reference electrodes. In corrosion engineering practice they are called secondary reference electrodes to differentiate them from the hydrogen electrode, which is a primary reference electrode. The following electrodes of the second kind are of interest in electrochemical and corrosion studies calomel electrode, silver-silver chloride electrode, and mercury-mercurous electrode. 

This redox process is used as a convenient internal, secondary reference electrode and, therefore, E° is defined as OV for reference purposes. (Relative to the standard hydrogen electrode, °fc+/fc= + 0.40 V.)... 

Although the reversible hydrogen electrode is the primary standard, there are several other reference electrodes that are used in practice. Some of these secondary reference electrodes are described below (see also Section 4.1.7). 

This book contains extensive tables of standard electrode potentials covering the periodic chart. For each electrode reaction Is given the standard potential, the temperature and the pressure, the solvent, and a literature reference. Occasionally the temperature coefficient of the electrode potential Is given together with an estimate of uncertainty. Much of the tabulated data Is taken from secondary sources (such as Item [149]). 

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