Convenient Hydrogen Electrode

The Convenient Hydrogen Electrode (21) (CHE) or the Self-Contained Hydrogen Electrode (22) (SCHE) is an RE with a hydrogen bubble contained within the electrode body that replaces the hydrogen gas supply required for typical hydrogen REs. The thin layer of solution drawn by capillary action onto the platinized electrode surface quickly... 

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. 

The Vacuum Reference The first reference in the double-reference method enables the surface potential of the metal slab to be related to the vacuum scale. This relationship is determined by calculating the workfunction of the model metal/water/adsorbate interface, including a few layers of water molecules. The workfunction, — < ermi. is then used to calibrate the system Fermi level to an electrochemical reference electrode. It is convenient to choose the normal hydrogen electrode (NHE), as it has been experimentally and theoretically determined that the NHE potential is —4.8 V with respect to the free electron in a vacuum [Wagner, 1993]. We therefore apply the relationship... 

The most convenient and reliable electrical biasing method for use with a hydrated SPE cell has been shown to be a three electrode potentiostatic circuit which maintains the sensing electrode at a predetermined potential vs. a stable reference (1 >3.>j0e The most reversible reference is a Pt/Hp, H+, static or dynamic. In practical instruments, however, good accuracy and convenience are achieved using a large surface area platinoid metal black/air (Op). All work reported in this study utilized the air reference which has a potential of approximately +1.05 V vs. a standard hydrogen electrode (SHE). For convenience, all potentials reported are vs. the SHE ... 

Because (like AG) refers to a difference in a state property, it can be evaluated in additive fashion along many alternative pathways. For this purpose, it is convenient to assign conventional ° values to each half-cell reaction [e.g., standard oxidation potentials as compiled in W. M. Latimer. Oxidation Potentials, 2nd edn (Prentice-Hall, New York, 1952)], such that the algebraic sum of the two half-reaction potentials equals the overall cell °. Such half-reaction ° values can in turn be obtained by choosing some standard electrode reaction as the conventional zero of the scale [such as the standard hydrogen electrode (SHE) for the l/2H(g) —> H+ aq) + e oxidation reaction, with she = 0]. Sidebar 8.2 illustrates a simple example of this procedure. 

The half-cell enclosed by the dashed line in Figure 15-1 is called a silver-silver chloride electrode. Figure 15-3 shows how the electrode is reconstructed as a thin tube that can be dipped into an analyte solution. Figure 15-4 shows a double-junction electrode that minimizes contact between analyte solution and KCI from the electrode. The silver-silver chloride and calomel reference electrodes (described soon) are used because they are convenient. A standard hydrogen electrode (S.H.E.) is difficult to use because it requires H2 gas and a freshly prepared catalytic Pt surface that is easily poisoned in many solutions. 

In Equation (18b), the activity quotient is separated into the terms relating to the silver electrode and the hydrogen electrode. We assume that both electrodes (Ag+/Ag and H+/H2) operate under the standard condition (i.e. the H+/H2 electrode of our cell happens to constitute the SHE). This means that the equilibrium voltage of the cell of Figure 3.1.6 is identical with the half-cell equilibrium potential E°(Ag+l Ag) = 0.80 V. Furthermore, we note that the activity of the element silver is per definition unity. As the stoichiometric number of electrons transferred is one, the Nemst equation for the Ag+/Ag electrode can be formulated in the following convenient and standard way ... 

The potentials of various metals shown in Table 1.5 are with respect to the standard hydrogen electrode (SHE). The hydrogen electrode is not very convenient to use in practice as a reference electrode in the context of measuring electrode potentials. Some of the reference electrodes used in practice are detailed below (see Table 1.6). 

A standard hydrogen electrode can easily be built from a platinum foil, coated by platinum black by an electrolytic process, and immersed in a solution of hydrochloric acid containing hydrogen ions of unit activity (a mixture of 1000 g water and 1-184 mol hydrogen chloride can be used in practice). Hydrogen gas at a pressure of 1 atm is passed over the foil. A convenient form of the standard hydrogen electrode is shown on Fig. 1.16. The gas is introduced... 

There are two other common versions of this half-cell the normal and tenth normal csAo-mel electrodes, in which the KCl concentration is either 1.0 or 0.1 N. The saturated electrode is the easiest to prepare and the most convenient to use but has the largest temperature coefficient. The half-cell potential for each of the calomel electrodes has a different value relative to the standard hydrogen electrode these emf valnes are given in Table 1. Calomel electrodes can be easily prepared in the laboratory and are also available commercially. Two typical calomel electrode designs are shown in Fig. 7. 

At equilibrium the ions O and together with the negative ions and solvent species (H2O, OH, H+), will reach some equilibrium concentration on the surface of the metal electrode. There will be a potential difference between the metal electrode and the bulk of the solution whose magnitude may be measured relative to some reference electrode such as the standard hydrogen or calomel electrodes. For convenience let us refer our working electrode to the standard hydrogen electrode taken as zero. Its potential is then related to the concentrations (O) and (R) in the solution by the Nemst equation... 

The copper fluoride electrode has been found to be a convenient reference electrode in hydrogen fluoride [213]. 

Several types of reference electrodes may be used. The hydrogen electrode has been shown to behave reversibly in HF [217,218] and has been employed as reference electrode both in electroanalytical and preparative work [218], and mercury fluoride [251,312] and copper fluoride [213,306] have also been used as such. A comparison of these reference electrodes pointed to the hydrogen electrode (H2/Pd) as the most convenient [313]. 

Nevertheless, it is convenient to be able to compare the relative affinities of redox couples for electrons as a tool for predicting reaction directions. This comparison is typically done by comparing the electron affinity of a couple to that of the Standard Hydrogen Electrode (SHE). As illustrated in Fig. 3.12, the SHE consists of a platinum electrode (redox reaction site) that is immersed in a water solution at 25 °C containing H" ions at an activity of 1 (pH = 0). Pure hydrogen gas is bubbled around the platinum electrode at a partial pressure of 1 atm, so that H2 also has an activity of 1. The reaction occurring in the SHE is the reduction of H+ to gaseous H2 ... 

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... 

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