Silver electrode, 6.30

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

Fleischmann M, Hendra P J and McQuillan A J 1974 Raman spectra of pryridine adsorbed at a silver electrode Chem. Phys. Lett. 26 163-6... 

Albrecht M G and Greighton J A 1977 Anomalously intense Raman spectra of pyridine at a silver electrode J. Am. Chem. Soc. 99 5215-17... 

Jeanmaire D L and Van Duyne R P 1977 Part I heterocyclic, aromatic and aliphatic amines adsorbed on the anodized silver electrode J. Electroanal. Chem. 84 1-20... 

Figure Bl.22.6. Raman spectra in the C-H stretching region from 2-butanol (left frame) and 2-butanethiol (right), each either as bulk liquid (top traces) or adsorbed on a rough silver electrode surface (bottom). An analysis of the relative intensities of the different vibrational modes led to tire proposed adsorption structures depicted in the corresponding panels [53], This example illustrates the usefiilness of Raman spectroscopy for the detennination of adsorption geometries, but also points to its main limitation, namely the need to use rough silver surfaces to achieve adequate signal-to-noise levels.

Horanyi G and Rizmayer E M 1984 Radiotracer study of anion adsorption at silver electrodes in acidic medium J. Electroanal. Chem. 176 339-48... 

Franke C, Piazza G and Kolb D M 1989 The influence of halide adsorption on the electronic surface states of silver electrodes Electrochim. Acta 34 67-73... 

Ldutzenkirchen-Flecht D and Strehblow FI-FI 1998 Surface analytical investigations of the electrochemical double layer on silver electrodes in alkaline media Electrochim. Acta 43 2957-68... 

Electrodes of the Second Kind An electrode of the first kind involving an M"+/M redox couple will respond to the concentration of another species if that species is in equilibrium with M"+. For example, the potential of a silver electrode in a solution of Ag+ is given by... 

Electrodes. AH of the finished silver electrodes have certain common characteristics the grids or substrates used in the electrodes are exclusively made of silver, although in some particular cases silver-plated copper is used. Material can be in the form of expanded silver sheet, silver wire mesh, or perforated silver sheet. In any case, the intent is to provide electronic contact of the external circuit of the battery or cell and the active material of the positive plate. Silver is necessary to avoid any possible oxidation at this junction and the increased resistance that would result. 

Silver electrodes prepared by any of the three methods are almost always subjected to a sintering operation prior to cell or battery assembly. 

Silver—Iron Cells. The silver—iron battery system combines the advantages of the high rate capabiUty of the silver electrode and the cycling characteristics of the iron electrode. Commercial development has been undertaken (70) to solve problems associated with deep cycling of high power batteries for ocean systems operations. 

Cells consist of porous sintered silver electrodes and high rate iron electrodes. The latter are enclosed with a seven-layered, controUed-porosity polypropylene bag which serves as the separator. The electrolyte contains 30% KOH and 1.5% LiOH. 

AppHcations have been found for these batteries in emergency power appHcations for telecommunications systems in tethered balloons. Unfortunately, the system is expensive because of the high cost of the silver electrode. AppHcations are, therefore, generally sought where recovery and reclamation of the raw materials can be made. 

The surface X-ray experiments by Toney et al. [151] give experimental evidence for voltage-dependent ordering of water on a silver electrode. They observed a shift of the silver-oxygen distance with applied potential. 

It can be seen from Table 1.16 that differences in composition (nature or concentration) of the environment can lead to localised attack, and in Section 1.4 it was shown how differences in the activity of silver ions can give rise to a reversible concentration cell in which the silver electrodes in contact with the solution containing the lower and higher concentration of Ag ions are the anode and cathode, respectively. Concentration cells of this type are rare in practice, but can occur during the corrosion of copper and copper alloys. 

In the experiments to be described, in each case the half-cell was coupled to a Ag/AgOl half-cell, which in each case formed the positive electrode of the cell that is to say, on closing the external circuit, electrons flowed in the external circuit from the amalgam electrode to the silver electrode. This was the situation whether the solvent was water, or methanol, or a mixture of methanol and water. 

Before examining the processes in a cell, we should name the parts of a cell and clear away some language matters. The electrons enter and leave the cell through electrical conductors—the copper rod and the silver rod in Figure 12-5— called electrodes. At one electrode, the copper electrode, electrons are released and oxidation occurs. The electrode where oxidation occurs is called the anode. At the other electrode, the silver electrode, electrons are gained and reduction occurs. The electrode where reduction occurs is called the cathode. 

Thus an almost complete separation is theoretically possible. The separation is feasible in practice if the point at which the iodide precipitation is complete can be detected. This may be done (a) by the use of an adsorption indicator (see Section 10.75(c)), or (b) by a potentiometric method with a silver electrode (see Chapter 15). 

Procedure. Place 50 mL of the supporting electrolyte in the beaker and add some of the same solution to the tube carrying the silver electrode so that the liquid level in this tube is just above the beaker. Pass nitrogen into the solution until the pH is 7.0. Pipette 10.00 mL of either 0.01 M or 0.001 M hydrochloric acid into the cell. Continue the passage of nitrogen. Proceed with the titration as described under (a) above. 

The silver wire can be regarded as a silver electrode with a potential given by the Nernst equation as... 

Commercial forms of the electrode are available and in general are similar to the calomel electrode depicted in Fig. 15.1(h) with the replacement of the mercury by a silver electrode, and calomel by silver chloride. The remarks concerning clogging of the sintered disc, and the use of ion exchange membranes and double junctions to reduce this are equally applicable to the silver-silver chloride electrode. 

The indicator electrode employed in a potentiometric titration will, of course, be dependent upon the type of reaction which is under investigation. Thus, for an acid-base titration, the indicator electrode is usually a glass electrode (Section 15.6) for a precipitation titration (halide with silver nitrate, or silver with chloride) a silver electrode will be used, and for a redox titration [e.g. iron(II) with dichromate] a plain platinum wire is used as the redox electrode. 

The indicator electrode must be reversible to one or the other of the ions which is being precipitated. Thus in the titration of a potassium iodide solution with standard silver nitrate solution, the electrode must be either a silver electrode or a platinum electrode in the presence of a little iodine (best introduced by adding a little of a freshly prepared alcoholic solution of iodine), i.e. an iodine electrode (reversible to I-). The exercise recommended is the standardisation of silver nitrate solution with pure sodium chloride. 

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