Voltaic potentials

Another interesting applications area for fullerenes is based on materials that can be fabricated using fullerene-doped polymers. Polyvinylcarbazole (PVK) and other selected polymers, such as poly(paraphcnylene-vinylene) (PPV) and phenylmethylpolysilane (PMPS), doped with a mixture of Cgo and C70 have been reported to exhibit exceptionally good photoconductive properties [206, 207, 208] which may lead to the development of future polymeric photoconductive materials. Small concentrations of fullerenes (e.g., by weight) lead to charge transfer of the photo-excited electrons in the polymer to the fullerenes, thereby promoting the conduction of mobile holes in the polymer [209]. Fullerene-doped polymers also have significant potential for use in applications, such as photo-diodes, photo-voltaic devices and as photo-refractive materials. 

Click Coached Problems for a self-study module on voltaic cell potentials. 

An interesting application of electrode potentials is to the calculation of the e.m.f. of a voltaic cell. One of the simplest of galvanic cells is the Daniell cell. It consists of a rod of zinc dipping into zinc sulphate solution and a strip of copper in copper sulphate solution the two solutions are generally separated by placing one inside a porous pot and the other in the surrounding vessel. The cell may be represented as ... 

The reaction may be regarded as taking place in a voltaic cell, the two half-cells being a C12,2C1 system and a Fe3+,Fe2+ system. The reaction is allowed to proceed to equilibrium, and the total voltage or e.m.f. of the cell will then be zero, i.e. the potentials of the two electrodes will be equal ... 

Voltaic cells 64. 504 Voltammetry 7, 591 anodic stripping, 621 concentration step, 621 mercury drop electrode, 623 mercury film electrode, 623 peak breadth, 622 peak current, 622 peak potential, 622 purity of reagents, 624 voltammogram, 622 D. of lead in tap water, 625 Volume distribution coefficient 196 Volume of 1 g of water at various temperatures, (T) 87... 

A voltaic cell consists essentially of three parts two electrodes, from which the positive and negative electricity leave the cell, and an electrolyte in which the electrodes are contained. Its form is therefore that of an electrolytic cell, and the difference between the two lies only in the condition that in the former we produce an electric current through the agency of the material changes, whereas in the latter we induce these material changes by a current supplied from an external source the same arrangement may therefore serve as either. The direction in which the current flows through the cell will depend on the potential difference between its terminals. 

Similar considerations apply of course to the opposing electromotive forces of polarisation during electrolysis, when the process is executed reversibly, since an electrolytic cell is, as we early remarked, to be considered as a voltaic cell working in the reverse direction. In this way Helmholtz (ibid.) was able to explain the fluctuations of potential in the electrolysis of water as due to the variations of concentration due to diffusion of the dissolved gases. It must not be forgotten, however, that peculiar phenomena—so-called supertension effects—depending on the nature of the electrodes, make their appearance here, and com-... 

The Volta potential is defined as the difference between the electrostatic outer potentials of two condensed phases in equilibrium. The measurement of this and related quantities is performed using a system of voltaic cells. This technique, which in some applications is called the surface potential method, is one of the oldest but still frequently used experimental methods for studying phenomena at electrified solid and hquid surfaces and interfaces. The difficulty with the method, which in fact is common to most electrochemical methods, is lack of molecular specificity. However, combined with modem surface-sensitive methods such as spectroscopy, it can provide important physicochemical information. Even without such complementary molecular information, the voltaic cell method is still the source of much basic electrochemical data. 

This review is arranged as follows. After a short review of the basic definitions and significance of the various potentials which are assumed to exist at free surfaces and interfaces, the nature and most important features of voltaic cells, including their measurement techniques, are... 

In addition, this review has been prepared to promote the term voltaic cell in honor of Alessandro Volta, the inventor of the pile, i.e., an electrochemical generator of electricity. Up to now this name has been used in only a few papers. This term is a logical analogue to the term galvanic cell, particularly in discussions of Volta potential and Gal-vani potential concepts. 

The surface potential of a liquid solvent s, %, is defined as the difference in electrical potentials across the interface between this solvent and the gas phase, with the assumption that the outer potential of the solvent is zero. The potential arises from a preferred orientation of the solvent dipoles in the free surface zone. At the surface of the solution, the electric field responsible for the surface potential may arise from a preferred orientation of the solvent and solute dipoles, and from the ionic double layer. The potential as the difference in electrical potential across the interface between the phase and gas, is not measurable. However, the relative changes caused by the change in the solution s composition can be determined using the proper voltaic cells (see Sections XII-XV). 

Volta potentials are measured by means of voltaic cells, i.e., systems composed of conducting, condensed phases in series, with a gas, liquid dielectric (e.g., decane) or a vacuum (in the case of solid conductors such as metals) gap situated between two condensed phases. The gap, g, may contain a gas such as pure air or nitrogen, saturated with vapors of the liquids present. Owing to the presence of a dielectric, special methods are necessary for the investigation of voltaic cells (see Section IV). 

The basic principle of every measurement of the Volta potential and generally of the investigations of voltaic cells too, in contrast to galvanic cells, may thus be presented for systems containing metal/solution (Fig. 2) and liquid/liquid interfaces (Fig. 3), respectively. This interface is created at the contact of aqueous and organic solutions (w and s, respectively) of electrolyte MX in the partition equilibrium. Of course, electrolyte MX, shown in Fig. 2 and other figures of this chapter, may be different in organic (s) and aqueous (w) phases. 

Thus the Volta potential may be operationally defined as the compensating voltage of the cell. Very often the terms Volta potential and compensation voltage are used interchangeably. It should be stressed that the compensating voltage of a voltaic cell is not always the direct measure of the Volta potential. 

Rabinovich et al. have shown that it is possible to propose an extrather-modynamic definition of single-ion activity, a, as a function of the real potentials of those particles. "" By carrying out the measurements of voltaic cells containing electrodes reversible to the same ionic species in solutions of different concentrations in the same solvent. 

The determination of the real energies of solvation from measurements of the voltaic cells (Section VI) makes it possible to find the absolute electrode potentials in nonaqueous solvents owing to the relation... 

The non situ experiment pioneered by Sass uses a preparation of an electrode in an ultrahigh vacuum through cryogenic coadsorption of known quantities of electrolyte species (i.e., solvent, ions, and neutral molecules) on a metal surface. " Such experiments serve as a simulation, or better, as a synthetic model of electrodes. The use of surface spectroscopic techniques makes it possible to determine the coverage and structure of a synthesized electrolyte. The interfacial potential (i.e., the electrode work function) is measured using the voltaic cell technique. Of course, there are reasonable objections to the UHV technique, such as too little water, too low a temperature, too small interfacial potentials, and lack of control of ionic activities. ... 

Figure 10. The potentials measured in various possible configurations of the voltaic cells with liquid/liquid interfaces.

The possibility of measuring a nonequilibrium liquid junction (diffusion) potential by using voltaic cells has been checked also ... 

The difference between the surface potentials of two solvents (e.g., an organic solvent and water) may also be measured by means of the following voltaic cells " (Fig. 12) ... 

This review has been restricted mainly to clarification ofthe fundamentals and to presenting a coherent view ofthe actual state of research on voltaic cells, as well as their applications. Voltaic cells are, or may be, used in various branches of electrochemistry and surface chemistry, both in basic and applied research. They particularly enable interpretations of the potentials of various interphase and electrode boundaries, including those that are employed in galvanic and electroanalytical cells. 

Every liquid interface is usually electrified by ion separation, dipole orientation, or both (Section II). It is convenient to distinguish two groups of immiscible liquid-liquid interfaces water-polar solvent, such as nitrobenzene and 1,2-dichloroethane, and water-nonpolar solvent, e.g., octane or decane interfaces. For the second group it is impossible to investigate the interphase electrochemical equilibria and the Galvani potentials, whereas it is normal practice for the first group (Section III). On the other hand, these systems are very important as parts of the voltaic cells. They make it possible to measure the surface potential differences and the adsorption potentials (Section IV). 

Another proposed procedure of finding the ionic data is the application of a special salt bridge, which provides practically constant or negligible liquid junction potentials. The water-nitrobenzene system, containing tetraethylammonium picrate (TEAPi) in the partition equilibrium state, has been proposed as a convenient liquid junction bridge for the liquid voltaic and galvanic cells. 

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