APPLICATION OF GALVANIC CELLS

The sacrificial anode method is the practical in-service application of galvanic cell. Hence, the corrosion potential for a sacrificial anode material must be lower than that of the structure to be protected. 

This chapter explains the fundamental principles and applications of galvanic cells, the thermodynamics of electrochemical reactions, and the cause and prevention of corrosion by electrochemical means. Some simple electrolytic processes and the quantitative aspects of electrolysis are also discussed. 

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 ... 

These equations are of great importance in chemistry and physics. They were first derived by Helmholtz and independently by Willard Gibbs in connection with the theory of galvanic cells. The former of the two equations is the more generally useful in chemistry, since it is easier as a rule to keep the pressure on a system constant than to maintain it at constant volume. We shall have frequent examples of the-application of equation (21) in the subsequent chapters of this book. 

The only assumption made in the derivation of equation (2) (p. 347), apart from the two laws of thermodynamics, was the validity of the simple laws of solution. The equation is therefore also applicable to reactions which proceed practically to completion, so that the equilibrium cannot be investigated directly. This is the case in the great majority of galvanic cells, especially those which are used in practice, as it is only under these conditions that the equilibrium constant and therefore the e.m.f. can assume considerable values. It is, of course, impossible to predict the value of the e.m.f. in such cases (as K is unknown),... 

The electrochemistry of galvanic cells leads to a variety of applications in industry and in everyday life. This section focuses on two of the most important batteries to store energy and fuel cells to convert chemical energy to electrical energy. 

In order to decide the suitability of a certain melt in technical practice, an in-depth knowledge of its physico-chemical properties is unavoidable. The present database of the properties of inorganic melts is relatively broad. Many properties are known, such as phase equilibria, enthalpies of fusion, heat capacities, density, electrical conductivity, viscosity, surface tension, emf of galvanic cells of many molten systems, the measurement of which was stimulated first by their technological application. 

Investigations of Braune and Koref.—We are indebted to Braune and Koref (77, and particularly 986) for an extremely careful and painstaking test of the application of the Heat Theorem to condensed systems. The test was made on a number of galvanic cells in exactly the same manner as that already employed by U. Fischer. In all the cells U was found to have nearly the same value, whether determined from the temperature coefficient of the potential or by direct thermochemical means this provides a guarantee that the cells under observation were actually controlled by the process assumed to be supplying the current, though as a matter of fact there was hardly any doubt of this in the cases examined. 

In the practical application of equation (157) in the above important case there arises, however, a difficulty in that the temperature coefficient of the E.M.F. of galvanic cells has always been measured at constant pressure the determination at constant volume is hardly practicable directly. 

One of the well-known applications of the electrochemistry is the use of galvanic cells in batteries. A battery is in principle just a group of galvanic cells in series, in which the potential of each cell is summed up to give a higher voltage across the batteiy. Batteries are used for a variety of purposes in our daily life. There are several different principles of how a battery may be build. In the following examples we will look at three types of batteries. 

Galvanic cells and batteries also played a decisive role in the development of such branches of physics as electrodynamics and electromagnetism. They were used in experiments resulting in the discovery of the well-known laws of Ampere, Ohm, Joule, Faraday and many others. The existence of galvanic cells favored practical application of electric current. Electrical telegraph and electric motors were invented and galvano-plastics was developed. 

Counting Electrons Coulometry and Faraday s Law of Electrolysis 21-7 Commercial Applications of Electrolytic Cells Voltaic or Galvanic Cells 21-8 The Construction of Simple Voltaic Cells... 

Voltaic cells use spontaneous oxidation-reduction reactions to convert chemical energy into electrical energy. Voltaic cells are also called galvanic cells. The most common application of voltaic cells is in batteries. 

Galvanizing iron sheet is an example of useful application of galvanic action or cathodic protection. Iron is die cathode and is protected against corrosion at the expense of the sacrificial zinc anode. Alternatively, a zinc or magnesium anode may be located in the electrolyte close to the structure and may be coimected electrically to the iron or steel. This method is referred to as cathodic protection of the structure. Iron or steel can become the anode when in contact with copper, brass, or bronze however, iron or steel corrode rapidly while protecting the latter metals. Also, weld metal may be anodic to the basis metal, creating a corrosion cell when immersed (Fig. 5). 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

Fig. 5.24. The electrochemical properties of the galvanic cell shown have been studied under high pressure shock compression. The cell is composed of anode, electrolyte, and cathode materials studied in independent applications of thermal batteries.

Although iron pipes suffer from the same corrosion risk as steel pipelines, associated with the generation of a galvanic cell with a small anode and a large cathode, the risk is mitigated for iron pipelines because the electrical continuity is broken at every pipe joint. For this reason long-line currents are uncommon in iron lines and cathodic protection is rarely necessary. It also accounts for the ability to protect iron lines by the application of nonadherent polyethylene sleeving . 

As in aqueous electrochemistry it appears that application of a potential between the two terminal (Au) electrodes leads to charge separation on the Pt film so that half of it is charged positively and half negatively8 thus establishing two individual galvanic cells. The Pt film becomes a bipolar electrode and half of it is polarized anodically while the other half is polarized cathodically. The fact that p is smaller (roughly half) than that obtained upon anodic polarization in a classical electrochemical promotion experiment can be then easily explained. 

Lithium metal had few uses until after World War II, when thermonuclear weapons were developed (see Section 17.11). This application has had an effect on the molar mass of lithium. Because only lithium-6 could be used in these weapons, the proportion of lithium-7 and, as a result, the molar mass of commercially available lithium has increased. A growing application of lithium is in the rechargeable lithium-ion battery. Because lithium has the most negative standard potential of all the elements, it can produce a high potential when used in a galvanic cell. Furthermore, because lithium has such a low density, lithium-ion batteries are light. 

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