Galvanic cells defined

At this stage it should be pointed out that the original definition of pH = —log cH (due to Sorensen, 1909 and which may be written as pcH) is not exact, and cannot be determined exactly by electrometric methods. It is realised that the activity rather than the concentration of an ion determines the e.m.f. of a galvanic cell of the type commonly used to measure pH, and hence pH may be defined as... 

Galvanic cells of the Nerst type are also termed cells with dissolution membranes or solvent type membranes [3]. Such systems are defined by the distribution equilibria in which all ions, present in aqueous and in organic solvents, participate (Section III.A). The general examples of the liquid concentration and chemical galvanic cells of this type are presented in the form of Schemes 8 and 9. 

Two types of methods are used to measure activity coefficients. Potentiometric methods that measure the mean activity coefficient of the dissolved electrolyte directly will be described in Section 3.3.3. However, in galvanic cells with liquid junctions the electrodes respond to individual ion activities (Section 3.2). This is particularly true for pH measurement (Sections 3.3.2 and 6.3). In these cases, extrathermodynamical procedures defining individual ion activities must be employed. 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

Define cell, half-cell, anode, cathode, electrolytic cell, and galvanic cell. 

A battery is defined as a set of galvanic cells connected in series. The negative electrode of one cell is connected to the positive electrode of the next cell in the set. The voltage of a set of cells connected in series is the sum of the voltages of the individual cells. Thus, a 9-V battery contains six 1.5-V dry cells connected in series. Often, the term battery is also used to describe a single cell. For example, a 1.5-V dry cell battery contains only a single cell. 

The standard potential for a redox reaction is defined for a galvanic cell in which all activities are unity. The formal potential is the reduction potential that applies under a specified set of conditions (including pH, ionic strength, and concentration of complexing agents). Biochemists call the formal potential at pH 7 E° (read "E zero prime"). Table 14-2 lists E° values for various biological redox couples. 

Define anode and cathode with reference to a specific galvanic cell. 

The negative logarithm of hydrogen ion concentration was defined by Sorensen S as the pH.t Sorensen did not actually measure hydrogen ion concentrations, but something more nearly related to activities. He measured the emf of galvanic cells such as... 

Electroanalytical chemistry has been defined as the application of electrochemistry to analytical chemistry. For the determination of the composition of samples, the three most fundamental measurements in electroanalytical chemistry are those for potential, current, and time. In this chapter several aspects relating to electrode potentials are considered current and time as well as further consideration of potentials are treated in Chapter 14. The electrode potentials involved in the classical galvanic cell are of considerable theoretical and practical significance for the understanding of many aspects not only of electroanalytical chemistry but also of thermodynamics and chemical equilibria, including the measurement of equilibrium constants. 

There is another way in which electrons can be rearranged in a chemical reaction, and that is through a wire. Electrochemistry is redox chemistry wherein the site for oxidation is separated from the site for reduction. Electrochemical setups basically come in two flavors electrolytic and voltaic (also known as galvanic) cells. Voltaic cells are cells that produce electricity, so a battery would be classed as a voltaic cell. The principles that drive voltaic cells are the same that drive all other chemical reactions, except the electrons are exchanged though a wire rather than direct contact. The reactions are redox reactions (which is why they produce an electron current) the reactions obey the laws of thermodynamics and move toward equilibrium (which is why batteries run down) and the reactions have defined rates (which is why some batteries have to be warmed to room temperature before they produce optimum output). 

By the principles of thermodynamics discussed in Chapter V., a galvanic cell will yield the maximum amount of work when the production of electricity takes place reversibly, that is to say, when the changes which take place both inside and outside the cell are completely reversed when an equally strong current is sent in the opposite direction through the cell. This can only occur when the current flowing through the cell is infinitely small, so that the irreversible production of Joule heat inside the cell is avoided. The electrode potential of the cell on open circuit (measured by the compensation method, for example) is therefore a measure of the maximum electrical work which the cell can do. It is also a measure of the chemical affinity of the reaction as defined on p. 318, Chapter IX. 

Define the terms anode and cathode and give the convention that is used to represent a galvanic cell (Section 17.1, problems 1-2). 

A galvanic cell consists, as we have seen, in principle of an oxidation component in a container that draws electrons through a wire from a reduction component in another solution. The driving force causing the transfer is called the cell potential or sometimes the electromotoric force (in short EMK). The unit for cell potentials is volt (in short V) defined as joules of work coulomb charge transferred. The electromotoric force is defined as ... 

Even though we are capable of measure the galvanic cell potential by using the voltmeter placed between the two electrodes, it is not possible to ensure the two half cell potentials. We thereby have to define a half cell reaction that we may determine the other half cell potentials from. As mentioned above we have chosen to let the half cell potential for the standard hydrogen electrode be the zero point. On the basis on this it is possible to determine all the other half cell potentials. In the present case we may thereby say that the half cell reaction ... 

To define a unique solution, we must specify the corresponding boundary and initial conditions. Normally electrolyte solutions are in contact with or bounded by electrodes. An electrode in its simplest form is a metal immersed in an electrolyte solution so that it makes contact with it. For example, copper in a solution of cupric sulfate is an example of an electrode. A system consisting of two electrodes forms an electrochemical cell. If the cell generates an emf by chemical reactions at the electrodes, it is termed a galvanic cell, whereas if an emf is imposed across the electrodes it is an electrolytic cell (Fig. 6.1.1). If a current is generated by the imposed emf, the electrochemical or electrolytic process that occurs is known as electrolysis. Now whether or not a current flows, the electrolyte can be considered to be neutral except at the solution-electrode interface. There a thin layer, termed a Debye sheath or electric double layer, forms that is composed predominately of ions of charge opposite to that of the metal electrode. We shall examine this double layer in Section 6.4, but for our purposes here it may be neglected. 

When a current I flows in a galvanic cell, such as the one shown in Figure lb, between the catalyst, or working (W), electrode and the counter (C) electrode, then the potential difference deviates from its open-circuit value V. The galvanic cell overpotential q vc defined from ... 

A combination of any two dissimilar metallic conductors can be used to construct a galvanic cell. The cell potential defines the measure of the energy available in a cell. A high cell potential signifies a vigorous spontaneous redox reaction. The unit of potential is the volt, V. A Daniell cell, for example, has a potential of 1.1 V. 

Where ceii is the cell potential in volts, defined to be positive F is the Faraday constant, 96485 C mol and n is the number of moles of electrons that migrate from anode to cathode in the cell reaction. Thus a galvanic cell is also a Gibbs energy meter. 

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