Electromotive force potential

IR drop to obtain the real electrode potential, or in some cases one can compensate for the ir drop during potential control. During the measurement of an electromotive force (potential measurement without any current flowing), the IR drop is always zero, and the position of die reference electrode is immaterial. 

TABLE 4.2 Eixcerpted listing of standard electromotive force potentials"... 

Standard Electromotive Force Potentials Galvanic Series of Metals Galvanic Series of Metals in Sea Water Corrosion... 

TABLE 57.1 Standard Electromotive Force Potential (Reductions Potentials) for Elements Commonly Found in Electronic Assemblies... 

Ei electromotive force (potential) of the cell assembly Ei cell constant a. activity of primary ion I in sample solution a/, activity of interfering ion J in sample solution Zi, Zj. charges of I and J, respectively and Kij potentiometric selectivity coefficient of primary ion I against interfering ion. An ideal selective electrode would show all Kr = 0. 

A special example of electrical work occurs when work is done on an electrochemical cell or by such a cell on the surroundings -w in the convention of this article). Themiodynamics applies to such a cell when it is at equilibrium with its surroundings, i.e. when the electrical potential (electromotive force emi) of the cell is... 

Section 8 now combines all the material on electrolytes, electromotive force, and chemical equilibrium, some of which had formerly been included in the old Analytical Chemistry section of earlier editions. Material on the half-wave potentials of inorganic and organic materials has been thoroughly revised. The tabulation of the potentials of the elements and their compounds reflects recent lUPAC (1985) recommendations. 

When strips of reactive metals such as zinc are placed in water a potential difference, die electromotive force (emf), is set up die metal becomes negatively charged due to die transfer of zinc ions to die solution and die build-up of electrons on die metal. The metal strips or rods are termed die... 

As a result, the electromotive force (EMF) of the cell is zero In the presence of fluoride ions, cerium(IV) forms a complex with fluoride ions that lowers the cerium(IV)-cerium(IIl) redox potential The inner half-cell is smaller, and so only 5 mL of cerium(IV)-cenum (III) solution is added To the external half-cell, 50 mL of the solution is added, but the EMF of the cell is still zero When 10 mL of the unknown fluonde solution is added to the inner half-cell, 100 mL of distilled water IS added to the external half-cell The solution in the external half-cell is mixed thoroughly by turning on the stirrer, and 0 5 M sodium fluonde solution is added from the microburet until the null point is reached The quantity of known fluonde m the titrant will be 10 times the quantity of the unknown fluoride sample, and so the microburet readings must be corrected prior to actual calculations... 

If electron flow between the electrodes is toward the sample half-cell, reduction occurs spontaneously in the sample half-cell, and the reduction potential is said to be positive. If electron flow between the electrodes is away from the sample half-cell and toward the reference cell, the reduction potential is said to be negative because electron loss (oxidation) is occurring in the sample halfcell. Strictly speaking, the standard reduction potential, is the electromotive force generated at 25°C and pH 7.0 by a sample half-cell (containing 1 M concentrations of the oxidized and reduced species) with respect to a reference half-cell. (Note that the reduction potential of the hydrogen half-cell is pH-dependent. The standard reduction potential, 0.0 V, assumes 1 MH. The hydrogen half-cell measured at pH 7.0 has an of —0.421 V.)... 

Electric pofcTidfff, ), potential difference, or electromotive force (emf, E, e) have units of volts and refer to the energy change when a charge is moved from one point to another m an electric field. 

The theory of Kelvin (1854), developed in the preceding, section, stands midway between these two hypotheses, in that it assumes the existence of potential differences at the junctions, playing the role postulated by Clausius, and also admits the production of electromotive forces in the interior of the homo-, geneous wires due to inequalities of temperature in the latter, these inequalities giving rise to the flow of heat which is regarded as essential in the theory of Kohlrausch. 

The sign of the electrode potential is arbitrarily defined as follows. A kation electrode (e.g., Zn in ZnS04 aq.) is said to be positive when it is positive to a unimolar (f — 1) solution of its ions an anion electrode e.g., CI2 in KC1) is said to be positive when it is positive to a unimolar solution of its ions. If a cell is made up of electrodes reversible with respect to any kinds of ions, its electromotive force is the algebraic difference of its electrode potentials, provided the electromotive force at the contact of the two solutions, due to diffusion (cf. Jahn, Elcktro-chcmie) is neglected. 

The electromotive force of a cell with solutions of given concentrations may be calculated by subtracting the electrode potentials so obtained. 

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-... 

Each metal or metal area will develop an electrode with a measurable electrical potential. This potential can be referenced to that of a standard hydrogen electrode, which by convention is set at zero. Thus, all metals have either a higher or lower potential compared to hydrogen, and a comparative list of metals can be produced indicating their relative nobility. This list is the galvanic or electrochemical series and measured as an electromotive force (EMF). 

Cathodic protection apparatuses are well proven, widely used devices and are not to be confused with magnetic devices gadgets ) or other similar but generally less than satisfactory items of capital equipment. Cathodic protection devices reverse the tendency of a metal to go into solution at the anode (corrosion) by the application of a counter-potential. This counter-potential or electromotive force (EMF) is provided either from a permanent external source such as a battery or rectifier or from the installation of a sacrificial anode. 

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