Electromotive potential

From this experiment and the preceding one we should conclude that the non-metallic elements fall in the order F, Cl, Br, 0, I, S (fluorine being strongest) with respect to their activity in aqueous solutions containing free acid. This is approximately the order of the electromotive series for these non-metals. If the solution is made neutral the electromotive potential of oxygen is lowered so that the oxygen is no longer able to displace iodine. 

In a thermocouple, heating one junction of a bimetallic couple and cooling the other produces electromotive force in the circuit. This observation was originally was made by Seebeck in 1821. Besides the use of thermocouples, transistor electronics and semiconductors are important areas of interest for thermoelectric phenomena. Thermocouples made of semiconductors can develop relatively large electromotive potentials and are used to convert heat into electricity. 

The electromotive potential difference ( jj) measured in volts may be related to the corresponding contribution to the Gibbs free energy (4>i — 2) by... 

The amount of work required to add or remove electrons is called the electromotive potential or force (emf) and is designated E. It is measured in volts (joules per coulomb, where a coulomb is a unit of electric charge or a quantity of electrons). 

So, the overall reaction in the fuel cell is the combination of hydrogen and oxygen to yield water and an electromotive potential. Because the oxidation product of the fuel is essentially H+, the polyelectrolyte membrane is also addressed as a proton conducting membrane or a proton exchange membrane [90]. 

Copper(ll) ions begin to enter solution, resulting In a positive charge and an electromotive potential. 

The transport of co-ions is a function of the leakage flow of ions through the fiber wall. No flow occurs through a perfectly semi-permeable fiber wall and the electromotive potential observed corresponds to the Nernst potential. However, when leakage flow occurs, as it does with real membranes, one can assume that the flow occurs through water filled pores. The cross-sectional area of pores allowing such flow, divided by the total available cross-sectional area is then a measure of the semi-permeability of the membrane. On this basis the rejection of ionic species is estimated by ... 

In 1890 Ostwald introduced the normal calomel electrode ( ) as a reference electrode of fixed potential in equilibrium with aqueous potassium chloride solution. Ostwald calibrated a normal calomel electrode against a dropping mercury electrode and obtained a mean value of 0.560 volt. He referred to this value as the absolute value of the electrode potential of the normal calomel electrode. Ostwald recommended the use of the normal calomel electrode as the null electrode, or the standard electrode, in the measurement of the potential difference at a metal-solution junction. He suggested that the electromotive potential measurement with the normal calomel... 

Switching behavior of molecule systems can lead to changes in spectral characteristics, electromotive potentials, conductivity, polarization and optical rotatory dispersion. In all cases the physical properties exWbited are characterized by measurements of macroscopic properties. Such properties have been studied and understood for years. What is new is the introduction of additional materials and a possible better understanding of the microscopic factors that lead to the macroscopic properties. The goals of this research at the present time are scientific in nature. 

Examples of thermal detectors include thermocouples, bolometers and pyroelectric detectors. A thermocouple has a junction of two dissimilar metals. When incident radiation is absorbed at the junction, the temperature rise causes an increase in the electromotive potential developed across the junction leads. A bolometer is a detecting device which depends on a change of resistance with temperature. 

Electroless electrolytic cleaning relies on the difference in electromotive potentials to remove material from one surface and deposit it on another (i.e. displacement plating. Sec. 1.1.2). 

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. 

The most significant chemical property of zinc is its high reduction potential. Zinc, which is above iron in the electromotive series, displaces iron ions from solution and prevents dissolution of the iron. For this reason, zinc is used extensively in coating steel, eg, by galvanizing and in zinc dust paints, and as a sacrificial anode in protecting pipelines, ship hulls, etc. 

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). 

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