Force electromotive

Just as, in hydrodynamics, the energy of falling water is determined not only by the amount of water that falls, but also by the height of fall, so also in electricity, the electrical energy involves the two factors, amount of electridty, and fall of potential or electromotive force. This latter ctor constitutes the driving force, and its measurement is of the utmost importance. 

For this reason, and also on account of the problems which measurements of the electromotive force enable us to solve, the latter form one of the most important sections of physical chemical practice. 

Measurement of the E.M.F. of a Cell—Outline of the Method.— The method usually employed for the determination of the e.m.f. of a cell is essentially that known as the Poggendorff compensation method. 

If a source of electricity, A (Fig. 67), of constant e.m.f. is connected with the two ends of a wire, CD, of uniform resistance, then the fall of potential along the wire will be imiform. The difference of potential between C and any point E of the wire will be proportional to the distance CE, and will be equal to the fraction of the total fall of potential along the wire. If another 

Apparatus.—The Working Cell.—As working cell A, one must employ a cell with a greater e.m.f. than is to be measured. Since in all cases to be studied here, the e.m.f. is less than 2 volts, the most convenient working cell to use is a lead accumulator, which, when fully charged, has an e.m.f. of somewhat over 2 volts. In order that the cell shall not nm down too rapidly, and the e.m.f. therefore, fell, an accumulator of fairly large capacity should be employed. Where large fixed cells are not available, a portable accumulator of 30 to 40 ampere-hours capacity is very suitable. 

Multiplying the anode reaction by 2 and then summing the half-cell reactions gives 

EXERCISE 20.S Give the overall cell reaction for the voltaic cell 

To fully specify a voltaic cell, it is necessary to give the concentrations of solutions or ions and the pressure of gases. In the cell notation, these are written within parentheses for each species. For example, 

Electrical work = charge X potential difference Corresponding SI units for the terms in this equation are 

The Faraday constant, F, is the magnitude of charge on one mole of electrons it equals 9.65 X 10 C per mole of electrons (96,500 C/mol e ). The faraday is a unit of charge equal to 9.65 X 10 C. In moving this quantity of charge (one faraday 

---

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... 

In electrochemical cells (to be discussed later), if a particular gas participates in a chemical reaction at an electrode, the observed electromotive force is a fiinction of the partial pressure of the reactive gas and not of the partial pressures of any other gases present. 

A second source of standard free energies comes from the measurement of the electromotive force of a galvanic cell. Electrochemistry is the subject of other articles (A2.4 and B1.28). so only the basics of a reversible chemical cell will be presented here. For example, consider the cell conventionally written as... 

The changes, however, are both numerous and significant. First of all, there is a change in the organization of the subject matter. For example, material formerly contained in the section entitled Analytical Chemistry is now grouped by operational categories spectroscopy electrolytes, electromotive force, and chemical equilibrium and practical laboratory information. Polymers, rubbers, fats, oils, and waxes constitute a large independent section. 

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. 

Another ak pollutant that can have very serious effects is hydrogen sulfide, which is largely responsible for the tarnishing of silver, but also has played a destmctive role in the discoloration of the natural patinas on ancient bronzes through the formation of copper sulfide. Moreover, a special vulnerabihty is created when two metals are in contact. The electromotive force can result in an accelerated corrosion, eg, in bronzes having kon mounting pins. 

Fig. 5. Energy requirements of the HaH-Hfiroult cell (23—25). E, decomposition of alumina Eg, depolarization by carbon E, anode overvoltage E, counter electromotive force E, bath voltage drop E, bath bubble voltage F/, anode voltage drop Eg, cathode voltage drop E, external voltage drop ...

In the electromotive force series of the elements, silver is less noble than only Pd, Hg, Pt, and Au. AH provide high corrosion resistance. Silver caimot form oxides under ambient conditions. Its highly reactive character, however, results in the formation of black sulfides on exposure to sulfur-containing atmospheres. 

FIG. 24-14 Dissolved-oxygen electrodes a) polarographic (impress breakdown voltage for oxygen measure current) (h) voltametric (measure electromotive force). 

Mineev, V.N. and Ivanov, A.G., Electromotive Force Produced by Shock Compression of a Substance, Soviet Phys. Uspekhi 19 (5), 400-419 (1976). 

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... 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

ASTM E230-96el. Standard Specification for Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples. American Society for Testing and Materials, 1996. 

---