Measurement of Electric Quantities

Resistors whose values can be varied are termed potentiometers. They are made of the same materials as fixed resistors but have a movable wiper to contact a coil of resistance wire or a strip of resistive film at any point along the resistor. Potentiometers used only occasionally to adjust a circuit are called trimmers, while those employed for high-wattage applications such as control of heating mantles and ovens are called rheostats. Precision potentiometers have played an important role in the measurement of electrical quantities and are considered in detail in a later section. 

The different variants of the sensor application have become obvious from observing Figure 4.5. A mechanical load on piezoelectric material induces an electric field strength field when the electrodes are disconnected and an electric flux density field when the electrodes are connected. This corresponds to the possibilities of measurement of electric quantities. As with the actuator application, only the ideal cases shall be considered here, leaving the complications of the actual circuits, including the necessary amplification, to the competent electrical engineering literature, see for example Tichy and Gautschi [174]. 

Measurement of the quantity of electricity used in an electrochemical reaction at constant potential or constant current. 

COULOMETER. Also known as cnulombmeier, a device for the measurement of electric currem. Originally developed (1916) by the U.S. National Bureau of Standards, the silver coulometer consists of a small platinum vessel, acting as the cathode, into which a pure silver anode is immersed. An aqueous solution of silver nitrate (15% AgNO<. wl) of very high purity is used as the electrolyte, In use. both the quantity of silver deposited and the lime are carefully noted. These measurements permit a calculation of the average currcnl strength. 

Apparatuses called coulometers are used for the measurement of the quantity of electricity. These are in principle electrolytic cells in which a given electrochemical process is allowed to proceed under exactly defined conditions it must be ascertained that the process in question proceeds in one direction only thus guaranteeing 100 p. c. current efficiency of electrolysis. From the amounts of products obtained which must be suitably determined, the quantity of electricity which passed through the cell can be calculated by applying Faraday s law. 

Analogous results have been reported from the systematic measurements of electrical conductivity and transference numbers of ions (// and tf) in black foam films [336] and parallel measurements of these quantities in highly concentrated surfactant/water system [337], Furthermore, it has been found that while the electrical conductivity of CBF depends on the electrolyte concentration in the initial solution, that of NBF does not. The transference numbers of the ions measured for films and a gel obtained from NaDoS-NaCl-HCl system are given below... 

The electrolytic gas coulometer is useful for the approximate measurement of small quantities of electricity the total volume of hydrogen and oxygen liberated in the electrolysis of an aqueous solution of sulfuric acid or of sodium, potassium or barium hydroxide can be measured, and from this the quantity of electricity passed can be estimated. If the electrolyte is dilute acid it is necessary to employ platinum electrodes,... 

Measurements of the extent of H2 dissociation in flowing discharges operated at pressures up to a few mbar are available and have been analyzed in Ref.15). Uncer-tainities in the selection of rate constants and of electric quantities relevant to the discharge model are rather high. Rate constants for the dissociation process, calculated for DEM from v = 0 only (k ) and utilizing a maxwellian edf, can fit the experimental data. The accuracy of this fit is likely to be within one order of magnitude. 

The methods of coulometry are based on the measurement of the quantity of electricity involved in an electrochemical electrolysis reaction. This quantity is expressed in coulombs and it represents the product of the current in amperes by the duration of the current flow in seconds. The quantity of electricity thus determined represents, through the laws of Faraday, the equivalents of reactant associated with the electrochemical reaction taking place at the electrode of significance. In the analytical chemistry sense, the process of coulometry, carried out to the quantitative reaction of the analyte in question, either directly or indirectly, will yield the number of analyte equivalents involved in the sample under test. This will lead to a quantitative determination of the analyte in the sample. Analytical coulometry can be carried out either directly or indirectly. In the former the analyte usually reacts directly at the surface of either the anode or cathode of the electrolysis cell. In the latter, the analyte reacts indirectly with a reactant produced by electrolytic action at one of the electrodes in the electrolysis cell. In either case, the determination will hinge on the number of coulombs consumed in the analytical process. 

Having considered a few cases of affinity m which the determination is carried out by means of vapour pressure or concentration measurements, it is necessary to discuss the electrical method, especially as this may be employed in cases to which other methods are quite inapplicable As already pointed out, if we take the faraday (96,540 coulombs) as the unit of electrical quantity, ie the quantity of electricity associated with 1 gram equivalent of any ion, and if the valency of the ion of the chosen substance be n, the electrical energy connected with the transformation of 1 gram ion is n x E or E, where E is the electromotive VOL II 23... 

Electrolytic methods include some of the most accurate, as well as most sensitive, instrumental techniques. In these methods, an analyte is oxidized or reduced at an appropriate electrode in an electrolytic cell by application of a voltage (Chapter 12), and the amount of electricity (quantity or current) involved in the electrolysis is related to the amount of analyte. The fraction of analyte electrolyzed may be very small, in fact negligible, in the current-voltage techniques of voltammetry. Micromolar or smaller concentrations can be measured. Since the potential at which a given analyte will be oxidized or reduced is dependent on the particular substance, selectivity can be achieved in electrolytic methods by appropriate choice of the electrolysis potential. Owing to the specificity of the methods, prior separations are often uimecessary. These methods can therefore be rapid. 

This study presents results of measurements of electrical conductivity of high density ceramics (porosity about 2 %) made of high purity OIL perovskite mentioned above as well as the OIL doped with oxides of calcium CaO and hafnium Hf02 in quantity up to 0,5 mol. %. 

The experimental tools of electrochemists were, until a few years ago, mainly rather simple measurements of electrical, physical and chemical quantities. Using a broad variety of experimental methods today called classical electrochemical methods , they were able to provide models of electrified interfaces with respect to both structure and dynamics. Unfortunately their results were in many cases of a very macroscopic nature, any interpretations of the model with respect to the microscopic structure and mechanistic aspects of the dynamics and reaction were only more or less reasonable derivations. This gap, which caused many misunderstandings of puzzling features in electrochemical processes and interfaces, has started to close. The use of an enormous variety of spectroscopic and surface analytical tools in investigations of these interfaces has considerably broadened our knowledge. In many cases microscopic models based on the results of these studies with non-traditional electrochemical methods have enabled us to understand many hitherto strange phenomena in a convincing way. 

The transport of mass in the form of ions through the electrolyte can often be attributed to a chemical reaction or a transport process at an electrode. In this way reaction rates can be measured electrically. It is often possible to analyze reaction mechanisms in detail by a combination of rate measurements by means of the electrical current with measurements of thermodynamic quantities— in particular, chemical potentials—by means of the emf of the galvanic cell. More details on kinetic investigations using galvanic cells will be given in Section V.B. 

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