Coulometry coulometer

Coulometry Coulometer background electrolyte, and that the other components be nonconductive Requires the quantitative removal of water from sample by... 

In coulometry, one must define exactly the amount of charge that was consumed at the electrode up to the moment when the endpoint signal appeared. In galvanosta-tic experiments (at constant current), the charge is defined as the product of current and the exactly measured time. However, in experiments with currents changing continuously in time, it is more convenient to use special coulometers, which are counters for the quantity of charge passed. Electrochemical coulometers are based on the laws of Faraday with them the volume of gas or mercury liberated, which is proportional to charge, is measured. Electromechanical coulometers are also available. 

The way in which these alternatives with their particular measuring characteristics are carried out can be best described by (1) controlled-potential coulometry and (2) coulometric titration (controlled-current coulometry). Both methods require an accurate measurement of the number of coulombs consumed, for which the following instrumental possibilities are available (a) chemical coulometers, (b) electrochemical coulometers and (c) electronic coulometers. 

Coulometry carried out with a proper coulometer is usually highly accurate. We need to note, however, that it is an extremely common experimental practice to obtain Q as the integral of current 1 with time t An even more approximate method is to draw a graph of current (as y ) against time (as x ) and then ascertain the area under the curve. If the current is constant, then charge is obtained as Q = l xt This latter... 

Reduction of analyte occurs at the cathode (on the right-hand side of the cell). Once formed, however, the reduced form of the analyte couple diffuses across the cell - it may also be swept along by the stirred solution - and/or be re-oxidized again at the anode. Clearly, a single molecule of analyte could be oxidized and reduced many times, thus leading to an artificially high charge at the coulometer. For this reason, the two halves of the coulometry cell should be separated if possible, e.g. with a semipermeable membrane or frit, or we should ensure that the product of electron transfer should be a solid, i.e. it is immobilized as soon as it is formed. 

This integrating circuit is used to give linear and cyclic voltage scans in polarography and voltammetry. It is also used as a coulometer in coulometry. 

D. N. Craig. J. I. Hoffman. C. A. Law. and W. J. Hamer, Determination of the Value of the Faraday with a Silver-Perchloric Acid Coulometer, J. Res. Natl. Bur. Stds. 1960, 64A, 381 H. Diehl, High-Precision Coulometry and the Value of the Faraday, Anal. Chem. 1979,51, 318A. 

Coulometer — (previously coulombmeter, or also voltameter) A coulometer is an instrument to measure charge, i.e., to perform -> coulometry. Richards and Heimrod [i] suggested in 1902 the name coulometer to replace the previously used term voltameter . Modern coulometers perform electronic integration of - current over time. However, the first coulometers utilized - Faraday s law, e.g., by weighing the amount of silver that has been deposited on a silver electrode in a silver electrolyte solution the charge could be calculated (silver... 

Coulometers, like the balance, are basic instruments for absolute analysis and they are still used as the most reliable and precise instruments for the analysis of absolute standards. Coulometers are frequently used in elucidating electrochemical reactions because they allow determining the number of transferred electrons when the molar amount of electrolyzed compound is known (-> Faraday s law). When the charge is measured as a function of time, the technique is called chrono-coulometry. See also coulometric titration. 

A reaction coulometer has been used to determine the rate of heat release from these combustible volatiles (65). Table VIII shows these results on the effect of inorganic additives that were obtained by using reaction coulometry. The treated cellulose samples decomposed at lower temperatures and produced less heat than the untreated. Addition of 5% NaOH reduced the heat of combustion of cellulose volatiles at 500 °C to less than one-half of untreated (65). 

PotentiostatS and Coulometers For controlled-potential coulometry, we use a potentiostat similar in de.sign to that shown in Figure 22-8. Generally, however, the potentiostat is automated and equipped with a computer or an electronic current integrator that gives the charge in coulombs necessary to complete the reaction, as shown in Figure 22-12. 

Coulometry. Faraday s laws of electrolysis, enunciated in 183 form the basis of coulometric techniques. By the beginning of the present century the silver coulometer had been shown to provide an accurate means for the measurement of quantities of electricity. An excellent survey of various chemical and other coulometers is available ( ). The electronic digital coulometer, first described in 1962 ( ), was a major practical advance. 

The development of electronic instrumentation has effectively displaced mechanical devices as the prime element of coulometers. Those in use today almost universally utilize a capacitor charged by the current being measured. The capacitor is placed in the feedback loop of an operational amplifier (op amp), the output of which registers the time-integral of the current, a principle previously used in analog computation circuitry (20). One of the first to use this method for chemical coulometry was Booman in 1957 (21). The difficulty with this approach is that a capacitor of reasonable size (Booman used a 30-yF non-electrolytic capacitor, a giant of its kind) cannot accomodate sufficient charge at the potentials suitable for chemical use. This limitation has been overcome in several ways. 

A complete system providing both a sensor and an actuator would be ideal in the field of process control, but because of a lack of truly reliable chemical sensors on the market the concept has not been widely implemented. One exception relates to the analytical method of coulometry, a technique that offers great potential for delivering chemical compounds to a controlled reaction. Especially attractive in this context is the method of constant- current coulometry, which can be carried out with an end-point sensor and a coulomet-ric actuator for maintaining a generator current until the end-point has been reached. In this case both of the required devices can be miniaturized and constructed with the same technology. 

During coulometry at constant potential, the total amount of charge (g) is obtained by integration of the current (7) - time (0 curve or g can be determined directly by using a coulometer (electronic integrator). In principle, the end point 1 = 0, i.e., when the concentration of the species under study becomes zero, can be reached only at infinite time, however, in practice the electrolysis is stopped when the current has decayed to a few percent of the initial values. The change of I and g as a function of time at a constant potential >> e. for a stirred solutions and for an uncomplicated electrolysis, is as follows ... 

A platinum button and a platinum wire served as the working and auxilary electrodes respectively for cyclic and differential pulse voltam-metric measurements made using a conventional three-electrode configuration and an IBM Model EC225 voltammetric analyzer. A saturated calomel electrode (SCE), separated from the bulk of the solution by a fritted glass disk, was used as the reference electrode. A Princeton Applied Research Model 173 potentiostat with Model 179 digital coulometer was used for controlled-potential electrolysis (CPE) and coulometry. All potentials are reported versus aqueous SCE. 

Bard and Solon (13) have recently described a new electronic coulometer for use in high speed coulometry. The instrument consists basically of a voltage-to-frequency converter which reduces the input potential across a precision resistor into a signal suitable for measurement by a scaler-counter. It appears to have the important advantages of excellent linearity and extremely rapid response time and can be adjusted to read directly in coulombs, micro-equivalents, etc. 

Alfonsi (62, 92-95) has applied controlled-potential coulometric techniques to the determination of copper in a wide variety of brasses, bronzes, solders, and other alloys. Yamada (96) has constructed a novel capillary type micro-coulometer to estimate traces of copper in eluates from paper chromatograms. In an effort to characterize quantities of trace elements in living bodies, Suzuki and Yamamoto (97) used controlled-potential coulometry to determine 100-600 jxg of copper in plant materials. Farrar and co-workers (98) used similar techniques to recover copper from other fission elements in partially spent reactor fuels. Controlled-potential coulometry can be of considerable value here for remote-control monitoring of high radiation-level facilities (99). 

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