High coulometry

From this equation we see that increasing k leads to a shorter analysis time. For this reason controlled-potential coulometry is carried out in small-volume electrochemical cells, using electrodes with large surface areas and with high stirring rates. A quantitative electrolysis typically requires approximately 30-60 min, although shorter or longer times are possible. 

High pressure Hquid chromatography (qv) (138) and coulometry can be used to detect and quantify anthraquinones and thek derivatives in a hydrogen peroxide process working solution. 

Coulometry and amperometry can be distinguished by the extent to which the analyte undergoes a Faradaic reaction at the working electrode, namely complete and partial, respectively. Coulometry is essentially high-efficiency amperometry with working electrodes of large surface area. Successful coulometric or amperometric detection can result only if the applied potential is chosen correctly. 

Electrogravimetry is one of the oldest electroanalytical methods and generally consists in the selective cathodic deposition of the analyte metal on an electrode (usually platinum), followed by weighing. Although preferably high, the current efficiency does not need to be 100%, provided that the electrodeposition is complete, i.e., exhaustive electrolysis of the metal of interest this contrasts with coulometry, which in addition to exhaustive electrolysis requires 100% current efficiency. 

Precision of analytical techniques should only be described verbally or comparatively (e.g., the precision of coulometry is high or the precision of spectrophotometry is better then that of OES ). 

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... 

Figure 5.8 Schematic representation of a coulometry cell, showing how a single molecule of analyte A can be repeatedly oxidized and reduced, thereby giving an overly high charge Q. The subscripts ox and red relate to the oxidized and reduced forms, respectively, while the open-headed arrows represent movement of species A through the solution.

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. 

Chronocoulometric data have been used for accurate determination of the composition of solid phases, as described by Scholz et al. [74-78, 224-228]. It should be noted, however, that coulometric measurements are always more demanding than voltammetric ones, because coulometry offers opportunity for more pronounced systematic errors. However, its use is highly recommended for cases where voltammetry fails [77, 78, 224]. 

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. 

A variation of in-situ volumetry (or manometry) is its combination with high temperature coulometry as shown in Figure 16-4. The A n( ) change in the gas volume due to the reaction is compensated for by a corresponding flux of ions across an appropriate solid electrolyte. This coulometric transport is potentiostatically controlled with a reference electrode (Fig. 16-4). Since 10 p A times 1 s = 10 pC corresponds to ca. 10-11 mol, the sensitivity of the combined volumetry-coulometry matches that of tensiometry. Limitations of this method are leaks and the small electronic transference in the electrolyte. 

The applied current must be 1000 times the residual current to achieve a current efficiency of 99.9%. In many cases, a ratio of applied current to a residual current of 103 is reasonable for applied currents down to about 10 pA using generator electrode areas of about 0.1 cm2. Currents in excess of a few hundred milliamperes are seldom used in constant-current coulometry because the solubility limit of the precursor is reached and/or the experiment may be over too quickly to permit accurate measurement of the time. Heating effects (i2R) are also a problem when high currents are used. 

For monitoring catalytic (enzymatic) products, various techniques, such as spectrophotometry [32], potentiometry [33,34], coulometry [35,36] and amperometry [37,38], have been proposed. An advantage of these sensors is their high selectivity. However, time and thermal instability of the enzyme, the need of a substrate use and indirect determination of urea (logarithmic dependence of a signal upon concentration while measuring pH) cause difficulties in the use and storage of sensors. 

Determination of the amount of substance is thus in direct relation to basic units of the SI system and does not need a RM for comparison. The Faraday constant is one of the fundamental constants (it can be expressed as the product of the electron charge and the Avogadro constant). It enables the attainment of high precision and accuracy and is independent of the atomic weights of the elements in the sample. Its drawback is lower selectivity, a feature common to titration methods. This makes coulometry especially suitable for determination of relatively pure substances used as standards by other (relative) methods. The Faraday constant has been proposed as an ultimate standard in chemistry [3],... 

Three-dimensional electrode — This term is used for electrodes in which the electrode-solution interface is expanded in a three-dimensional way, i.e., the - electrode possesses a significantly increased surface area due to nonplanarity, so that it can be housed in a smaller volume. This can be achieved by constructing corrugated electrodes, reticulated electrodes, -> packed bed electrodes (see also - column electrodes), -> carbon felt electrodes, or fluidized bed electrodes. Three-dimensional electrodes are important for achieving high conversion rates in electrochemical reactions. Therefore they are especially important in technical electrochemistry, wastewater cleaning, and flow-through analytical techniques, e.g., - coulometry in flow systems. However, the - IR-drop within three-dimensional electrodes is an inherent problem. 

Usually, after a reaction is about 98% complete, a current of about 0.1 of the normal value is used. In the apparatus developed for coulometry of high accuracy, the voltage of the regulated power supply seldom changes by more than 0.001% in a day, the current is accurately measurable through the use of a standard resistor (known to better than 1 ppm) of high precision, and adjustment of the iR drop is made equal to that of a Weston cell. The time can be measured to an accuracy of 1 ppm or better. ... 

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