Potentiostatic coulometry

Coulometry. Even in water, controlled potential or potentiostatic coulometry is a difficult and often time-consuming technique, as the analyte must participate in a direct electrode reaction. Therefore, in non-aqueous media there are only a few examples of its application, e.g., the potentiostatic coulometry of nitro and halogen compounds in methanol (99%) with graphical end-point prediction, as described by Ehlers and Sease153. 

Potentiostatic coulometry is mostly employed in the investigation of electrode processes. In this case, the current has to be directly proportional to the concentration of the electroactive substance ... 

The electronic adsorption spectra for the complexes [Ir(OH)6]", where n = 0-2, have been resolved and peak maxima locations, molar extinction coefficients, oscillator strengths, and band half-widths calculated.44 Bands have been assigned in the main part to be one-electron MLCT transitions. Spectrophotometrically determined rate constants for the OH reduction of the IrVI and Irv complexes at 25 °C in 3M NaOH are (2.59 0.09) x 10—3 s—1 and (1.53 0.05) x 10 4 s 1 respectively. The activation energy for the reduction, Irv—>IrIV, is nAkcalmoC1. Cyclic voltammetry and potentiostatic coulometry of [Ir(OEI )r,]2 in 3M NaOH on a Pt electrode show that during the electro-oxidation compounds of Irv and IrVI are formed.45... 

In coulometry, the analyte is quantitatively electrolyzed and, from the quantity of electricity (in coulombs) consumed in the electrolysis, the amount of analyte is calculated using Faraday s law, where the Faraday constant is 9.6485309 xlO4 C mol-1. Coulometry is classified into controlled-potential (or potentiostatic) coulometry and controlled-current (or galvanostatic) coulometry, based on the methods of electrolysis [19, 20]. 

Coulometry comes in several flavors constant-potential or potentiostatic coulometry, constant-current or amperostatic coulometry, coulometric titrations, and electrogravimetry. 

Q is the total charge passed, n is the number of electrons passed, F is Faraday s constant(96 485 Cmol ), and E is the potential of the electrolysis. This technique, the most coimnon BE method, is also referred to as potentiostatic coulometry. Another BE method used is amperostatic coulometry, whereby the current is kept constant. This technique requires the use of an amperostat instead of a potentiostat which limits its use. 

Two methods have been developed that are based on measuring the quantity of charge controlled-potential (potentiostatic) coulometry and controlled-cur-rent coulometry, often called coulometric titrimetry. Potentiostatic methods are performed in much the same way as controlled-potential gravimetric methods, with... 

The instrumentation for potentiostatic coulometry consists of an electrolysis cell, a potentiostat, and a device for determining the charge consumed by the analyte. 

Figure 22-11 Electrolysis cells for potentiostatic coulometry. Working electrode (a) platinum gauze, (b) mercury pool. (Reprinted with permission from J. E. Harrar and C. L. Pomernacki, Anal. Chein., 1973, 45, 57. Copyright 1973 American Chemical Society.)...

Coulometry is the name given to a group of other techniques that determine an analyte by measuring the amount of electricity consumed in a redox reaction. There are two categories referred as potentiostatic coulometry and amperostatic coulometry. The development of amperometric sensors, of which some are specific for chromatographic detection, open new areas of application for this battery of techniques. Combining coulometry with the well known Karl Fischer titration provides a reliable technique for the determination of low concentrations of water. 

The potentiostatic coulometry, involves holding the electric potential (the voltage) of the working electrode constant during the reaction. This avoids parasitic reactions. The current decreases as fast as the analyte disappears from the solution. A galvanostat (or amperostat) is used to compute the quantity of current used. 

FIGURE 10a. Potentiostatic coulometries of tosyl chloride in acetonitrile/LiC104. Variation of electricity consumption (in faraday per mole) with the concentration in perchloric acid is shown. Electrolyses conducted with about 0.5 g substrate at mercury pool electrode. (Reproduced by permission of the Societe Frangaise de Chimie from Reference 50)... 

There are essentially two different coulometric processes, namely potentio-static and galvanostatic coulometry. The former functions with constant, controlled electrode potential, whereas the galvanostatic method - also called coulometric titration - functions with constant current strength and uncontrolled potential. Fig. 13 shows the basic circuit diagram for potentiostatic coulometry. 

This behavior is illustrated by the followings. First, the complex [ Fe Lj was quantitatively reduced ( potentiostatic coulometry ) to [ Fe Lj H, ... 

This decoordination was observable on the time scale of the potentiostatic coulometry whereas it was not detected by voltammetry in this method... 

Coulometric Detectors. Coulometric detectors are based on potentiostatic coulometry [30]. The signal of the constant-potential measurements, as in amperometric detection, is the current resulting from an electron-transfer process (reduction or oxidation of the analyte arriving with the eluent) while the working electrode is held at constant potential. 

Controlled-current coulometry uses a constant current. which passes through a cell until an indicator signals completion of the analytical reaction. The quantity of charge required to reach the end point is then calcuiated from the magnitude of the current and the time that the current passes. This method has enjoyed wider application than potentiostatic coulometry. It is frequently called a coulometric titration for reasons that we discuss in Section 24D. 

There are basic electroanalytical characterization techniques that are consistently used to evaluate performance characteristics of BFCs. Standard electroanalytical techniques include linear sweep voltammetry, cyclic voltammetry, amperometry, and both galvanostatic and potentiostatic coulometry [1-5]. 

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