Coulometry potentiostat

Time, Cost, and Equipment Controlled-potential coulometry is a relatively time-consuming analysis, with a typical analysis requiring 30-60 min. Coulometric titrations, on the other hand, require only a few minutes and are easily adapted for automated analysis. Commercial instrumentation for both controlled-potential and controlled-current coulometry is available and is relatively inexpensive. Low-cost potentiostats and constant-current sources are available for less than 1000. 

For coulometric analysis, the substance being examined must react in 100% current yields [i.e., other (secondary) reactions must be entirely absent]. In efforts to avoid side reactions, coulometry most often is performed potentiostatically (amperometrically) (i.e., the electrode potential is kept constant during the experiment), and the current consumed at the electrode is measured. The current is highest at the start of the... 

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... 

Electrochromic materials (either as an electroactive surface film or an electroactive solute) are generally first studied at a single working electrode, under potentiostatic or galvanostatic control, using three-electrode circuitry.1 Traditional electrochemical techniques such as cyclic voltametry (CV), coulometry, and chronoamperometry, all partnered by in situ spectroscopic measurements... 

For their characterization, electrochromic compounds are initially tested at a single working electrode under potentiostatic control using a three-electrode arrangement. Traditional characterization techniques such as cyclic voltammetry, coulometry, chronoamperometry, all with in situ spectroscopic measurements, are applied to monitor important properties [27]. From these results, promising candidates are selected and then incorporated into the respective device. 

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]. 

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. 

Ohmic drop in three-electrode circuits. In modem coulometry and voltammetry the use of a potentiostat and a three-electrode configuration is the routine practice. The three electrodes are usually called the working, reference, and counter (or auxiliary) electrodes (see Figure 5.2). The cell current passes between the working electrode immersed in the test solution and the counter electrode, which may be in the test solution but is usually isolated from it by a single- or double-junction glass frit. 

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

Using modem electronics and/or well-designed potentiostats, both controlled-potential coulometry and controlled-current coulometry can be performed, as can gravimetric determinations of analytes. Figure 11.68 shows a potentiostat that uses three operational amplifiers. 

Amperostatic control is easier than potentiostatic. Therefore they predominated, preferably in coulometry, before the appearance of the potentiostat in 1942. 

Lingane was a leader in the field of - electro analytical chemistry and wrote, with Kolthoff, the definitive, two volume monograph, Polarography [i] that remains a useful reference work. He also helped develop other electroanalytical techniques, like controlled potential electrolysis, -> coulometry, -> coulometric titrations, and developed an early electromechanical (Lingane-Jones) potentiostat, He wrote the widely-used monograph in this field, Electroanalytical Chemistry (1st edn., 1953 2nd edn., 1958). Lingane received a number of awards, including the Analytical Chemistry (Fisher) Award of the American Chemical Society in 1958. Many of his Ph.D. students, e.g., -> Meites, Fred Anson, Allen Bard, Dennis Peters, and Dennis Evans, went on to academic careers in electrochemistry. 

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. 

The simplest methods of HTSC analysis are based on the determination of the products of sample dissolution in acidic media. Potentiometric, amperometric, or coulometric titrations are frequently used (mainly for YBCO ceramics [525-527] and their analogs with other rare-earth elements [528, 529], and also for BSCCO [530]). We note particularly the method of potentiostatic coulometric analysis [531], which allows one to analyze thallium cuprate samples over a wide range of the Tl/Cu ratio, and also the method of flow-through coulometry for determining the effective valence of copper [532]. The polarographic determination of Cu content in the samples obtained by dissolving HTSCs in concentrated alkaline solutions with special... 

Yahnke et al. [99] have obtained spectra for adsorbed from solution onto a commercial Pt/carbon fuel-cell electrode. They could vary the coverage in situ by connection to a potentiostat, but the actual NMR measurements were performed under open-cell conditions. They found an excellent correlation between the quantities of CO determined from the NMR spectra and by coulometry. 

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.)...

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 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)... 

Instrumentation for Electrogravimetry and Coulometry The basic apparatus required is a power supply (potentiostat with a DC output voltage), an inert cathode and anode (usually platinum foil, gauze, or mesh), and arrangements for stirring. Sometimes a heater is used to facilitate the processes. 

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. 

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