Coulometry, constant-current

Curran, D. J. Constant-Current Coulometry. Chapter 20 in Kissinger, P. T. Heineman, W. R., eds. Eaboratory Techniques in Electroanalytical Chemistry. Marcel Dekker, Inc. New York, 1984, pp.539—568. 

Figure 4.19 A constant current coulometry titration cell. The reagent is produced at the working electrode and reacts with the sample. The indicator electrodes detect the changing potential or conductivity of the solution and the amount of change that takes place is measured and related to the concentration of the reactant in the sample.

Controlled-potential coulometry in a three-electrode cell is more selective than constant-current coulometry. Because the working electrode potential is constant, current decreases exponentially as analyte concentration decreases. Charge is measured by integrating current over the time of the reaction ... 

Controlled-current electrolysis in flowing solution has been extremely useful for analytical purposes. The prevalent techniques are constant-current coulometry and coulometric titrations, which are discussed in Chapter 25. 

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. 

The uses of constant-current coulometry for the determination of drugs in biological fluids are few, basically due to sensitivity restriction. Monforte and Purdy [46] have reported an assay for two allylic barbituric acid derivatives, sodium seconal and sodium sandoptal, with electrogenerated bromine as the titrant and biamperometry for endpoint detection. Quantitative bromination required an excess of bromine hence back titration with standard arsenite was performed. The assay required the formation of a protein-free filtrate of serum with tungstic acid, extraction into chloroform, and sample cleanup by back extraction, followed by coulometric titration with electrogenerated bromine. The protein precipitation step resulted in losses of compound due to coprecipitation. The recoveries of sodium seconal and sodium sandoptal carried through the serum assay were approximately 81 and 88%, respectively. Samples in the concentration range 7.5-50 pg/mL serum were analyzed by this procedure. 

Fig. 6.26 Values of /°x recorded by voltammetry at 1-minute intervals during constant current coulometry...

The following table lists some common coulometric (also known as constant-current coulometry) titrations.1 Since the titrant is generated electrolitically and reacted immediately, the method gets widespread applications. The generating electrolytic concentrations need to be only approximate, while unstable titrants are consumed as soon as they are formed. The technique is more accurate than methods where visual end points are required, such as in the case of indicators. The unstable titrants in the table below are marked with an asterisk. 

Figure 42. Constant current coulometry (25 mA) of 3,4-diphenyl-1,2-dithiolylium ion (0.1 mmol). Cyclic voltammograms, horizontally displaced, are shown after reduction for 0, 1,2, 3, 4, 5, and 6 min, respectively.

Under conditions where the primary electrode product undergoes a slow chemical reaction, that is, ti/2 is of the order of seconds, the value of n determined by a relatively fast technique like LSV may differ from that obtained by a slow experiment like coulo-metry. This type of behavior was observed in the anodic oxidation of 2,3,5,6-tetraphenyl-1,4-dithiin in MeCN [278]. During CV the reversible oxidation to the radical cation is observed. However, when constant-current coulometry was carried out as described earlier, this time at i = 50 mA, 6.44 min was required to oxidize completely 0.1 mmol of the substrate to a product electroinactive in the potential region of interest, indicating an overall two-electron process (Fig. 43). Thus, apparently contradictory results may be obtained due to the difference in time scale between the two types of experiment. 

Differentiate between controlled-potential coulometry and constant-current coulometry. 

In controlled-potential coulometry, the working electrode potential is maintained at a constant value with respect to a reference electrode. In constant-current coulometry, the cell is operated so that the current is maintained at a constant value. 

In constant-current coulometry, what current would be required so that the time in seconds would be equal to the number of microequivalents 7... 

It is critical for these techniques that the current efficiency be as close to 100% as possible. The percent current efficiency tells us how much of the applied current results in the reaction of interest. The percent current efficiency is defined as being equal to 100 x ( appiied - residuai)/FppUed. where 4ppUed IS the cuiTent applied to the cell and rVesiduai is the background current. All real cells have some residual current. To achieve a 99.9% current efficiency, the applied current must be approximately lOOOx the residual current. This is readily achieved for currents in the p,A to 100 mA range used in constant-current coulometry. 

There are two options. Coulometry can be conducted in the constant current and the constant potential modes. The former is inherently simpler as in this case the total charge is obtained directly from the measured time to the completion of the reaction. However, it can only be used successfully if there is only one redox-active species present in the sample or if at least the redox potentials of species present are succinctly different. In constant current coulometry, the cell voltage needs to follow the depletion of the concentration of the analyte according to Nemst s law (for an oxidation) ... 

Figure 1 Potential change during constant current coulometry. The secondary reaction is due to oxidation or reduction of a further electroactive species in the sample or decomposition of the electrode or solvent.

Figure 3 Standard galvanostat circuitry for constant current coulometry consisting of a voltage source, a resistor, and an operational amplifier.

Before operational amplifiers became available, constant current coulometry (CCC) was used more often than controlled potential coulometry (CPC) because the instrumentation was so much simpler. CCC continued to be used for highly precise U determinations in very pure materials (Goode et al. 1967 Malinowski 1967 Merciny et al. 1981). 

Malinowski J (1967) Precise determination of uranium in pure metal and uranium compounds by constant current coulometry Talanta 14 283 Marsh SF, Ortiz MR, Abernathey RM, Rein JE (1974) Improved 2-column ion exchange separation of plutonium, uranium and neodymium in mixed U/Pu fuels for burn up measurements, LA5568 Marsh SF, Abernathey RM, Rein JE (1981) Evaluation of treatments to attain isotopic equilibration of plutonium preceding the resin bead technique for mass spectrometric assay analysis of spent reactor fuel. In Minutes 4th SALE participants meeting, Argonne, p 218 Anal Lett 13 1487... 

The essential difference between voltammetric and other potentiodynamic techniques, such as constant current coulometry, is that in voltammetry an electrode with a small surface area (< 10mm ) is used to monitor the current produced by the species in solution reacting at this electrode in response to the potential applied. Because the electrode used in voltammetry is so small, the amount of material reacting at the electrode can be ignored. This is in contrast to the case in coulometry where large area electrodes are used so that all of a species in the cell may be oxidized or reduced. 

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. 

Ziyatdinova G, Ziganshina E, Budnikov H (2012) Surfactant media for constant-current coulometry. Application for the determination of antioxidants in pharmaceuticals. Analytica Chimica Acta 744 23-28... 

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