Coulometry current efficiency

In coulometry, current and time are measured, and equation 11.24 or equation 11.25 is used to calculate Q. Equation 11.23 is then used to determine the moles of analyte. To obtain an accurate value for N, therefore, all the current must result in the analyte s oxidation or reduction. In other words, coulometry requires 100% current efficiency (or an accurately measured current efficiency established using a standard), a factor that must be considered in designing a coulometric method of analysis. 

In controlled-potential coulometry, accuracy is determined by current efficiency and the determination of charge. Provided that no interferents are present that are easier to oxidize or reduce than the analyte, current efficiencies of greater than 99.9% are easily obtained. When interferents are present, however, they can often be eliminated by applying a potential such that the exhaustive electrolysis of the interferents is possible without the simultaneous electrolysis of the analyte. Once the interferents have been removed the potential can be switched to a level at... 

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. 

In controlled-potential coulometry, the analyte is electrolyzed quantitatively with 100% current efficiency and the quantity of electricity Q is measured with a coul-ometer ... 

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. 

Quiescent Solutions. Coulometry at constant current provides a simple method for measuring the quantity of electrogenerated species as long as the reaction proceeds with 100% current efficiency. However, this condition breaks down with depletion of the electroactive material in the diffusion layer (cf. chronopotentiometric transitions see Fig. 4.3). For low values of the applied current, the thermal and density gradients supplement diffusion sufficiently to sustain electrolysis without the potential shifting to a different reaction. This mode of radical generation has been employed successfully in the study of stable species. 

Coulometry is generally difficult with in situ EPR experiments because of convection and iR drop in the long, thin layer of solution. In order to determine the current efficiency of carbazole anion formation, an in situ measurement was carried out [29,30], Carbazoles can either form soluble products or polymerize forming a conductive layer on the electrode. Results of the measurement showed that there was between 0.2 and 2% current efficiency for radical formation depending on the carbazole used. The system described, however, can only be used for coulometry over short periods of time because of the problems discussed earlier. 

Faradaic efficiency (or coulometric efficiency) — Relates the moles of product formed in an -> electrode reaction to the consumed -> charge. The faradaic efficiency is 1.00 (or 100%) when the moles of product correspond to the consumed charge as required by -> Faraday s law. See also -> current efficiency, -> coulometry. 

In coulometry the stoichiometry of the electrode process should be known and should proceed with 100% current efficiency, and the product of reaction at any other electrode must not interfere with the reaction at the electrode of interest. If there are intermediate reactions, they too must proceed with the desired accuracy. In practice the electrolytic cell is designed to include isolation chambers. Losses of solute through diffusion, through ionic or electrical migration, and simply through bulk transfer must be minimal. Finally, the end point has to be determined by one of the many techniques used in titrations generally, whether coulometric or not. Both indeterminate and determinate end-point errors limit the overall accuracy achieved. Cooper and Quayle critically examined errors in coulometry, and Lewis reviewed coulometric techniques. 

A fundamental requirement for all coulometric methods is 100% current efficiency that is, each faraday of electricity must bring about chemical change in the analyte equivalent to one mole of electrons. Note that 100% current efficiency can be achieved without direct participation of the analyte in electron transfer at an electrode. For example, chloride ion may be determined quite easily using poten-tiostatic coulometry or using coulometric titrations with silver ion at a silver anode. Silver ion then reacts with chloride to form a precipitate or deposit of silver chloride. The quantity of electricity required to complete the silver chloride formation serves as the analytical variable. In this instance, 100% current efficiency is realized because the number of moles of electrons is essentially equal to the number of moles of chloride ion in the sample despite the fact that these ions do not react directly at the electrode surface. 

Bulk electrolysis methods are also classified according to purpose. For example, one form of analysis involves determination of the weight of a deposit on the electrode (electrogravimetry). In this case 100% current efficiency is not required, but the substance of interest must be deposited in a pure, known form. In coulometry, the total quantity of electricity required to carry out an exhaustive electrolysis is determined. The quantity of material or number of electrons involved in the electrode reaction can then be determined by Faraday s laws, if the reaction occurred with 100% current efficiency. For electroseparations, electrolysis is used to remove, selectively, constituents from the solution. 

In controlled-potential coulometry the total number of coulombs consumed in an electrolysis is used to determine the amount of substance electrolyzed. To enable a coulometric method, the electrode reaction must satisfy the following requirements (a) it must be of known stoichiometry (b) it must be a single reaction or at least have no side reactions of different stoichiometry (c) it must occur with close to 100% current efficiency. 

There have been numerous applications of controlled-potential coulometry to analysis. Many electrodeposition reactions that are the basis of electrogravimetric determinations can be employed in coulometry as well. However, some electrogravimetric determinations can be used when the electrode reactions occur with less than 100% current efficiency, for example, the plating of tin on a solid electrode. Coulometric determinations can, of course, also be based on electrode reactions in which soluble products or gases are formed (e.g., reduction of Fe(III) to Fe(II), oxidation of 1 to I2, oxidation of N2H4 to N2, reduction of aromatic nitro compounds). Many reviews concerned with controlled-potential coulometric analysis have appeared (1, 20-22) some typical applications are given in Table 11.3.2. 

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. 

Of the few electroanalytical monitors the ones used in the chloralkali industry are worth mentioning.Sulphate was determined in brines. Oflf-line conductometry was used to determine sulphate in the concentration range 25 - 500 mM with Ba " as titrant, or Pb " " as titrant when potentiometric measurement was used. These methods can, however, not compete with infrared spectrophotometry in this application. Water was determined in chlorine gas by coulometry with 100 % current efficiency. In this case the analyzer should be installed very close to the production plant. 

In the conductivity, potentiometric, and voltam-metric measurements the response is correlated to concentration or activity of the analyte usually by using calibration curves. In coulometry, however, the charge measured gives directly the amount of substance and therefore no calibration is needed. However, in coulometry the sample is consumed in the measurements and the problem is that the method requires 100% current efficiency to be reliable. Conductimetry and potentiometry are sample nonconsuming methods. In voltammetry, only an insignificant amount of the sample is consumed and therefore the measurement can be repeated. Only in voltammetric stripping methods of very low concentrations of the analyte the amount consumed at the electrode reaction has to be considered if repeated measurements are to be done. 

Figure 6.39 Galvanostatic polarization of Ni in the transpassive potential region in sodium nitrate solution (a) anodic polarization curve in the transpassive potential region (b) the current efficiency for metal dissolution and (c) the apparent thickness of the film as measured by coulometry [36].

Coulometry. Two methods of coulometry are used coulometry at controlled potential and coulometric titrations. The main advantage of the coulometric method is the elimination of the necessity of standardization as the Faraday constant is a standard. In analysis of complicated samples encountered in environmental analysis the coulometric titrations are more advantageous where 100% current efficiency can be more readily attained by suitable choice of the reagent-solvent system. Coulometric titrations are suitable for determining the amount of substance in the range 0.01 to 100 mg (and sometimes below 1 iJg). Under optimum conditions these titrations can be carried out with a precision and accuracy of 0.01%. Automatic coulometric analyzers for the determination of gaseous pollutants (SO2, O3, NO, etc.) have proven to be useful in environmental chemistry. 

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