Constant potential coulometry electrolysis

In the preceding section, we mostly considered cases wherein only a thin segment of the electroactive region (whether the solution or the film phase) was electro-chemically altered. This situation must be contrasted with those in which exhaustive electrolysis is involved. An example is constant-potential coulometry (Fig. 20.4) wherein the entire solution contained within the cell is electrolyzed. As mentioned earlier, this is ensured by the use of a large A/V ratio and efficient solution agitation. The underlying coulometric equation derives from Faraday s law of electrolysis and can be expressed as... 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

Controlled-potential coulometry involves nearly complete reduction or oxidation of an analyte ion at a working electrode maintained at a constant potential and integration of the current during the elapsed time of the electrolysis. The integrated current in coulombs is related to the quantity of analyte ion by Faraday s law, where the amps per unit time (coulomb) is directly related to the number of electrons transferred, and thus to the amount of analyte electrolyzed. 

The number of electrons exchanged on a time scale similar to that of a preparative electrolysis is determined by coulometry. A coulometry experiment involves the complete conversion of the substrate to product(s) and, accordingly, C 0 decreases with time, in principle to zero. This is in contrast to the electro analytical methods where C 0 stays essentially constant during the experiments. Coulometry is carried out at either constant potential or constant current and, usually, the solution is stirred magnetically. 

Coulometry at constant current is often considered as being less attractive than coulometry at constant potential. However, when the current density is low, the potential of the working electrode stays almost constant until approximately 90% of the substrate is consumed. Control of the current rather than the potential has, however, a number of advantages. First, the charge consumed during the reaction is directly proportional to the electrolysis time,... 

A different way of determining n values is based on the measurement of the amount of charge necessary for the exhaustive electrolysis of a known amount of substrate. This type of experiment, traditionally called coulometry, may be carried out either at constant potential or at constant current. 

In controlled-potential coulometry, the potential of the working electrode is maintained at a constant level such that only the analyte is responsible for conducting charge across the electrode/solution interface. The charge required to convert the analyte to its reaction product is then determined by recording and integrating the current-versus-time curve during the electrolysis. 

During coulometry at constant potential, the total amount of charge (g) is obtained by integration of the current (7) - time (0 curve or g can be determined directly by using a coulometer (electronic integrator). In principle, the end point 1 = 0, i.e., when the concentration of the species under study becomes zero, can be reached only at infinite time, however, in practice the electrolysis is stopped when the current has decayed to a few percent of the initial values. The change of I and g as a function of time at a constant potential >> e. for a stirred solutions and for an uncomplicated electrolysis, is as follows ... 

Three electroanalytical methods are based on electrolytic oxidatiorior reduction of an anqlyte fora sufficient period to assure its quantitatwg. conversion to a new oxidation state. These methods are constant-potential coulomelry constant-current coulometry, or coulometric. titrationsf imd electiogravimeiiry. In electrogravimetric methods, the product of the electrolysis is weighed as a dep osit on one of the electrodes. In the two coulometric procedures, on the other hand, the quantity of electricity needed to complete the electrolysis is a measure of the amount of analyte present. 

Because controlled-potential coulometry involves heterogeneous processes, the time required for completion of electrolysis will depend on the ratio of solution volume to electrode area. In fact, the rate of electrolysis is inversely proportional to the volume-electrode area ratio provided all other variables are held constant. This has the important experimental consequence that electrolysis rates may be varied without adjusting any of the other parameters, such as temperature or potential, which might influence the mechanism of the electrolytic process. Bard (29) has taken advantage of this fact to design cells of extremely low volume to electrode area ratios for high speed coulometry. 

Coulometry. If it can be assumed that kinetic nuances in the solution are unimportant and that destmction of the sample is not a problem, then the simplest action may be to apply a potential to a working electrode having a surface area of several cm and wait until the current decays to zero. The potential should be sufficiently removed from the EP of the analyte, ie, about 200 mV, that the electrolysis of an interferent is avoided. The integral under the current vs time curve is a charge equal to nFCl, where n is the number of electrons needed to electrolyze the molecule, C is the concentration of the analyte, 1 is the volume of the solution, and F is the Faraday constant. 

Now returning to the coulometric analysis proper we can. say that any determination that can be carried out by voltammetry is also possible by coulometry whether it should be done by means of the controlled-potential or the titration (constant-current) method much depends on the electrochemical properties of the analyte itself and on additional circumstances both methods, because they are based on bulk electrolysis, require continuous stirring. 

Kihara et al. employed flow coulometry to study the electrode reactions for Np ions in various acidic media [49]. Flow coulometry has an inherent advantage over the conventional hulk coulometry methods in that the electrolysis can be achieved rapidly to aid in the characterization of unstable electrode products. The resulting coulopo-tentiograms for the Np02 /Np02 and Np /Np " " couples indicate reversible processes in nitric, perchloric, and sulfuric acids. The differences in potentials between the various acids are attributed to the associated stability constants of the electrode products with the anion of the acid in each case. Table 2 contains the half-wave potentials for each couple in the various acids. 

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]. 

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. 

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. 

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