Controlled-Potential Bulk Electrolysis

Figure 11.3.1 Current-potential curves at different times during a controlled-potential bulk electrolysis at =...

In an effort to determine if the redox electron that is added to [1-Na] is localized in one bipy unit, as it is in the crystalline state, or delocalized, as in the case of [Ru(bpy)3] in solution, ESR experiments were recently conducted. Reduction of [1-Na] was done by controlled potential bulk electrolysis of a solution of the corresponding Br salt in DMF. The corresponding solution was analyzed using the X-band of a Bruker ER-200 SRC spectrometer. A variable temperature sequence that corresponds to the spectrum of the monoreduced species is shown in figure 5. Notice that the spectrum consists of a single, relative sharp ESR line, indicative of fast electron delocalization over the three bipy units. 

Controlled potential bulk electrolysis or coulometry, as it is often called, is widely used to determine the overall number of electrons involved in an electrode process. It is also used to prepare a sufficient quantity of the reaction products to enable them to be identified by the application of conventional analytical techniques. 

Figure 11.3 Controlled potential bulk electrolysis (a) current and charge transients and (b) current vs. charge plot used to determine...

Signals A and B were assigned, respectively, as the [3Fe-4S]+ and [4Fe-4SJ2+/+ couples, based on EPR spectra of samples prepared by controlled potential bulk electrolysis [47], What was striking at the time the first experiments were carried out in the similar protein from Azotobacter chroococcum (1988) was the revelation that the [4Fe-4S + cluster has an unusually negative reduction potential,... 

The results previously discussed clearly demonstrate that electrochemical techniques are powerful tools to study compoimds of interest to human and animal health. Particularly, linear, cyclic, convolution, square wave voltammetries, and controlled potential bulk electrolysis allow inferring the reaction mechanism and perform a full thermodynamic and kinetics of redox couples controlled by diffusion, adsorption as well as those which show a mixed control diffusion/adsorption. On the other hand, the square wave voltammetry coupled to adsorptive accumulation of redox couples which are both electroactive, and show specific interactions with the electrode smface allows detecting and quantifying substrates at trace levels. 

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. 

The oxidation state of Au in both Au-oxo complexes 3 and 4 was thoroughly investigated by several chemical and physicochemical methods 44). First, bulk electrolysis (coulometry at controlled potential) confirms the Au(III) oxidation state assignment in both 3... 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

A single TBP droplet is laser-trapped and positioned above an Sn02 electrode without direct contact (Figure 11). With the droplet being laser-trapped, Fe(II) in water is oxidized by potential-controlled bulk electrolysis. As is characteristic of bulk electrolysis in a thin-layer cell, Fe(II) can be oxidized almost completely to Fe(III) during the first 10s [75]. Upon electrolysis, ET across the single droplet/water interface and subsequent ion transfer (IT) of FeCp-X+ from the droplet to water are induced, as in... 

Several oxides with perovskite related stmctures can also be intercalated with oxygen ions by an electrochemical method. The oxide Sr2Fe20s with the brownmillerite stmcture has been electrochemically oxidized to SrFeOs. The reaction was carried out by controlled potential electrolysis at a potential below that for oxygen evolution in 1 M aqueous KOH at room temperature. Bulk oxidation was confirmed by Mossbauer spectroscopy and X-ray difflaction. Similar results have been obtained for electrochemical oxidation... 

Bulk controlled-potential electrolysis experiments have shown that most of the (O) ester anion radicals decay via a simple bond cleavage mechanism to form car-boxylate anions in very high yield, while the (S) ester radicals decay via a very complicated mechanism often involving aromatic substitution reactions [442], Using CV, many of the compounds are shown to display chemically (and electrochemi-cally) reversible behavior at slow scan rates, in the sense that the ratios... 

Bulk electrolyses are used to prepare one-electron reduction or oxidation products. If cyclic voltammetry (CV) reveals reversible redox, the bulk preparation of the reduced (or oxidized) product may be attained. The overall electrode process may be different in controlled-potential electrolysis and in CV because of the time factor (see below). The iron cluster, (h -CjHjFeCO), in nonaqueous electrolytes undergoes a four-membered electron-transfer CV series through three steps. The potentials measured (in CHjCN/O.l M [n-Bu N] [PFJ) are ... 

The methods can be classified by the controlled parameter (E or i) and by the quantities actually measured or the process carried out. Thus in controlled-potential techniques the potential of the working electrode is maintained constant with respect to a reference electrode. Since the potential of the working electrode controls the degree of completion of an electrolytic process in most cases, controlled-potential techniques are usually the most desirable for bulk electrolysis. However, these methods require potentiostats with large output current and voltage capabilities and they need stable reference electrodes, carefully placed to minimize uncompensated resistance effects. Placement of the auxiliary electrode to provide a fairly uniform current distribution across the surface of the working electrode is usually desirable, and the auxiliary electrode is often placed in a separate... 

For slow electron-transfer (irreversible) processes, the eventual extent of the electrode process will be governed by equilibrium considerations and the Nemst equation, but the rate of electrolysis will be small at the potentials predicted in the previous sections and long-duration electrolyses would result. For these processes, reduction must be carried out at somewhat more negative potentials the actual potential is usually selected on the basis of experimental current-potential curves taken under conditions near those for the intended bulk electrolysis. Processes that are controlled by the rate of a homogeneous reaction, such as... 

Figure 11.2.2 Typical cells for bulk electrolysis, (a) Undivided cell for controlled-potential separations and electrogravimetric analysis at a solid cathode. [From J. J. Lingane, Anal Chim. Acta, 2, 584 (1948), with permission.] ib) Undivided cell for coulometric analysis at mercury cathode with a silver anode. [Reprinted with permission from J. J. Lingane, J. Am. Chem. Soc.,...

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