Electrolysis at controlled potential

In the simplest case, the current flowing during a controlled-potential electrolysis is limited by the amount of the reducible species arriving at the electrode, and is proportional to its concentration both the concentration and the current will decrease exponentially as the electrolysis proceeds [2]  

The diffusion-layer thickness, S, depends primarily on the rate of stirring. Usually, Equation 4.8 is written more simply as 

In this form, it is obvious that the expression is simply that for a first-order chemical reaction. 

Equation 4.8 indicates that, provided the electrode reaction is mass-transfer limited and uncomplicated by coupled chemical reactions, the electrolysis time may be reduced by making the electrode large and the volume small, and by stirring the solution as fast as possible to decrease the diffusion-layer thickness. In a well designed cell it is possible to carry an electrolysis to 99.9% completion in about 10 to 20 min, although times down to 1 min can be achieved by proper cell design. 

In cases where a metal plated on a platinum cathode is to be quantitatively determined by weighing, the deposit may not adhere well to the cathode because of the high initial current-densities. Hence, it may be desirable to begin the electrolysis at a potential less negative than that required to reach the limiting-current plateau. The potential is later shifted to a more negative value to complete the electrolysis. 

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In electrolysis at controlled potential, the quantity of electricity Q (coulombs) passed from the beginning of the determination to time t is given by... 

Fig. 8. Plots of the corrected electron equivalents, n = (C-Cb)/FN, passed during btdk electrolysis at controlled potential versus time. (I) 3.37 mM (NH4)AuCl4, 50 mM Na2S04 buffer, pH ca. 2, 0.4 M NaCl, = 320 mV ...

A thorough electrochemical characterization of new metalloporphyrins is nowadays state of the art for the synthetic inorganic chemist. In many of the papers cited in Sects. 3 and 4, a characterization of the new complexes by cyclic voltametry and electrolysis at controlled potential has been done. Thin-layer spectroelectrochemistry is very fruitful [346]. Fortunately, apart from classical articles of Davis et al. [347], Felton et al. [292], Fuhrhop et al. [293], Buchler et al. [190], more recent reviews of Kadish et al. are available which systematically cover the field of general metalloporphyrins [294] or organometallic porphyrin complexes [306]. Therefore, a short, update of these articles will be given in the form of Table 7. For details, the reader is referred to the original literature. 

Fig. 6. Cell for macroscale electrolysis at controlled potential consisting of a 2-liter beaker covered with a glass plate G, containing holes for a silver/silver chloride referenoe electrode R, the anode compartment, a cooling coil S, a thermometer, an inlet for nitrogen, and one for withdrawing of samples. The mercury cathode C has an area of 125 cm2. The diaphragm D consists of two porous clay cylinders separated by agar containing KC1. The anolyte (15% aqueous NaOH) is continuously renewed through T. Anode A is of stainless steel. From Iversen and Lund.55...

Electrolysis at controlled potential can also serve as an elegant method of removing interfering metals from samples to be analyzed by other methods such as spectrophotometry or polarography. The electrogravimetric and coulometric procedures mentioned above represent such separations. The electrolysis can, however, be carried out primarily as a selective separation, with the actual determination being... 

The mechanistic details of the anodic alkylbenzene oxidation have been under intense investigation. In the early stages of this research the major issue was the order of the electron and proton transfer steps leading to the benzyl cations and whether the second electron-transfer reaction takes place at the anode (ECE) or in solution (DISP) see Eq. (7). By the application of a combination of voltammetry and specular reflectance spectroscopy [173,192-195], it has been possible to monitor the electrode surface by UV-visible spectroscopy during electrolysis at controlled potential or to monitor the absorption at a predetermined value as a function of potential. In this way absorption spectra, which were attributed to the radical cation, XIII, the benzyl radical, XIV, formed by deprotonation, Eq. (60), the benzyl cation XV, and the N-benzylnitrilium ions, XVI, have been recorded in MeCN. 

Electrolysis at controlled potential is the most efficient method of carrying out a bulk electrolysis, because the current is always maintained at the maximum value (for given cell conditions) consistent with 100% current efficiency. Note that the rate of electrolysis is independent of Cq(0), so that electrolysis of a 0.1 M solution of O and a 10 M solution of O should require the same amount of time, given the same values of E, A, V, and thq. 

Electrochemical study based on the combination of cyclic voltammetry and electrolysis at controlled-potential enabled the mechanistic aspects and synthetic scopes of the reaction to be established (Scheme 18). 

Anodic stripping analysis The accumulation is usually carried out by the electrolysis at controlled potential for a definite time, t cc under reproducible mass-transport conditions. The deposition potential, E, imposed to the working electrode should be adjusted at the potential adequate to the plateau of the reduction voltammetric wave characteristic for the test metal (see Figs. 62A and 62B for a mixture of two tested species). Before adjusting the experiment, it is recommended to know the solubility data and diffusion coefficients of metals in mercury and their half-wave potentials (see Table 4). 

Lingane JJ (1945) Automatic apparatus for electrolysis at controlled potential. Ind Eng Chem 17 332-333... 

The inflection of the Ej/2 -pH plot was attributed [84] to the existence of two forms of these oximes protonated in different ways, and this conclusion was confirmed by isolation of the products from electrolysis at controlled potential. 

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... 

V/Fc) was reversible at room temperature, while the second ( 1/2 = —2.20V/Fc) showed some reversibility only at low temperature. Whereas controlled potential reduction at the first step afforded the stable monoanion, electrolysis at the potential of the second process was found to generate the highly reduced species [Mo(NO)(l,2-C6H4S2)2] in... 

The choice of the electrolysis procedure either working at controlled potential (potentiostatic or controlled-potential electrolysis, cpe) or at a fixed current density (galvanostatic)... 

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... 

TABLE 6. Relative intensity (%) from GC-MS peaks of products arising from electrolysis of 104 at controlled potential E = 1.200 V vs. Ag/Ag+ in unbuffered... 

When a solution of volume 100 cm containing metal ion, at a concentration 0.010 M is electrolyzed with a rapid scan at a large-area (10 cm ) rotating disk electrode, a limiting current of 193 mA is observed for reduction to metal M. Calculate the value of the mass transport coefficient mj 2+, in cm/s. If an electrolysis of the solution is carried out at this electrode at controlled potential in the limiting current region, what time will be required for 99.9% of the to be plated out How many coulombs will be required for this electrolysis ... 

Several finishing modes are applicable. The carbon dioxide can be retained in an absorption tube packed with Ascarite (sodium hydroxide mixed with asbestos) and weighed, or absorbed in a solution and titrated. The iodine produced in Eq. (14) can also be determined by titration. Alternatively, the iodine vapor can be led by a stream of nitrogen into an electrolysis cell, where iodine is reduced at controlled potential and the amount of electricity is recorded. Figure 6 shows, from right to left, the complete equipment for oxygen determination, which comprises the combustion train, the furnace for the oxidation of carbon monoxide, and the assembly for electrometric finish. 

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