Controlled-potential electrolysi

Studies aimed at characterizing the mechanisms of electrode reactions often make use of coulometry for determining the number of electrons involved in the reaction. To make such measurements a known amount of a pure compound is subject to a controlled-potential electrolysis. The coulombs of charge needed to complete the electrolysis are used to determine the value of n using Faraday s law (equation 11.23). 

A 0.3619-g sample of tetrachloropicolinic acid, C6HNO2CI4, is dissolved in distilled water, transferred to a 1000-mL volumetric flask, and diluted to volume. An exhaustive controlled-potential electrolysis of a 10.00-mL portion of this solution at a spongy silver cathode requires 5.374 C of charge. What is the value of n for this reduction reaction ... 

Anodic stripping voltammetry consists of two steps (Figure 11.37). The first is a controlled potential electrolysis in which the working electrode, usually a hanging mercury drop or mercury film, is held at a cathodic potential sufficient to deposit the metal ion on the electrode. For example, with Cu + the deposition reaction is... 

The electrochemical route to duoroaromatics (90) based on controlled potential electrolysis in the absence of hydrogen duoride (platinum anode, +2.4 V acetonitrile solvent tetraalkylammonium duoride electrolyte) has not been commercialized. However, considerable industrial interest in the electrochemical approach stiU exists (91—93). 

One possible reason for the reluctance of non-electrochemists to venture into this field is that in contrast to the electrochemists claim that controlled potential electrolysis offers a method for the selective introduction of energy into molecules, many electrode reactions carried out at a controlled potential have still been reported to give low yields and a diversity of products. The electrode potential is, however, only one of several variables and the lack of selectivity in the electrode process may be attributed to a failure to understand and to control all the parameters of the overall electrode reaction. 

Controlled potential electrolysis of the substrates, at —1.4V vs. SCE, at a carbon felt electrode on which only 3.6x10" mol Bjj were immobilized (approx. 1 X 10 ° mol cm" ) resulted in the production of 76pmol of valeronitrile 16, corresponding to a turnover number of 2100 This example shows that the combination of inner sphere redox mediators and high surface electrodes is promising. 

Figure 3. Controlled-potential electrolysis cell for generation of radical ions in the cavity of esr spectrometer [from (16) by permission of the authors and the American Chemical Society]. 

The kinetics of the reduction of chlorate ion by Ir(III) have been determined by controlled-potential electrolysis to be ... 

The evolution of CO2 was also observed when controlled potential electrolysis was carried out by applying a definite potential difference at the W/NB interface in the cell of Eq. (6)... 

Summarizing the results obtained by controlled potential electrolysis and polarography, the reaction process for the electrolytic evolution of CO2 was estimated to be as follows the first step was one electron transfer from DMFC in NB to FMN in W as in Eq. (7). The second step was the catalytic reduction of O2 by FMNH as in Eq. (8). The final step was the oxidation of pyruvic acid by the reduction product of O2, H2O2, in W as in Eq. (9), well-known as an oxidative decarboxylation of a-keto acids [43] ... 

Controlled potential electrolysis was carried out for 4 h by applying a definite potential difference, iiappb at the stationary interface between W containing 10 M NADH and 0.01 M borate buffer and DCE containing 10 M CQ, and the concentration of NADH in W after the electrolysis was determined spectrophotometrically. Ratios of concentrations of NADH reacted (and hence decreased) by the electrolysis are plotted as curve 1 in Fig. 6 as the function of itappi-... 

Taking into account the results obtained by polarography as well as controlled potential electrolysis, the reaction which proceeded in Range A and gave the polarographic wave was estimated to be composed of two-electron oxidation of NADH and one-electron reduction of CQ at the W/DCE interface. The oxidation of NADH is accompanied by the dissociation of one H in W. 

The controlled potential electrolysis was carried out for 4 h by applying a definite i appi at the stationary interface between W and DCE for which the compositions were as Eq. (17), and concentrations of CQ and CQ produced in DCE were determined as curves 1 and 2, respectively, in Fig. 9. The dominant species produced in DCE by the reaction between... 

When the controlled potential electrolysis had been carried out at iiappi in Range Al by employing a system the same as Eq. (17) but in the absence of phosphate buffer in W, the decrease of pH in W to around 3 was found after the electrolysis, though oxidation products of CQH2 were the same as those in the presence of the... 

Controlled-potential electrolysis (CPE) represents an improvement over the previous constant-potential method this is attained by the application of an emf across the electrodes that yields a cathodic potential as negative as is acceptable in view of current density limitations and without taking the risk that the less noble metal is deposited hence the technique requires non-faradaic control of the cathodic potential versus the solution. 

Fig. 3.84. Controlled potential electrolysis on behalf of electrogravimetric determination of Ag in presence of Cu. 

The attraction of controlled-potential electrolysis now is that it cam start with a cathode potential well below 0.818 V, because this increases considerably the current throughput and, as a consequence, the rapidity of depostion however, there are two limiting conditions (1) the current density should not be too high and (2) in particular the final potential must remain well above 0.340 V in order to prevent Cu deposition. 

In many instances electrogravimetry must be preceded by a separation between metals suitably this can be an electroseparation by means of constant-current electrolysis as previously described, but more attractively an electroseparation by means of controlled-potential electrolysis at a mercury pool or sometimes at an amalgamated Pt or brass gauze electrode. In this way one can either concentrate the metal of interest on the Hg or remove other metals from the solution alternatively, it can be a rougher separation, i.e., the concentration of a group of metals such as Fe, Ni, Co, Cu, Zn and Cd on the Hg whilst other metals such as alkali and alkaline earth metals, Be, Al, Ti and Zr remain in solution151. In all these procedures specific separation effects can be... 

A crystal structure of the C02 derivative of (8), K[Co(salen)( 71-C02)], haso been reported in which the Co—C bond is 1.99 A, the C—O bonds are both equivalent at 1.22 A and the O-C-O angle is 132°.125 Carboxylation of benzylic and allylic chlorides with C02 in THF-HMPA was achieved with (8) electrogenerated by controlled-potential electrolysis,126 in addition to reductive coupling of methyl pyruvate, diethyl ketomalonate and / -tolylcarbodiimide via C—C bond formation. Methyl pyruvate is transformed into diastereomeric tartrates concomitant with oxidation to the divalent Co(salen) and a free-radical mechanism is proposed involving the homolytic cleavage of the Co—C bond. However, reaction with diphenylketene (DPK) suggests an alternative pathway for the reductive coupling of C02-like compounds. 

Chelating phosphines are effective ligands for Co11. The [Co(mtriphos)2]2+ cation (85) was formed by controlled potential electrolysis of its trivalent relative and characterized by EPR... 

This was confirmed by experiment controlled-potential electrolysis of 25 at -1.0 V, i.e., on the plateau of the first four-electron wave, produced 2,7-di-iodofluoroscein (26) as the major product. 

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