Electrolysis constant-current

In the common method of electro-gravimetric analysis, a potential slightly in excess of the decomposition potential of the electrolyte under investigation is applied, and the electrolysis allowed to proceed without further attention, except perhaps occasionally to increase the applied potential to keep the current at approximately the same value. This procedure, termed constant-current electrolysis, is (as explained in Section 12.4) of limited value for the separation of mixtures of metallic ions. The separation of the components of a mixture where the decomposition potentials are not widely separated may be effected by the application of controlled cathode potential electrolysis. An auxiliary standard electrode (which may be a saturated calomel electrode with the tip of the salt bridge very close to the cathode or working electrode) is inserted in the... 

A mercury cathode finds widespread application for separations by constant current electrolysis. The most important use is the separation of the alkali and alkaline-earth metals, Al, Be, Mg, Ta, V, Zr, W, U, and the lanthanides from such elements as Fe, Cr, Ni, Co, Zn, Mo, Cd, Cu, Sn, Bi, Ag, Ge, Pd, Pt, Au, Rh, Ir, and Tl, which can, under suitable conditions, be deposited on a mercury cathode. The method is therefore of particular value for the determination of Al, etc., in steels and alloys it is also applied in the separation of iron from such elements as titanium, vanadium, and uranium. In an uncontrolled constant-current electrolysis in an acid medium the cathode potential is limited by the potential at which hydrogen ion is reduced the overpotential of hydrogen on mercury is high (about 0.8 volt), and consequently more metals are deposited from an acid solution at a mercury cathode than with a platinum cathode.10... 

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... 

Constant current electrolysis of triphenylphosphine in dichloromethane in the presence of amides and N,N -disubstituted... 

Figure 2. Partial currents for C02 reduction at various electrodes during constant-current electrolysis at 16 mA/cm2 in a 0.5 M NaHC03 solution as a function of hydrogen overvoltage of the metal used.20 Values21 of hydrogen overvoltage of the metals were those obtained in acidic solutions at 1 mA/cm2.

Various cation radical salts [(ppy)Au(S-S) 2 anion 11solvent (S-S = CgH4Sg or CgH4Sg02, anion = PF6, BF4 , AsFfi. TaFfi. solvent = PhCl, n = 0-0.5) were prepared by constant current electrolysis of their benzonitrile or chlorobenzene solutions containing (Bu Xanion) as electrolyte [26, 27, 35, 37]. 

The passage of l.OF/mol charge (constant current electrolysis) proved sufficient the yield of coupling product was 92%. Lower yields were observed with higher charge. Several examples of such homo-couplings75 were given. 

In constant-reactant concentration-constant current electrolysis,... 

In constant reactant concentration-constant current electrolysis, the bulk concentration of B also increase proportionally with time, but the proportionality factor is i/FV instead of C°/tc [equation (2.34)]. 

Constant current electrolysis is an easy way to operate an electrochemical cell. Usually, it is also applied in industrial scale electrolysis. For laboratory scale experiments, inexpensive power supplies for constant current operation are available (also a potentiostat normally can work in galvanostatic operation). The transferred charge can be calculated directly by multiplication of cell current and time (no integration is needed). 

Electrochemical processes are conducted under what one refers to as either constant current (CC) or controlled potential (CP) conditions [1,2]. The constant current electrolysis (CCE) is often preferred it is less expensive to implement since it does not require the acquisition of a potentiometer, is readily amenable to scale-up, and reaction times are often short. The disadvantage is that in order to maintain a constant current, the potential changes, become more negative or positive, depending on whether... 

A typical Kolbe electrolysis experiment calls for the constant current electrolysis of... 

Both inter- and intramolecular [5 + 2] cycloaddition modes have been utilized in the synthesis of natural products. Successful intermolecular cycloaddition depends on making an appropriate selection of solvent, supporting electrolyte, oxidation potential, and current density. This is nicely illustrated in Schemes 23 to 25. For example, in methanol the controlled potential oxidation of phenol (101) affords a high yield (87%) of (102), the adduct wherein methanol has intercepted the reactive intermediate [51]. In contrast, a constant current electrolysis conducted in acetonitrile rather than methanol, led to an 83% yield of quinone (103). 

The first coupling reaction of this type studied utilized a 3-methoxyphenyl ring as the aryl coupling partner (Scheme 36) [47a, c]. The reaction employed constant current electrolysis conditions and a reticulated vitreous carbon anode (RVC). A good yield of cyclized material was obtained. However, the reaction was plagued by the formation of secondary products derived from over-oxidation (35 and 36) of the initially formed cyclization products (33 and 34). The amount of over-oxidized material could be greatly reduced with the use of controlled potential electrolysis conditions. 

Anodic oxidation of JV,iV-disubstituted trifluoroethanimidamide 45 in dry and in aqueous acetonitrile gave the imidazole 46 and quinoneimine 47 as the reaction products (Scheme 24). The constant current electrolysis on a glassy carbon anode and a platinum cathode was performed in an undivided cell [74]. 

Mechanistic aspects of the intermolecular cyclization reaction in the anodic oxidation of catechol in the presence of 4-hydroxycoumarin were discussed in Sect. 2.2. This reaction is a synthetically simple and versatile method for the preparation of formally [3 + 2] cycloadducts between a -diketo compound and catechol [44,45]. Anodic oxidation of catechol using controlled potential electrolysis (E = 0.9-1.1 V vs SCE) or constant current electrolysis (i = 5 mA/cm ) was performed in water solution containing sodium acetate (0.15 mol/1) in the presence of various nucleophiles such as 4-hydroxycoumarin,... 

In the direct synthesis of thiophosphonium salts, the oxidizing agent can be a disulphide using either constant-current electrolysis (c.c.e) conditions or by simple addition with benzoic acid433 (reaction 122). 

By constant-current electrolysis of triphenylphosphine with alcohol and benzoic or succinic acid the alkoxyphosphonium perchlorates can be obtained434 (reaction 123). 

Electrochemical oxidation of formazans is a particularly advantageous preparative route to tetrazolium salts, which can be performed by controlled-potential or constant-current electrolysis. Tetrazolium salts with widely differing anions can be prepared by merely using a supporting electrolyte carrying the desired anion. The two-electron oxidative cyclization of forma-zan to tetrazolium salt may occur through an ECPE(d) mechanism. 

Typical concentration profiles at various instants during a constant current electrolysis are shown in Fig. 5. Note that [0co(x, t)/dt]x=0 is the same for all curves and that eqns. (23) are identical to the results of a diffusion layer approach , see eqn. (11), with at = (7rZ)0/4f)1/2 ... 

The general procedure for the electrochemical preparation of (10) is as follows. A solution of (9) (3 mmol) in wet acetonitrile (40 ml, 5 vol.% of H20) containing sodium perchlorate (0.25 m) was placed in an undivided electrolysis cell equipped with a platinum plate anode and a platinum plate cathode. The system was subjected to a constant current electrolysis (300 mA current density, 20mAcnr2) at ambient temperature. After 4 faradays per mole of (9) had been consumed, the electrolysed solution was poured into water (50 ml) and extracted with dichloromethane (3 X 30 ml). The organic layer was dried with magnesium sulfate and concentrated under reduced pressure. The residue was chromatographed on silica gel to afford (10) in an excellent yield. 

During the course of the simulation, the most important variables are the electrode surface concentrations of A and B because they determine E(t). These may be calculated directly by placing the electrode in the center of the first volume element in the model as in previous simulations. In this case, however, it turns out to be more straightforward to place the electrode at the exterior edge of the first volume and to calculate the electrode surface concentrations by extrapolating the concentration profiles to x = 0. This is illustrated in Figure 20.8. The extrapolation is made easier by the fact that one boundary condition in constant-current electrolysis requires that the concentration gradient at the electrode surface be constant ... 

Figure 29.17 Simulation results showing the fraction of the total current at each of 25 segments of the working electrode at different times during the constant-current electrolysis of 2.5 mAf anthraquinone and in 0.1 M tetrabutylammonium iodide-dimethyl-formamide solution. Total current 100 / A electrode dimensions 0.5 cm x 3 cm flat cell dimensions 0.5 cm x 0.9 cm x 3 cm.

Electrochemical bromo-lactonization of the triterpenoid oleanolic acid 80 has been performed in an MeOH—E NBr/fPhSe —(Pt) system to give 12a/p/in-bromo-28,13-he/a-oleanolide 81 in quantitative yield. On the other hand, constant current electrolysis of 80a or hederagenin 80b in an MeOH/AcOEt—Et4NBr—(C) system forms 82 in 90-92 % yields (Schetne 3-30) 78a>. 

The point at which the sample ion is depleted in the vicinity of the working electrode is called the transition time r this quantity is related to a number of variables including the sample-ion concentration. In 1901 Sand21 derived the equation that describes the functional dependence of the transition time for a constant-current electrolysis of a diffusion-controlled process... 

Constant current electrolysis also is applicable to thin-layer cells such that... 

Constant Current Electrolysis and Constant Potential Electrolysis [6]... 

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