Constant cathode potential electrolysis

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... 

The course of electrical reduction, like that of purely chemical reduction, depends decisively upon whether the reduction is carried out in an alkaline or acid solution. But these relations are of a positive nature in electrolysis only so long as they are not compensated by the electrical factors, such as cathode material and potential. To avoid a complication, it is necessary to limit the considerations primarily to unattackable cathodes and to take no account of an adjustment to certain and constant cathode potentials, and to exclude a secondary interference of the solvent, for instance by molecular rearrangements. In this general comprehension of the problem it can be said that the well-known chemical rule reoccurs in electrolytical... 

Experimental details for performing a controlled-cathode-potential electrolysis are presented in Section 24C-1. For the present, it is sufficient to note that the potential difference between the reference electrode and the cathode i.s measured with a voltmeter. The voltage applied between the working electrode and its counter electrode is controlled with a voltage divider so that the cathode potential is maintained at a level suitable for the separation. Figure 24-3 is a schematic of a manual apparatus that permits deposition at a constant cathode potential. 

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... 

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. 

Torsi et al. [395] have carried out a systematic investigation to establish the potential value of such an apparatus. The apparatus is basically an electrothermal device in which the furnace (or the rod) is replaced by a small crucible made of glassy carbon. Figure 5.10 provides an overall view of the apparatus. Figure 5.11 shows a block diagram of the electrolysis circuit the crucible (6) acts a cathode, while the anode is a platinum foil dipped into either the sample solution reservoir (1) or the washing solution reservoir (2). Pre-elecrolysis was performed at constant current with a 500 V dc variable power supply (5). Under these conditions, the cathode potential is not controlled, so that other metals can be codeposited with lead. 

Section 2 (Fig. 1, curves A and B), usually performed at the rotating platinum electrode (anode reactions) or the dropping mercury electrode (cathode reactions), should ideally suffice to define the electroactive species and determine its half-wave potential. It may be that systems in which acid-base equilibria exist are somewhat more laborious to study due to the necessity of recording voltammetric curves over a wide pH range, but in most cases the task can be accomplished with some effort. Once the voltammetric characteristics are known, it remains to carry out preparative constant potential electrolysis (cpe) at a suitable potential in order to make sure that the electroactive species is connected with the reaction of interest. 

The present synthesis is an adaptation of a previously reported synthesis4 in a divided cell (i.e., separate anode and cathode compartments). The overriding consideration in making this modification has been to simplify the operations involved and render the synthesis more attractive to chemists not well acquainted with electrochemical procedures. The main simplification achieved is that the pH is controlled internally via the anodic generation of protons as noted above (in the reported procedure4 this is achieved by periodic addition of acetic acid to the cathode compartment). A further simplification has been to run the reaction with a constant current rather than at controlled cathode potential. After the electrolysis has been initiated, the reaction requires no special attention. A small price is paid for the simplicity of the present synthesis in that the yield is somewhat lower than that obtained previously.4 The major by-product formed is diethyl succinate, which results from a 2e reduction of diethyl fumarate or diethyl maleate ... 

The scope of the reaction type has been explored in particular by Grimshaw and coworkers [276-286]. Since the rate constant for cleavage of the carbon-halogen bond decreases in the order I > Br > Cl > F, yields of coupling products are usually higher using chloro rather than bromo compounds. Most of the studies have been carried out by controlled potential electrolysis —2.1 V for the chloro compounds) in DMF using divided cells and Hg cathodes [276-285]. 

The pre-electrolysis of 1 mmol of MesSiCl for about 1.5 h preceded the main electrolysis to eliminate formed fiom hydrolyses of the chlorosilane by residual water. The silanone precursor (50 mmol) and a trap (55 mmol) were then injected into the cell using a syringe, and the electrolysis was continued with constant flow of O2 through the solution (5 mL min ) and the current density j 5 - 7 mA cmT, the cathode potential not to exceed -2 V. From THF and DMF solutions, the products were isolated as described earlier [6, 7] when the process was carried out in IL, thorough extraction with diethyl ether, evaporation of the latter and distillation of the organic residue were effected to isolate the products. 

In practice, electrolysis at a constant cell potential is limited to the separation of easily reduced cations from those that are more difficult to reduce than hydrogen ion or nitrate ion. The reason for this limitation is illustrated in Figure 22-7, which shows the changes of current, IR drop, and cathode potential during electrolysis in the cell in Figure 22-6. The analyte here is copper(II) ions in a solution containing an excess of sulfuric or nitric acid. Initially, R is adjusted so that the potential applied to the cell is about — 2.5 V, which, as shown in Figure 22-7a, leads to a current of about 1.5 A. The electrolytic deposition of copper is then completed at this applied potential. 

Figure 22-8 Apparatus for controlled-potential electrolysis. The digital voltmeter monitors the potential between the working and the reference electrode. The voltage applied between the working and the counter electrode is varied by adjusting contact C on the potentiometer to maintain the working electrode (cathode in this example) at a constant potential versus a reference electrode. The current in the reference...

Current Changes during an Electrolysis at Constant Applied Potential It is useful l4) consider the changes in current in the cell under discussion when the potential is held constant at -2.5 V throughout the electroly,sis. l. iider these conditions. the current decreases with time as a result of the depletion of copjter ions in the solution as well as the increase in cathodic concentration polarization. In... 

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