Electrolysis requirement

Membranes in chlor-alkali electrolysis require highly pure brine feed the water used in membrane water electrolyzers must also be rather pure. 

Electrogravimetry is one of the oldest electroanalytical methods and generally consists in the selective cathodic deposition of the analyte metal on an electrode (usually platinum), followed by weighing. Although preferably high, the current efficiency does not need to be 100%, provided that the electrodeposition is complete, i.e., exhaustive electrolysis of the metal of interest this contrasts with coulometry, which in addition to exhaustive electrolysis requires 100% current efficiency. 

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. 

In both regimes, speeding up electrolysis requires diminishing the time constant of the cell [equation (2.32)] by decreasing the volume-to-surface area ratio and/or the thickness of the diffusion layer by increasing the rate of stirring or of electrolyte circulation. 

Successful handling of the cell and performance of the electrolysis requires ... 

One of the most important electrolytic processes is the extraction of aluminum from an ore called bauxite. This ore is mainly composed of hydrated aluminum oxide, AI2O3 XH2O. (The x in the formula indicates that the number of water molecules per formula unit is variable.) In industry, the scale of production of metals is huge. The electrolytic production of aluminum is over two million tonnes per year in Canada alone. As you know from Faraday s law, the amount of a metal produced by electrolysis is directly proportional to the quantity of electricity used. Therefore, the industrial extraction of aluminum and other metals by electrolysis requires vast quantities of electricity. The availability and cost of electricity greatly influence the location of industrial plants. 

The term water electrolysis implicitly means that the electrochemical reactor does not contain pure water only. Conventional electrolysis requires that the solution should be electrically conducting for the process to proceed. This implies that an electrolyte should be dissolved in water. Whereas in other cases, for example electrochemical organic or inorganic processes, the presence of an inert electrolyte may constitute a problem for the separation of products, this is not the case for water electrolysis since gaseous products are obtained. Nevertheless, the electrolyte can give other kinds of problems, such as corrosion phenomena, poisoning of electrodes and so on. 

The average conditions extracted from Figure 7.17 are summarized in Table 7.2. Overall, actual alkaline electrolysis requires an energy consumption of 4.0-4.9 kW hm of H2. Consumption somewhat lower than 4.0kWhm has recently been claimed for SPE cells [73]. The current yield and H2 purity are seen to be close to 100% in alkaline electrolysis. 

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 ... 

Problems of the Electrical Field The application of electrodes to a curtain is a difficult problem. To obtain a homogeneous field and at the same time to eliminate products of electrolysis requires paradoxically contradictory conditions. If the field is applied outside, at the bottom of the curtain, the field is com-... 

A common controlled potential electrolysis requires three electrodes—a cathode, an anode, and a reference electrode, of which either the cathode or the anode is the working electrode. 

Theoretical considerations and actual measurements of the working electrode potential under working conditions show that a uniform potential distribution is possible only with concentric electrodes of the same area [7, 8]. There is thus a limitation to how exactly the potential indicated by the potentiostat reflects the working potential at different parts of the electrode. In actual practice, a controlled potential electrolysis (CPE) is carried out in a small potential range around some average value. In the author s experience, a selective electrolysis requires a separation of 0.15-0.2 V in Ei/o between the two reactions. 

The excess silver ion is then removed electrochemically at a cylindrical platinum net electrode (1.37 0.01 V)4 in an apparatus such as presented in Fig. 1. In a typical run, the electrolysis requires 110-120 hours. During this time, the current initially is 6-8 mA, rises to —9-12 mA after 60 hours, and drops to 3-5 mA after 110 hours. The resulting yellow [Pt(H20)4](C104)2 stock solution5 has been demonstrated to produce bis[2,4-pentanedionato(l-)]-platinum(II) and other platinum(II) complexes uncontaminated by other metal ions. 

Explain why producing a kilogram of silver from its ions by electrolysis requires much less electric energy than producing a kilogram of aluminum from its ions. 

The chemical reaction occurring in electrolysis requires a minimum voltage. Sufficient voltage also must be supplied to overcome internal resistance in the electrolyte. Because an electrolyte may contain a variety of ions, alternative reactions are possible. In aqueous electrolytes, positively charged hydrogen ions are always present and can be reduced at the cathode. In a water solution of sodium chloride, hydrogen ions are reduced to the exclusion of sodium ions, and no sodium metal forms. The only metals that can be liberated by electrolysis from aqueous solution are those such as copper, silver, cadmium, and zinc, whose ions are more easily reduced than hydrogen ions. 

Water electrolysis requires deionized water, electricity and heat. 

Hot Electrolysis. Electrolysis (Dutta 1990 Quandt 1986) can be operated at high temperatures (700 to 900 C) to replace some of the electrical input with thermal energy. Because heat is cheaper than electricity, the H2 costs via this production method could ultimately be lower than for traditional electrolysis. However, the technology (Sheffield June 2000) is currently in an early state of development with high capital costs (> 1300/kW). Hot electrolysis requires collocation of H2 production with the nuclear reactor to provide the heat. 

Electrolysis requires a source of freshwater, which, globally, is also a scarce resource. 

This is admittedly an unusual way of presenting metal deposition. At first sight it might be argued that the electrode is charge positively, but in this sense Eq. (19.22) is not different from Eq. (19.1) or (19.2), where electrons are removed from the metal to neutralize the positive ion in solution. In both cases one must bear in mind that electrolysis requires two electrodes. The electrons removed from the metal in Eq. (19.1) are replenished through the external circuit (not across the interface) from the anode, where an oxidation reaction takes place and electrons are released. The only difference between Eq. (19.1) and Eq. (19.22) is that in the former it is tacitly assumed that the cation is neutralized when it is still in the solution phase (at the OHP, to be exact) while in the latter it is assumed that the metal cation first crosses the interface and is neutralized after it has reached the metal phase. 

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