Chlorine electrode process

At higher potentials, the anode surface is covered by carbon oxide leading to the extinction of active sites for the chlorine electrode process and to potential overshoot [37,40],... 

The Chlorine Electrode Process. Chlorine exists in various oxidation states [5-7] as shown in Table 4.1.3. Of these, the chloride ion and elemental chlorine couple... 

Reversible operation of the cell requires that no other process occurs in the cell than that connected with the current flow. An electrochemical process that need not be always connected with the passage of current is the dissolution of a metal in an acid (e.g. zinc in sulphuric acid in the Volta cell) or the dissolution of a gas in an electrolyte solution (e.g. in a cell consisting of hydrogen and chlorine electrodes, hydrogen and chlorine are dissolved... 

In an individual molten carbamide, the electrode processes are feebly marked at melt decomposition potentials because of its low electrical conductivity. Both electrode processes are accompanied by gas evolution (NH3, CO, C02, N2) and NH2CN (approximately) is formed in melt. In eutectic carbamide-chloride melts electrode processes take place mainly independently of each other. The chlorine must evolve at the anode during the electrolysis of carbamide - alkali metal and ammonium chloride melts, which were revealed in the electrolysis of the carbamide-KCl melt. But in the case of simultaneous oxidation of carbamide and NH4CI, however, a new compound containing N-Cl bond has been found in anode gases instead of chlorine. It is difficult to fully identify this compound by the experimental methods employed in the present work, but it can be definitely stated that... 

The committee notes that the electrode damage may become more severe when feeds containing fluorine and chlorine are processed. In the fluoride transfer test discussed earlier, the titanium electrodes were severely corroded and the Pt coating was pitted or peeling off. This more severe damage would be unacceptable during full-scale plant operation. 

In an alkali-chlorine cell a saturated (about 6 N) solution of sodium chloride is electrolyzed at ordinary temperatures, between a steel cathode (hydrogen overvoltage 0.2) and a graphite anode (oxygen overvoltage 0.6 volt chlorine overvoltage negligible). The nature of the electrode process. Explained ... 

The photoelectrochemical production of chlorine at nanocrystalline titanium dioxide thin film electrodes exposed to U V light has been reported [96]. In this process, the energy from photons substantially reduces the overpotential required for the chlorine evolution process and therefore less harsh conditions are required. Metal doping of the Ti02 photoelectrocatalyst was explored but found to be not beneficial for this process. In future, this kind of process could be of practical value, in particular, for water treatment and disinfection applications requiring low levels of chlorine. 

Passage of 1.0 mol of electrons (one faraday, 96,485 A s) will produce 1.0 mol of oxidation or reduction—in this case, 1.0 mol of Cl- converted to 0.5 mol of Cl2, and 1.0 mol of water reduced to 1.0 mol of OH- plus 0.5 mol of H2. Thermodynamically, the electrical potential required to do this is given by the difference in standard electrode potentials (Chapter 15 and Appendix D) for the anode and cathode processes, but there is also an additional voltage or overpotential that originates in kinetic barriers within these multistep gas-evolving electrode processes. The overpotential can be minimized by catalyzing the electrode reactions in the case of chlorine evolution, this can be done by coating the anode with ruthenium dioxide. 

Development of chlorine electrode materials has benefited from the experience of chlor-alkali electrolysis cell technology. The main problem is to find the best compromise between cycle life and cost. Porous graphite, subjected to certain proprietary treatments, has been considered a preferable alternative to ruthenium-treated titanium substrates. The graphite electrode may undergo slow oxidative degradation, but this does not seem to be a significant process. 

When the current flows, the chloride ions, Cl (aq), are attracted to the anode. Chlorine gas is produced by the electrode process. 

In the cell formed by combining with the hydrogen electrode the chlorine electrode emits anions into the solution and is charged positively. This means that a spontaneous reduction process takes place ... 

The theoretical decomposition voltage of chlorides can be calculated from the value of equilibrium oxidation potential of the chlorine electrode e t. cia1 ci-in the anolyte and the reversible reduction potential of the hydrogen electrode 7Toh- Ha. pt in alkaline catholyte. If we apply the Nemst equations for the corresponding electrochemical processes [see (XI-9) and (XI-10)] we obtain the... 

The anodic partial current may be a sum of several partial currents when two or more electrode processes take place simultaneously (see partial current) for instance, the evolution of chlorine and oxygen from aqueous hydrochloride acid solutions at high positive potentials. 

The Oads species could actually be RuO ,. Augustinski et al. [198] concluded that Ru03, as well as two different chloride species, were present on the surfaces of Ru02 and Ru02/Ti02 electrodes which had been used as anodes for chlorine evolution. These authors suggested that Ru(VI) species may play a role in the chlorine evolution process and concluded that reaction... 

In a recent study, Harrison et al. [485] used steady-state j-E and Z(co)-E data to characterize the chlorine evolution reaction at Ru02/Ti02 electrodes using a simple redox reaction description of the chlorine evolution process with HOC1 and CR as reactant and product, respectively. The impedance potential data were analyzed by the equivalent circuit method parameter curves such as CiX-E and Rct-E. It has been suggested by the authors [485] that this type of parametric analysis of impedance data can be useful for comparison of the activity of various types of electrodes. 

Radiolysis was also used to graft metal nanoparficles onto anodes or cathodes involved in the electrochemical chlorine-soda process. Bimetallic nanoparficles Pt-Ru and Ni-Ru grafted onto bulk metal electrodes (Ti or Ni) displayed a remarkable electro-catalytic efficiency for this reaction and a drastic decreasing of the overpotential was observed for Pt-Ru alloys (2 1 atomic ratio). 

Amalgam Cell Starting with the chlorine electrode, the reversible standard potential is +1.37 V (all potentials are given versus NHE), due to the sodium concentration of 6 mol L-1 we have to note +1.33 V. The current-overpotential curve represents an unhindered process... 

The conclusion just reached may be utilized to determine the dissociation pressure of chlorine gas in equilibrium with solid silver chloride for this purpose it is necessary to consider the problem of a gas electrode in which the pressure differs from the standard value of 1 atm. For such electrodes, the general equation (45.15) is still applicable, but allowance must be made for the activity, i.e., approximately the pressure, of the chlorine gas, as well as of the reversible ions. For a chlorine electrode, the gas is the oxidized state and the chloride ion the reduced state, and hence the electrode process may be written as... 

The kinetics of the chlorine electrode in different chloride melts was studied in the range 190-430°C. Different controlled processes involving the participation of chlorine atoms on graphite have been proposed [82, 83]. 

Another important field of membrane appUcatimi is the production of chlorine by the chlor-aUcali-electrolysis process. Here the mercury-electrode process has been replaced by the so-called membrane process and diaphragm cell electrolysis. 

Finally, we consider the membrane cells in Fig. 6.5. The electrode processes are the same as those in the diaphragm cells (Eqs. 1 and 2). Anolyte processing is quite similar to that practiced with mercury cells. We saw above in the discussion on brine treatment that membrane cells had stricter requirements. The same is true regarding dechlorination of the depleted brine. After vacuum dechlorination, the residual active chlorine content is high enough to damage the ion-exchange resin in the brine purification... 

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