Electrodes Chlorine

Indicator electrodes for anions may take the form of a gas electrode (e.g. oxygen electrode for OH- chlorine electrode for Cl-), but in many instances consist of an appropriate electrode of the second kind thus as shown in Section 15.1, the potential of a silver-silver chloride electrode is governed by the chloride-ion activity of the solution. Selective-ion electrodes are also available for many anions. 

The theoretical OCV of cells consisting of a hydrogen electrode (in alkaline solution) and a chlorine electrode is 2.17 V. Practical cell voltages of most modem... 

Figure 6.17 Production of an emf in a cell consisting of a zinc electrode and a chlorine electrode in zinc chloride solution.

A definite decomposition voltage occurs for the following reason. As soon as there is a potential difference between the electrodes, H+ ions move to the cathode and Cl ions to the anode. The ions are discharged, forming layers of adsorbed gas on the inert metal surfaces. This essentially amounts to having a hydrogen electrode and a chlorine electrode in place of the two platinum electrodes. The outcome is a typical chemical cell ... 

Since the oxygen overvoltage on a platinum electrode is equal to 0.45 V, the minimum potential required for oxygen evolution is (+ 0.813 + 0.45) V or 1.263 V. Since the voltage required to implement this reaction is lower than that for a normal chlorine electrode, it follows that oxygen will be evolved at the anode in preference to chlorine. In contrast to the two possible reactions at the anode, there are five reactions that are possible at the cathode as shown below ... 

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... 

The chlorine electrode contains gaseous chlorine in equilibrium with atomic chlorine adsorbed on the platinum black and a solution of chloride ions. Its potential is given by the equation... 

The chlorine electrode behaves reversibly, i.e. electrode equilibrium is... 

In chloride melts, the chlorine electrode (with graphite instead of platinum) is used as a reference electrode (see Table 3.3). 

Other methods for the determination of chlorine in seawater or saline waters are based on the use of barbituric acid [13] and on the use of residual chlorine electrodes [ 14] or amperometric membrane probes [15,16]. In the barbituric acid method [12], chlorine reacts rapidly in the presence of bromide and has completely disappeared after 1 minute. This result, which was verified in the range pH 7.5-9.4, proves the absence of free chlorine in seawater. A study of the colorimetric deterioration of free halogens by the diethylparaphenylene-diamine technique shows that the titration curve of the compound obtained is more like the bromine curve than that of chlorine. The author suggests... 

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],... 

At pH 2 (the lowest pH value maintained in chloralkali electrolysis anolytes), the equilibrium potential of the oxygen anode is more cathodic than that of the chlorine electrode (+0.96 V vs NHE compared to +1.26 V). Therefore it is of ultimate importance that Ru02 catalyzes chlo-... 

In redox electrodes an inert metal conductor acts as a source or sink for electrons. The components of the half-reaction are the two oxidation states of a constituent of the electrolytic phase. Examples of this type of system include the ferric/ferrous electrode where the active components are cations, the ferricyanide/ferrocyanide electrode where they are anionic complexes, the hydrogen electrode, the chlorine electrode, etc. In the gaseous electrodes equilibrium exists between electrons in the metal, ions in solution and dissolved gas molecules. For the half-reaction... 

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. 

A chlorine electrode shows a similar behavior as a hydrogen electrode because its potential is a function of the pressure of the gas surrounding the electrode and of the activity of chloride ions in the solution. The oxidation potential will be calculated by the already known method from the corresponding equilibrium... 

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 ... 

We can, therefore put the equation (VI-18) equal to the equation for the potential of the chlorine electrode (VI-16), whereby the following formula will be obtained ... 

The calomel electrode has, therefore, the same function as the chlorine electrode. It does not work at atmospheric pressure of chloride but at a very low pressure which corresponds to the equilibrium pressure of chlorine set free by the dissociation of calomel. 

In a cell formed by the combination of hydrogen and chlorine electrodes, dipped into diluted hydrochloric acid possessing the moan activity a ,... 

Should the order of the electrodes in both cells change so that the chlorine electrodes become the outer ones, the resultant EMF would be negative because the right hand electrode becomes the negative pole of the system. 

When dipping two identical chlorine electrodes into two hydrochloric acid solutions of different concentrations [(a+)a > (a+)jJ instead of the hydrogen electrodes, a concentration cell with transference will be obtained, reversible with respect to the anions ... 

The influence of the relative values of the transference numbers, affecting the resultant value of the EMF of the concentration cell without transference, is clearly to be seen from the equation (VI-29) should t.. > <+ then eK is positive and in a concentration cell reversible with respect to cations the liquid junction potential is added to the sum of the electrode potentials should, however, < t+, then the liquid junction potential will lower the resultant EMF. In a concentration cell reversible with respect to anions (e. g. in a cell with chlorine electrodes) the EMF is decreased when ( >(+, and increased when t. < t+. 

Electrons drawn away from the chlorine electrode are replaced by new ones, produced in the reaction ... 

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... 

An anolyte containing 265 grams of NaCl per litre has 5.026 moles of NaCl in 1000 grams of water (i. e. mNa+ = Wei- = 5.026). In the Table 8 it will be seen that the mean activity coefficient of sodium chloride in this solution equals 0.877. This value will be further considered to coincide with th activity coefficient of chloride ions (ya-) in a solution of the same concentration. The potential of the chlorine electrode then equals ... 

The theoretical decomposition voltage of sodium chloride may be calculated as the sum of the reversible oxidation potential of the chlorine electrode and the reversible reduction potential of the amalgam electrode. 

The potential of a gas electrode X2/X results from the equilibrium between the electrons of an inert conductor, gas molecules (X2) dissolved in the electrolyte or adsorbed on the electrode and X ions in the electrolyte. If we presume the simultaneous existence of chlorine atoms and molecules adsorbed on the graphite electrode, the behavior of the chlorine electrode results from an equilibrium between chlorine on the electrode and in the electrolyte, according to the reaction... 

In acidic melts, the A12C17 ions are predominant. In basic melts, per is constant, in the presence of solid NaCl [401], and it is equal to 1.1 at 175°C. Likewise, in the molten chloroaluminate systems the terms acid and base denote a chloride ion acceptor and a chloride ion donor, respectively. The pCL may be measured with either an aluminum or a chlorine electrode immersed in the melt. 

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