Electrolysis electrolytic cell

Key Terms Electrolysis Electrolytic cell Cathode Anode Voltaic cell... 

Sodium hydroxide is manufactured by electrolysis of concentrated aqueous sodium chloride the other product of the electrolysis, chlorine, is equally important and hence separation of anode and cathode products is necessary. This is achieved either by a diaphragm (for example in the Hooker electrolytic cell) or by using a mercury cathode which takes up the sodium formed at the cathode as an amalgam (the Kellner-Solvay ceW). The amalgam, after removal from the electrolyte cell, is treated with water to give sodium hydroxide and mercury. The mercury cell is more costly to operate but gives a purer product. 

Conversion of aqueous NaCl to Cl and NaOH is achieved in three types of electrolytic cells the diaphragm cell, the membrane cell, and the mercury cell. The distinguishing feature of these cells is the manner by which the electrolysis products are prevented from mixing with each other, thus ensuring generation of products having proper purity. 

Chloiine is pioduced at the anode in each of the three types of electrolytic cells. The cathodic reaction in diaphragm and membrane cells is the electrolysis of water to generate as indicated, whereas the cathodic reaction in mercury cells is the discharge of sodium ion, Na, to form dilute sodium amalgam. 

In some cases, particularly with iaactive metals, electrolytic cells are the primary method of manufacture of the fluoroborate solution. The manufacture of Sn, Pb, Cu, and Ni fluoroborates by electrolytic dissolution (87,88) is patented. A typical cell for continous production consists of a polyethylene-lined tank with tin anodes at the bottom and a mercury pool (ia a porous basket) cathode near the top (88). Pluoroboric acid is added to the cell and electrolysis is begun. As tin fluoroborate is generated, differences ia specific gravity cause the product to layer at the bottom of the cell. When the desired concentration is reached ia this layer, the heavy solution is drawn from the bottom and fresh HBP is added to the top of the cell continuously. The direct reaction of tin with HBP is slow but can be accelerated by passiag air or oxygen through the solution (89). The stannic fluoroborate is reduced by reaction with mossy tin under an iaert atmosphere. In earlier procedures, HBP reacted with hydrated stannous oxide. 

Manganate(VI) formed in the initial oxidation process must first be dissolved in a dilute solution of potassium hydroxide. The concentrations depend on the type of electrolytic cell employed. For example, the continuous Cams cell uses 120 150 g/L KOH and 50 60 g/L K MnO the batch-operated Bitterfeld cell starts out with KOH concentrations of 150 160 g/L KOH and 200 220 g/L K MnO. These concentration parameters minimize the disproportionation of the K MnO and control the solubiUty of the KMnO formed in the course of electrolysis. 

Electrochemical processes require feedstock preparation for the electrolytic cells. Additionally, the electrolysis product usually requires further processing. This often involves additional equipment, as is demonstrated by the flow diagram shown in Figure 1 for a membrane chlor-alkali cell process (see Alkali AND chlorine products). Only the electrolytic cells and components ate discussed herein. 

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

Production of potassium permanganate in the CIS is beheved to be from potassium manganate. Electrolysis of potassium manganate in a continuous-flow electrolytic cell with turbulent electrolyte flow and continuous crystallization has been reported (72). 

Aluminum. Aluminum [7429-90-5] Al, is produced worldwide by the Bayer-HaH-Heroult process. This process involves the electrolysis of alumina [1344-28-1J, AI2O2, dissolved in molten cryoHte [15096-52-3] Na AIF (see Aluminumand aluminum alloys). The electrolytic cells or pots operate... 

Stress corrosion can arise in plain carbon and low-alloy steels if critical conditions of temperature, concentration and potential in hot alkali solutions are present (see Section 2.3.3). The critical potential range for stress corrosion is shown in Fig. 2-18. This potential range corresponds to the active/passive transition. Theoretically, anodic protection as well as cathodic protection would be possible (see Section 2.4) however, in the active condition, noticeable negligible dissolution of the steel occurs due to the formation of FeO ions. Therefore, the anodic protection method was chosen for protecting a water electrolysis plant operating with caustic potash solution against stress corrosion [30]. The protection current was provided by the electrolytic cells of the plant. 

The battery acts as an electron pump, pushing electrons into the cathode, C, and removing diem from the anode, A. To maintain electrical neutrality, some process within the cell must consume electrons at C and liberate them at A. This process is an oxidation-reduction reaction when carried out in an electrolytic cell, it is called electrolysis. At the cathode, an ion or molecule undergoes reduction by accepting electrons. At the anode, electrons are produced by the oxidation of an ion or molecule. 

An electrolysis experiment is performed to determine the value of the Faraday constant (number of coulombs per mole of electrons). In this experiment, 28.8 g of gold is plated out from a AuCN solution by running an electrolytic cell for two hours with a current of 2.00 A. What is the experimental value obtained for the Faraday constant ... 

Sodium perchlorate is produced by the electrolysis of sodium chlorate. If a current of 1.50 X 103 A passes through an electrolytic cell, how many kilograms of sodium perchlorate are produced in an eight-hour run ... 

Cathode. The cathode is the electrode at which reduction occurs. In an electrolytic cell it is the electrode attached to the negative terminal of the source, since electrons leave the source and enter the electrolysis cell at that terminal. The cathode is the positive terminal of a galvanic cell, because such a cell accepts electrons at this terminal. 

Similar considerations apply of course to the opposing electromotive forces of polarisation during electrolysis, when the process is executed reversibly, since an electrolytic cell is, as we early remarked, to be considered as a voltaic cell working in the reverse direction. In this way Helmholtz (ibid.) was able to explain the fluctuations of potential in the electrolysis of water as due to the variations of concentration due to diffusion of the dissolved gases. It must not be forgotten, however, that peculiar phenomena—so-called supertension effects—depending on the nature of the electrodes, make their appearance here, and com-... 

Tlie anhyd salt is obtained when samples are recrystd from w above 53° below this temp a monohydrate is obtained (see below). The pure salt is best obtained on a lab scale by dissolving pure Na carbonate in a slight excess of dil aq perchloric ac, the soln partly evapd, cooled to 50°, the solid centrifuged off, and dried in a current of air at 250°. Similar results were obtained starting with pure Na chloride (Ref 2). On a coml scale it is prepd by the electrolysis of Na chlorate (see Vol 2, C197-R). Processing details and economics of the prepn are given in Refs 5 11. Coned solns are used, and modern plants use continuous electrolytic cells. In 1960 prodn was estimated to be ca 10000 tons/year at a cost of 17.56 /lb (Ref 11, p 87)... 

An electrochemical cell in which electrolysis takes place is called an electrolytic cell. The arrangement of components in electrolytic cells is different from that in galvanic cells. Typically, the two electrodes share the same compartment, there is only one electrolyte, and concentrations and pressures are far front standard. As in all electrochemical cells, the current is carried through the electrolyte by the ions present. For example, when copper metal is refined electrolytically, the anode is impure copper, the cathode is pure copper, and the electrolyte is an aqueous solution of CuS04. As the Cu2f ions in solution are reduced and deposited as Cu atoms at the cathode, more Cu2+ ions migrate toward the cathode to take their place, and in turn their concentration is restored by Cu2+ produced by oxidation of copper metal at the anode. 

Aluminum is produced by electrolysis of its oxide dissolved in molten cryolite (Na AIFJ. Find the mass of aluminum that can be produced in 1.00 day (d) in an electrolytic cell operating continuously ai 1.00 X 105 A. The cryolite does nor react. 

The term chlor-alkali refers to those products obtained from the commercial electrolysis of aqueous sodium chloride. These are chlorine, sodium hydroxide, and sodium carbonate. The first two are produced simultaneously during the electrolysis while the latter is included because it is also produced in small quantities and shares many of the end uses of sodium hydroxide. Perfluorinated ionomer membranes are permeable to sodium ions but not the chloride ions, and hence they are useful for these electrolytic cells. The arrangement of a typical membrane cell is shown in Figure 10.2. 

As world deposits of petroleum and coal are exhausted, new sources of hydrogen will have to be developed for use as a fuel and in the production of ammonia for fertilizer. At present, most hydrogen gas is produced from hydrocarbons, but hydrogen gas can also be generated by the electrolysis of water. Figure 19-23 shows an electrolytic cell set up to decompose water. Two platinum electrodes are dipped in a dilute solution of sulfuric acid. The cell requires just one compartment because hydrogen and oxygen escape from the cell much more rapidly than they react with each other. 

The potential supplied to an electrolytic cell determines whether or not electrolysis can occur, but the current flow and the time of electrolysis determine the amount of material electrolyzed. Recall Equation from Section 19-1 ... 

Electrolytic cells are constructed of materials that can withstand the action of the electrolytes and of the electrode products. The cell may be of the open type or may be partially or fully closed, depending on the requirement of handling the electrode products. Some of these cells will be described while dealing with the production of specific metals. Very stringent requirements are imposed when considering the design of electrolytic cells for the deposition of refractory and reactive metals. Most of such metals are produced by using molten salt electrolytes. These metals are prone to atmospheric contamination at the electrolysis temperature, and it is thus necessary to operate the cell under an inert atmosphere. 

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