Cell, electrolytic

The cell employed for the electrolytic production of NF3 was a cylindrical nickel cell of 1.5 dm in volume. A BDD with boron concentrations of 2500, 5000, 7500, 8000,10 000, and 12 500 ppm was used as the anode for the electrolytic production of NF3. For galvanostatic measurement of the anode polarization curve, a carbon anode (FE-5 Toyo Tanso Co., Ltd.) was also used in addition to the BDD anode. The BDD thin film (Permelec Electrode Ltd.) was prepared by the hot filament chemical vapor deposition (HFCVD) method on a carbon substrate using a gas mixture composed of CH4-H2-B(CH30)3-Ar. The anode was located at the center of the cell and the cell wall was utilized as the cathode. A nickel rod of 0.1 cm surface area pre-treated with anodic oxidation in a dehydrated NH4F-2HF melt was used as the reference electrode. The nickel rod functions as NiF Oy/Ni (0.073 V versus RHE) [4]. A PTFE skirt was provided between the anode and the cathode compartments, and the anode gas was separated from hydrogen evolved at the cathode to prevent an explosion. The cell bottom was covered with a PTFE sheet to avoid hydrogen evolution. 

1 Electrolysis of NH4F-2HF melt with saturated concentration of Ni + ion 

Let s now consider an electrochemical system analogous to the galvanic Daniell cell but that differs from it by the presence of a power supply instead of the electrical 

It is just the opposite of that occurring in Daniell s galvanic cell. Reaction (13.6) cannot be achieved spontaneously in a pure chemical manner without receiving energy (heat, for example) from the surroundings. An electrochemical cell whose reaction cell is not spontaneous is called an electrolytic cell or a substance-producing device (Fig. 13.3). 

From the electrostatic standpoint, when the cell behaves as an electrolytic cell, the zinc wire brings a negative charge and the copper wire a positive charge as in the galvanic behavior mode, but in the first case the behavior is imposed, whereas in the second, it is spontaneous. 

The success of reaction (13.6) achieved in an electrochemical way is not miraculous from an energy standpoint. Both half-reactions (13.4) and (13.5) ask for external energy, the electrical energy coming from the power supply in the occurrence. Likewise, the equivalent redox reaction (13.6) achieved in a chemical manner asks for 

In brief, an electrolytic cell permits the preparation of some compounds that cannot spontaneously be obtained by a chemical means without a capture of energy, wherefrom the qualifier given above. 

Electrochemistry is the study of chemical reactions that result in the production of electric current, and chemical reactions that occur when subjected to electric current. Electrochemical applications are part of everybody s day-to-day life. Energy storing batteries that we use for TV remotes, flashlights, automobiles are a few examples. Electroplating is another achievement among many others. In this chapter, our discussion will revolve aroimd two types of electrochemical cells - the electrolytic cell and the galvanic cell. 

In an electrolytic cell, electric current drives the chemical reaction. The chemical reaction involved in an electrolytic cell is nonspontaneous. Electric current is used to drive the reaction. This process is called electrolysis and hence the name, electrolytic cell. The reaction involves the transfer of electrons and thus it is a redox reaction. For further understanding of the functioning of an electrolytic cell, we will look at an example of an electrolytic cell involving the electrolysis of molten sodium chloride. Molten sodium chloride is a good conductor of electricity. The melting point of NaCl is around 800° C. 

As the reaction proceeds, the sodium ions (Na ) are reduced to sodium (Na) at the cathode, and the sodium metal is deposited at the cathode. On the other hand, the chloride (Cl) ions are oxidized at the anode forming chlorine gas (Clj). The half-reactions and the overall reaction are represented below  

Section 5.5 dwelled on the transport of charged mixtures and the derivation of the basic transport equations. Recall that for an infinite diluted mixture, the transport of ions takes place due to their migration in the electric field, diffusion and convection. As in the Section 5.5, we limit ourselves to the study of a binary electrolyte mixture, for which (in the case of electrically neutral mixture) the distribution of reduced ion concentration is described by a convective diffusion equation, with the effective diffusion coefficient given by (5.96). The solution of Eq. (5.94) allows us to find the distribution of electric potential. In Eq. (5.98), we can form scalar products of both parts with dx, where x is the radius-vector, and then use the relation between diffusion coefficients of ions and their mobility D = ATi . Integrating the resultant expression, we then find the potential difference Ap between two points of the mixture  

The expression for potential difference consists of two terms. The first term has the meaning of ohmic potential drop caused by the resistance of the medium to propagation of electric current of density i. The second term, called the diffusion potential drop, is related to the gradient of concentration, that is, to the presence of regions of concentration polarization. This term is caused by the difference in diffusion rates of charged particles and the occurrence of diffusion flux (the second term in Eq. (5.98)). 

In order to solve the equation for electric potential (5.94) and the diffusion equation (5.93), it is necessary to specify initial and boundary conditions. Usually the electrolyte is located in the region confined by the electrodes, which are metal plates (one example is copper plate electrodes immersed in a solution of copper vitriol). 

The system consisting of two electrodes connected by a liquid conductor (electrolyte) is called an electrolytic cell. If electric current caused by an external EMF 

As an example, consider an electrolytic cell (Fig. 7.1) consisting of copper electrodes spaced by a layer of electrolyte - water solution of copper vitriol [3, 4]. 

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The preference is for a process based on ethylene rather than the more expensive acetylene and chlorine rather than the more expensive hydrogen chloride. Electrolytic cells are a much more convenient and cheaper source of chlorine than hydrogen chloride. In addition, we prefer to produce no byproducts. 

Castner-Kellner cell An electrolytic cell for the production of sodium hydroxide. ... 

Petroleum coke is an excellent fuel, and that is its main use, especially for the coke from fluid coking". There are some other markets that have to do with calcined coke electrodes for aluminum production or for all other electrolytic cells, carbons for electro-mechanical equipment, graphite, and pigments. 

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. 

The ammonium hydrogensulphate is returned to the electrolytic cell. A process such as this yields an aqueous solution containing about 30% hydrogen peroxide. The solution can be further concentrated, yielding ultimately pure hydrogen peroxide, by fractional distillation but the heating of concentrated hydrogen peroxide solutions requires care (see below). 

The Monsanto adiponitrile process, first commercialized in 1965 (65—67), involves the dimerization of acrylonitrile at the cathode in an electrolytic cell (eq. 7) ... 

This reaction has a positive free energy of 422.2 kj (100.9 kcal) at 25°C and hence energy has to be suppHed in the form of d-c electricity to drive the reaction in a net forward direction. The amount of electrical energy required for the reaction depends on electrolytic cell parameters such as current density, voltage, anode and cathode material, and the cell design. 

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. 

The catholyte from diaphragm cells typically analyzes as 9—12% NaOH and 14—16% NaCl. This ceUHquor is concentrated to 50% NaOH in a series of steps primarily involving three or four evaporators. Membrane cells, on the other hand, produce 30—35% NaOH which is evaporated in a single stage to produce 50% NaOH. Seventy percent caustic containing very Httie salt is made directiy in mercury cell production by reaction of the sodium amalgam from the electrolytic cells with water in denuders. 

Electrolytic Cell Operating Characteristics. Currently the greatest volume of chlorine production is by the diaphragm ceU process, foUowed by that of the mercury ceU and then the membrane ceU. However, because of the ecological and economic advantages of the membrane process over the other systems, membrane ceUs are currently favored for new production facHities. The basic characteristics of the three ceU processes are shown in Eigure 5. 

Fluorine was first produced commercially ca 50 years after its discovery. In the intervening period, fluorine chemistry was restricted to the development of various types of electrolytic cells on a laboratory scale. In World War 11, the demand for uranium hexafluoride [7783-81-5] UF, in the United States and United Kingdom, and chlorine trifluoride [7790-91 -2J, CIF, in Germany, led to the development of commercial fluorine-generating cells. The main use of fluorine in the 1990s is in the production of UF for the nuclear power industry (see Nuclearreactors). However, its use in the preparation of some specialty products and in the surface treatment of polymers is growing. 

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. 

Hexafluorozirconic acid is used ia metal finishing and cleaning of metal surfaces, whereas the fluorozirconates are used in the manufacture of abrasive grinding wheels, in aluminum metallurgy, ceramics industry, glass manufacturing, in electrolytic cells, in the preparation of fluxes, and as a fire retardant (see Abrasives Metal surface treati nts). 

Brine Treatment. The principal use of aqueous HCl is for the acidification of brine prior to feeding it to the electrolytic cells for producing chlorine and caustic soda. Almost all of this HCl comes from captive sources. An estimated 213 thousand metric tons of HCl (100% basis) was used for brine treatment in 1993 (74). 

The spray dried MgCl2 powder is melted ia large reactors and further purified with chlorine and other reactants to remove magnesium oxide, water, bromine [7726-95-6], residual sulfate, and heavy metals (27,28). The molten MgCl2 is then fed to the electrolytic cells which are essentially a modification of the LG. Farben cell. Only a part of the chlorine produced is required for chlorination, leaving up to 1 kg of chlorine per kg of magnesium produced. This by-product chlorine is available for sale. 

Liquid magnesium is removed from the electrolytic cells under vacuum and transferred to the cast house where it is refined, purified, and cast iato a wide variety of shapes, sizes, and alloys. 

Russian production may be going to a flow line cell concept (35). In this process, dehydrated camaOite is fed to a chamber where it is mixed with spent electrolyte coming from the electrolytic cells. The spent electrolyte first enters a metal collection chamber, where the molten magnesium is separated. The electrolyte is then enriched with camaOite and any iasoluble impurities are allowed to settle. The enriched electrolyte is then returned to the electrolytic cells. The result is that most of the remaining impurities are removed ia the first electrolytic cell. 

Molten anhydrous magnesium chloride is tapped from the bottom of the reactor. Iron, aluminum, and siUcon-based impurities are also converted to their chlorides, which volatili2e out of the reactor. Carbon monoxide is generated from coke, carbon dioxide, and oxygen. The magnesium chloride is sent to electrolytic cells. Russian diaphragmless cells purchased from the defunct American Magnesium Co. are used. 

Dead Sea Works Process. The Dead Sea Works, a subsidiary of Israel Chemicals Ltd., aimounced plans ia 1992 to constmct a 25,000 t/yr magnesium plant at Beer-Sheva, Israel. The plant, to be based on Russian camaHite technology, is designed to use an existing potash plant as the source of camaHte. The chlorine by-product can be either Hquefted and sold, or used ia an existing bromine plant. Waste streams from the camaHite process, as well as spent electrolyte from the electrolytic cells, can be returned to the potash plant. 

When magnesium oxide is chlorinated in the presence of powdered coke or coal (qv), anhydrous magnesium chloride is formed. In the production of magnesium metal, briquettes containing CaCl2, KCl, NaCl, MgO, and carbon are chlorinated at a temperature such that the electrolyte or cell melt collects at the bottom of the chlorinator, enabling the Hquid to be transferred directly to the electrolytic cells. 

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