Concentric cathode cell

The use of a concentric cathode cell [10] has been described for the recovery of precious metal on a small scale (Wilson Process Systems), from spent photographic solutions and from electroplating and refining wastes. The recovery of the metal is by manually scraping from the cathode or by furnace refining in the case of gold. 

The development of a rechargeable polymer battery is being pursued worldwide. Its attraction lies in the specific weight of polymers, which is considerably lower than that of ordinary inorganic materials, as well as potential environmental benefits. In principle there are three different types of battery. The active polymer electrode can be used either as cathode (cell types 1, 2), or as anode (cell type 3), or as both cathode and anode (cell type 4) (Fig. 14). As the most common polymer materials are usually only oxidizable, recent research has concentrated on developing cells with a polymer cathode and a metal anode. 

The water concentration in the paint and in the paint film has been determined using a Mitsubishi moisture meter. The anode cell was filled with Karl-Flscher reagent and the cathode cell with a mixture of pyridine, formamlde and Karl-Flscher reagent (70/30/6Z (v/v)). Paint samples were injected directly into the cathode solution. 

In summary, when the concentration of the substance at the anode is greater than the concentration at the cathode, the cell emf decreases. When the value of the concentration at the anode is lower than the concentration at the cathode, cell emf increases. 

If the cell metal concentrated solution dilute solution metal is capable of yielding a current, the direction of the current must be such that the concentrated solution becomes more dilute and the dilute solution more concentrated. The positive current must therefore fiow from the dilute to the concentrated solution inside the cell, so that the electrode dipping into the concentrated solution becomes the cathode. As 1 —i/ equivalents of the electrolyte = 2(l —j/) equivalents of the ions are transferred by unit quantity of electricity from the more concentrated (cathode) solution to the more dilute (anode) solution, the E.M.F. of this concentration cell can be calculated by the same two methods (p. 354) which Helmholtz and Nemst employed in the calculation of the e.m.f. of concentration cells without transference. Thus, for dilute solutions of an w-valent metallic salt, we have the equation... 

We saw above that the concentration gradient at an electrode will be linear with respect to the spatial coordinate perpendicular to the electrode surface if the anode/cathode cell were operated at a constant current density and if the fluid velocity were zero. In actuality, there will always be some bulk liquid electrolyte stirring during current flow, either an imposed forced convection velocity or a natural convection fluid motion due to changes in the reacting species concentration and fluid density near the electrode surface. In electrochemical systems with fluid flow, the mass transfer and hydrodynamic fluid flow equations are coupled and the solution of the relevant differential equations is often a formidable task, involving complex mathematical and/or numerical solution techniques. The concept of a stagnant diffusion layer or Nemst layer parallel and adjacent to the electrode surface is often used to simplify the analysis of convective mass transfer in... 

The cell performances were estimated in correlation to the electrochemical surface area (ESA) of the catalyst and mass-transport limitations in the cathode catalyst layer. The experimental data are discussed in terms of temperature, methanol concentration, cathode gas humidity and flow rate. 

In the voltaic cells we have looked at thus far, the reactive species at the anode has been different from the reactive species at the cathode. Cell emf depends on concentration, however, so a voltaic cell can be constructed using the same species in both half-cells as long as the concentrations are different. A cell based solely on the emf generated because of a difference in a concentration is called a concentration cell. 

The largest of these processes remains those for ammonium and sodium persulphate and there are several plants operating on the 2000—10000 ton yr scale. The process involves the oxidation of the sulphate in a sulphuric acid medium at a Pt-based anode and the medium must be free of heavy-metal ions which catalyse the decomposition of persulphate. The current density is high, about 1 A cm" and the current efficiency 60—80%. Two cell technologies are used one uses a cell with an asbestos diaphragm while the other is an undivided concentric tube cell. To avoid reduction of the persulphate at the cathode, the conversion per pass is kept low and the persulphate is crystallized between passes through the cell. 

Table 9.2 Current efficiency for the formation of ozone as a function of HBF concentration for cells with glassy carbon anodes and air GDE cathodes.

As the redox reaction proceeds consuming the reactants, the cell voltage decreases. When the cell voltage reaches zero, the reaction has reached equilibrium and no further net reaction occurs. At this point, however, the Cu + ion concentration in the cathode cell is not zero. This description applies to any voltaic cell. 

In the cathode (concentrated) half-cell, Cu " ions gain the electrons and the resulting Cu atoms plate out on the electrode, which makes that solution less concentrated. 

One of the major limitations of this basic cell is poor mass transfer, particularly at low ion concentrations. Many cell improvements have concentrated on improving mass transfer through, for example, injecting a fine stream of air across the surface of the cathode. 

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. 

In potentiometry, the concentration of analyte in the cathodic half-cell is generally unknown, and the measured cell potential is used to determine its concentration. Thus, if the potential for the cell in Figure 11.5 is measured at -1-1.50 V, and the concentration of Zn + remains at 0.0167 M, then the concentration of Ag+ is determined by making appropriate substitutions to equation 11.3... 

Redox Electrodes Electrodes of the first and second kind develop a potential as the result of a redox reaction in which the metallic electrode undergoes a change in its oxidation state. Metallic electrodes also can serve simply as a source of, or a sink for, electrons in other redox reactions. Such electrodes are called redox electrodes. The Pt cathode in Example 11.1 is an example of a redox electrode because its potential is determined by the concentrations of Ee + and Ee + in the indicator half-cell. Note that the potential of a redox electrode generally responds to the concentration of more than one ion, limiting their usefulness for direct potentiometry. 

Data, for a 32% caustic concentration at 90°C and a current efficiency of 96.0%, obtained in laboratory cells using a DSA anode and an activated cathode, where the membrane is against the anode at a 3-mm gap. 

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