Electrolyte concentration cell

Biological Corrosion The metabohc activity of microorganisms can either directly or indirectly cause deterioration of a metal by corrosion processes. Such activity can (1) produce a corrosive environment, (2) create electrolytic-concentration cells on the metal surface, (3) alter the resistance of surface films, (4) have an influence on the rate of anodic or cathodic reaction, and (5) alter the environment composition. 

Concentration cells may be classified into two main groups (i) electrode-concentration cells and (ii) electrolyte-concentration cells. Of the two, those belonging to the second category are by far the most important. 

Electrolyte-concentration cells are based on electrolyte dilution, and have two identical electrodes that are immersed in two solutions of the same electrolyte containing ions of the electrode material at two different activities. Electrolyte concentration cells are classified as (i) cells without liquid junctions and (ii) cells with liquid junctions. 

Present attention is first focused on the electrolyte concentration cells without liquid junctions. Such cells can be explained with two cells of the type... 

The next focus is on the electrolyte-concentration cells with a liquid junction. The dilution of HC1, which was the subject of the discussion above, can also be realized in a cell with a liquid junction, as shown in Figure 6.13. It is presupposed that the two HC1 solutions of different concentration can be brought together and averted from mixing. The flowing of two streams of solution synchronously sometimes attains this. One then can establish the cell ... 

The emf of the cell, contrary to that in the absence of a liquid junction, depends on the transference numbers. Such cells are usually identified as concentration cells with presence of transference, the second one in the electrolyte concentration cell classification list. This system, as has been seen, contains a liquid junction across which it is possible for direct transport of ions to occur. 

A cell in which the emf is derived only from the free energy change of dilution of die electrolyte is called the electrolyte concentration cell. 

Note that there is no direct transference of the electrolyte (HC1) from one side to the other. HCI is removed from the left-hand side by the left-hand electrode reaction and it is added to the right-hand side by the right-hand electrode reaction. This cell is an example of a electrolyte concentration cell without transference. 

The difference in concentration which causes the potential difference within the cell is a result of either the difference in concentration of the electromotively active substance in the electrodes (with gas and amalgam concentration cells), or of the different concentration of solutions surrouding the electrodes (with electrolyte concentration cells). As will be seen later electrolyte concentration cells must be adjusted in special way in order to exclude liquid junction or diffusion potential. 

Tho general formula for calculating the EMF of an electrolyte concentration cell without transference is ... 

The technical process of copper refining is an important example of an electrolytic concentration cell. Two copper electrodes operate in the same Cu++-containing electrolyte, one as anode, the other as cathode. Clearly, at equilibrium, E - 0. The current flow at the anode will dissolve Cu as Cu++, raising the concentration there. On the other hand, at the cathode, Cu++ deposits as Cu. Consequently, near the cathode, the Cu++ concentration decreases. Thus, a cell voltage will establish that opposes the applied voltage and leads to a loss of energy. Note that the aim of the process is the... 

An electrolyte concentration cell may also be set up with a liquid junction between two electrolyte solutions with different concentrations. An example based on the Zn... 

D7.7 Electrode combinations that produce identical cell compartments with differing concentrations only (electrolyte concentration cells) have a cell potential dependence upon the liquid junction potential and the concentration difference. If the cell has identical compartments with either gaseous or amalgam electrodes (electrode concentration cell), the cell potential will depend upon the gas pressure differences or the amalgam concentration differences but will not have a liquid junction potential. Other electrode combinations produce cells for which the cell potential depends upon the half-reaction reduction potentials. 

The simplest type of galvanic cell has a single electrolyte common to both electrodes (as in Fig. 5.3). In some cases it is necessary to immerse the electrodes in different electrolytes, as in the DanieU cell (Fig. 5.7), in which the redox couple at one electrode is Cu VCu and at the other is Zn VZn. In an electrolyte concentration cell, which would be constructed Hke the cell in Fig. 5.4, the electrode compartments are of identical composition except for the concentrations of the electrolytes. In an electrode concentration cell the electrodes themselves have different concentrations, either because they are gas electrodes operating at different pressures or because they are amalgams (solutions in mercury) with different concentrations. 

D. (1973) High temperature aqueous electrolyte concentration cells and the ionization of liquid water to 200 °C. Can. 

---