Cell, amalgam with single electrolyte

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. 

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. 

Concentration Cells with a Single Electrolyte Amalgam Concentration Cells.—In the concentration cells already described the e.m.p. is a result of the difference of activity or chemical potential, i.e., partial molal free energy, of the electrolyte in the two solutions it is possible, however, to obtain concentration cells with only one solution, but the activities of the element with respect to which the ions in the solution are reversible are different in the two electrodes. A simple method of realizing such a cell is to employ two amalgams of a base metal at different concentrations as electrodes and a solution of a salt of the metal as electrolyte thus... 

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