Again, the by products of the chlor-alkali process are sodium hydroxide and hydrogen gas, Although the mercury is cycled back into the cell for reuse, some of it is always discharged with waste solution into thee environment, resulting in mercury pollution. This is a major drawback of the mercury cell. Figure 22.21 shows the industrial manufacture of chlorine gas.
The half-cell reaction in a diaphragm cell are shown in Figure 22.22. The asbestos diaphragm is permeable to the ions but not to the hydrogen and chlorine gases and so prevents the gases from mixing. During electrolysis a positive pressure is applied on the anode side of the compartment to prevent the migration of the OH- ions from the cathode compartment. Periodically fresh brine solution is added to the cell and the sodium hydroxide solution is run off as shown. The diaphragm cell presents no pollution problems. Its main disadvantage is that the sodium hydroxide solution is contaminated with un reacted sodium chloride.