Abstract: In a system for the electrolysis of sodium chloride in an electrolytic cell divided by a cation exchange membrane into the anode and cathode chambers, hydrogen chloride gas is synthesized from the chlorine gas and the hydrogen gas generated in said electrolytic cell, the hydrogen chloride gas synthesized is directly absorbed in the aqueous sodium chloride solution fed into the anode chamber to neutralize the hydroxyl ions in the anolyte migrated from the cathode chamber and the reaction heat as well as the absorption heat of the hydrogen chloride gas are also utilized for concentration of the dilute aqueous sodium chloride solution and/or the aqueous sodium hydroxide solution produced in the cathode chamber.

Abstract: An aqueous sodium chloride solution for use in production of caustic soda in an electrolytic cell having a cation exchange membrane is purified by adding to said solution a chemical reagent for precipitation separation of impurities remove silica through co-precipitation with a slurry of the precipitates of impurities which are circulated through said solution.