Electrochemical Desalination of a 50% w/w Sodium Hydroxide Solution, a Pharmaceutical Sterilization Agent

Sodium hydroxide solutions are often employed as sterilization agents in the pharmaceutical industry. Here, the chloride content is considered as a critical impurity. In this study, an electrochemical method was developed to remove chloride ions (Cl⁻) through the oxidative deposition of AgCl on a Ag anode. The Cl⁻ content in the commercially available 50% w/w NaOH solution employed was approximately 100 mg Cl⁻/kg NaOH. As the OH⁻ content is approximately 18,000 times higher than the Cl⁻ content, the formation of AgCl may be expected to be thermodynamically less favorable than the formation of Ag₂O. However, activation energies for AgCl and Ag₂O formation have been reported to be approximately 3.8 and 31.2 kJ/mol, respectively, and indicate that AgCl formation is favored. AgCl can be selectively produced by controlling the anode potential. Here, the Cl⁻ concentration was reduced to less than 50 mg Cl⁻/kg NaOH when an anode potential of 0.18 or 0.19 V vs. Hg/HgO (reference electrode) was applied for one hour at 50°C. XRD analysis and visual monitoring of the Ag anode confirmed the oxidative deposition of AgCl on the anode surface as well as the electrochemical desalination of the concentrated NaOH solution.