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

Abstract

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<sup>−</sup>) through the oxidative deposition of AgCl on a Ag anode. The Cl<sup>−</sup> content in the commercially available 50% w/w NaOH solution employed was approximately 100 mg Cl<sup>−</sup>/kg NaOH. As the OH<sup>−</sup> content is approximately 18,000 times higher than the Cl<sup>−</sup> content, the formation of AgCl may be expected to be thermodynamically less favorable than the formation of Ag<sub>2</sub>O. However, activation energies for AgCl and Ag<sub>2</sub>O 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<sup>−</sup> concentration was reduced to less than 50 mg Cl<sup>−</sup>/kg NaOH when an anode potential of 0.18 or 0.19 V <italic>vs</italic>. 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.