Cobalt Precipitation by Reduction with Sodium Borohydride

The reaction of cobalt reduction with borohydride is very complicated, with various authors reporting different reaction stoichiometries and proposing a number of mechanisms. There are conflicting claims regarding the removal of cobalt from zinc electrolyte using sodium borohydride. This research focuses on the stoichiometry of cobalt reduction with borohydride and the removal of cobalt from zinc sulfate solution. The study observes that cobalt reduction with borohydride releases hydrogen ions, leading to a decrease in the pH of the solution. The efficiency of cobalt reduction increases with higher concentrations of NaOH in the reducing solution, as the NaOH neutralizes hydrogen ions released during cobalt reduction. The optimal reduction efficiency without precipitating cobalt hydroxide is achieved with a molar ratio of one mole of sodium borohydride to one mole of cobalt (II). When the pH is maintained at 4, the maximum reduction efficiency is approximately 81% over a ten-minute addition time, indicating that adding more sodium borohydride does not further enhance reduction efficiency.