Pourbaix Diagrams for Copper in Aqueous Lithium Bromide Concentrated Solutions

Pourbaix diagrams (electrode potential-pH diagrams) for Cu-Br−-H2O systems at 25°C were developed in 400-g/L and 700-g/L (4.61-M and 8.06-M) lithium bromide (LiBr) solutions, common concentrations in different parts of refrigeration plants. The diagrams were compared with the simple Cu-H2O system at 25°C. Pourbaix diagrams were constructed from standard Gibbs free energy data (ΔG°) of all the species considered. Conventional procedures were followed to calculate the electrochemical and chemical equilibria from standard Gibbs free energy data. Equilibria for Cu-Br−-H2O systems at 25°C were determined for bromide ion activities of 15.61 and 194.77, which correspond to 400-g/L and 700-g/L LiBr solutions, respectively. Activities of all the ion species containing copper were plotted for 10−6, 10−4, 10−2, and 100. Comparison of the simple Cu-H2O system with the diagrams for Cu-Br−-H2O systems at 25°C showed that the formation of CuBr2− complexes extended the copper solubility range to both higher pH values and lower potentials by destabilizing the formation of copper oxides and promoting more active behavior of the metal. The effect was enhanced at higher bromide ion activities.