Pourbaix Diagrams for Nickel in Concentrated Aqueous Lithium Bromide Solutions at 25°C

Pourbaix diagrams (electrode potential-pH diagrams) for the Ni-Br−-H2O system at 25°C were developed in 400-g/L, 700-g/L, and 850-g/L (4.61-M, 8.06-M, and 9.79-M) lithium bromide (LiBr) solutions, which are common concentrations found in different parts of absorption devices. The diagrams were compared with the simple Ni-H2O system at 25°C. Pourbaix diagrams were constructed from standard Gibbs free energy of formation (ΔGfo) data at 25°C for all the species considered. Conventional procedures were followed to calculate the electrochemical and chemical equilibria from ΔGfo data Equilibria for the Ni-Br−-H2O system at 25°C were determined for bromide ion activities of 15.61, 194.77, and 650.06, which corresponded to the 400-g/L, 700-g/L, and 850-g/L LiBr solutions, respectively. Activities of all the dissolved species containing nickel were plotted for 10−6, 10−4, 10−2, and 100. Comparison of the simple Ni-H2O system at 25°C with the diagrams illustrating the effect of Br− activity showed that the formation of aqueous NiBr2(aq) extended the nickel solubility range to both higher pH values and lower potentials, particularly in acid, neutral, and weak alkaline areas of the diagrams, as a result of destabilization of β-Ni(OH)2, and promotion of a more active behavior of nickel. The effect was enhanced at higher bromide ion activities.