Solubility of Nickel Oxide and Hydroxide in Water

The solubility of NiO(cr) was measured using flow techniques and a potentiometric method from ca. 0 to 350°C in aqueous solutions as a function of pH. A more limited study of the solubility of -Ni(OH)2(cr) was conducted by both techniques to 200°C. These results indicate that the transition temperature at which nickel hydroxide dehydrates to form the oxide is ca. 75°C, much lower than previously reported. However, -Ni(OH)2(cr) can persist as a metastable phase to 200°C for a sufficient period of time to conduct solubility measurements. The oxide was also found to persist metastably below ambient temperatures, at least in acidic solutions. Independent calculations of the thermodynamic properties of these two nickel phases support the claim of a 75°C transition temperature. Only two aqueous nickel(II) species were identified over the entire pH range studied, in contrast to the results reported previously. However, in general, excellent agreement was found with previous investigations for the solubility constant of NiO in acidic solutions where Ni is stable. At higher pH, the stable solution species is Ni(OH)2 and reasonable agreement was obtained with one previous study for the thermodynamic stability of this species. The first comprehensive set of results of the solubility of -Ni(OH)2(cr) is reported here.