Nickel(II) oxide solubility and phase stability in high temperature aqueous solutions

A platinum-lined, flowing autoclave facility was used to investigate the solubility behavior of nickel(II) oxide (NiO) in deoxygenated ammonium and sodium hydroxide solutions, between 21 and 315degreesC. Solubilities were found to vary between 0.4 and 400 nmol-kg(-1). The measured nickel ion solubilities were interpreted via a Ni(II) ion hydroxo- and ammino-complexing model, and thermodynamic functions for these equilibria were obtained from a least-squares analysis of the data. Two solid phase transformations were observed: at temperatures below 149degreesC, the activity of Ni( II) ions in aqueous solution was controlled by a hydrous Ni( II) oxide (theophrastite) solid phase rather than by anhydrous NiO (bunsenite); above 247degreesC, Ni( II) activities were controlled by cubic rather than by rhombohedral bunsenite.