Solubility Measurements of Crystalline NiO in Aqueous Solution as a Function of Temperature and pH

Results of solubility experiments involving crystalline nickel oxide (bunsenite) in aqueous solutions are reported as functions of temperature (0 to 350 °C) and pH at pressures slightly exceeding (with one exception) saturation vapor pressure. These experiments were carried out in either flow-through reactors or a hydrogen-electrode concentration cell for mildly acidic to near neutral pH solutions. The results were treated successfully with a thermodynamic model incorporating only the unhydrolyzed aqueous nickel species (viz., Ni 2+ ) and the neutrally charged hydrolyzed species (viz., . The thermodynamic quantities obtained at 25 °C and infinite dilution are, with 2 σ uncertainties: kJ⋅mol −1 ; kJ⋅mol −1 ; J⋅K −1 ⋅mol −1 ; J⋅K −1 ⋅mol −1 ; and ; kJ⋅mol −1 ; kJ⋅mol −1 ; J⋅K −1 ⋅mol −1 ; J⋅K −1 ⋅mol −1 . These results are internally consistent, but the latter set differs from those gleaned from previous studies recorded in the literature. The corresponding thermodynamic quantities for the formation of Ni 2+ and are also estimated. Moreover, the anion was never observed, even in relatively strong basic solutions ( mol⋅kg −1 ), contrary to the conclusions drawn from all but one previous study.