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

Abstract

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., Ni²⁺) and the neutrally charged hydrolyzed species (viz., $\mathrm{Ni(OH)}_{2}^{0})$. The thermodynamic quantities obtained at 25 °C and infinite dilution are, with 2σ uncertainties: $\log_{10}K_{\mathrm{s0}}^{\mathrm{o}} = (12.40 \pm 0.29),\varDelta_{\mathrm{r}}G_{m}^{\mathrm{o}} = -(70. 8 \pm 1.7)$ kJ⋅mol⁻¹; $\varDelta_{\mathrm{r}}H_{m}^{\mathrm{o}} = -(105.6 \pm 1.3)$ kJ⋅mol⁻¹; $\varDelta_{\mathrm{r}}S_{m}^{\mathrm{o}} =-(116.6 \pm 3.2)$ J⋅K⁻¹⋅mol⁻¹; $\varDelta_{\mathrm{r}}C_{p,m}^{\mathrm{o}} = (0 \pm 13)$ J⋅K⁻¹⋅mol⁻¹; and $\log_{10}K_{\mathrm{s2}}^{\mathrm{o}} = -(8.76 \pm 0.15)$; $\varDelta_{\mathrm{r}}G_{m}^{\mathrm{o}} = (50.0 \pm 1.7)$ kJ⋅mol⁻¹; $\varDelta_{\mathrm{r}}H_{m}^{\mathrm{o}} = (17.7 \pm 1.7)$ kJ⋅mol⁻¹; $\varDelta_{\mathrm{r}}S_{m}^{\mathrm{o}} = -(108\pm 7)$ J⋅K⁻¹⋅mol⁻¹; $\varDelta_{\mathrm{r}}C_{p,m}^{\mathrm{o}} = -(108 \pm 3)$ 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 Ni²⁺ and $\mathrm{Ni(OH)}_{2}^{0}$ are also estimated. Moreover, the $\mathrm{Ni(OH)}_{3}^{ -}$ anion was never observed, even in relatively strong basic solutions ($m_{\mathrm{OH}^{ -}} = 0.1$ mol⋅kg⁻¹), contrary to the conclusions drawn from all but one previous study.