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., Ni²⁺) and the neutrally charged hydrolyzed species (viz., {Ni(OH)}₂⁰}). The thermodynamic quantities obtained at 25°C and infinite dilution are, with 2σ uncertainties: log ₁₀ K_s0^o = (12.40 ± 0.29), Δ_r G_m^o = -(70. 8 ± 1.7) kJ·mol^-1; Δ_r H_m^o = -(105.6 ± 1.3) kJ·mol^-1; Δ_rS _m^o = -(116.6 ± 3.2) J·K ^-1·mol^-1; ΔrC_p,m^o = (0 ± 13) J·K^-1·mol^-1; and log ₁₀ K_s2^o = -(8.76 ± 0.15); Δ_rG_m^o = (50.0 ± 1.7) kJ·mol^-1; Δ_rH_m^o = (17.7 ± 1.7) kJ·mol^-1; Δ_rS_m ^o = -(108 ± 7) J·K^-1·mol ^-1; Δ_rC_p,m^o = -(108 ± 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 Ni(OH)}₂⁰ are also estimated. Moreover, the Ni(OH)₃^- anion was never observed, even in relatively strong basic solutions (m_OH^- = 0.1 mol·kg^-1), contrary to the conclusions drawn from all but one previous study.