Solubility measurements of crystalline β-Ni(OH)2 in aqueous solution as a function of temperature and pH

Abstract

Experimental results on the solubility of crystalline nickel hydroxide (theophrastite), according to the dissolution reaction, β-Ni(OH)2(cr) + 2H+ ⇄ Ni2+ + 2H2O(l), are reported as functions of temperature (0–200°C) and pH at pressures slightly exceeding saturation vapor pressure. The experiments were carried out in either various flow-through cells or a hydrogen-electrode concentration cell (HECC). The results were treated with a thermodynamic model incorporating only the unhydrolyzed aqueous nickel species (namely, Ni2+) whose solubility constants were fitted as a function of temperature. The thermodynamic quantities obtained at infinite dilution are: (25°C) = (11.67 ± 0.20), (25°C) = −(66.6 ± 1.1) kJ mol−1, (mean value 5–200°C) = −(82.1 ± 0.8) kJ mol−1, = (mean value 5–200°C) = −(52 ± 5) J K−1 mol−1, (mean value 5–200°C) = 0 J K−1 mol−1, where the uncertainties quoted are 2σ. These results are internally consistent, but differ from most of those reported in the literature. A significantly lower temperature for the thermodynamic stability of β-Ni(OH)2(cr) versus NiO(cr), namely, 77°C, is proposed based on the current measurements. Additional batch experiments at 50°C and 80°C within aqueous NaClO4 media established and quantified a particle-size dependence of the solubility constant of β-Ni(OH)2(cr) that extended to larger particle sizes than normally observed for metal oxide/hydroxides.