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

Experimental results on the solubility of crystalline nickel hydroxide (theophrastite), according to the dissolution reaction, beta-Ni(OH)(2)(cr) + 2H(+)reversible arrow Ni2+ + 2H(2)O(l), are reported as functions of temperature (0-200 degrees 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: log(10) K-s0(o) (25 degrees C) = (11.67 +/- 0.20), Delta G(s0)(o) (25 degrees C) = -(66.6 +/- 1.1) kJ mol(-1),Delta H-s0(o) (mean value 5-200 degrees C) = -(82.1 +/- 0.8) kJ mol(-1), Delta S-diss(s0)o = (mean value 5-200 degrees C) = -(52 +/- 5) JK(-1) mol(-1), Delta C-p(o) (mean value 5-200 degrees C) = 0 JK(-1) mol(-1), where the uncertainties quoted are 2 sigma. These results are internally consistent, but differ from most of those reported in the literature. A significantly lower temperature for the thermodynamic stability of beta-Ni(OH)(2)(cr) versus NiO(cr), namely, 77 degrees C, is proposed based on the current measurements. Additional batch experiments at 50 degrees C and 80 degrees C within aqueous NaClO4 media established and quantified a particle-size dependence of the solubility constant of beta-Ni(OH)(2)(cr) that extended to larger particle sizes than normally observed for metal oxide/hydroxides.