Hydroxo complexes in acidic or neutral solutions

The Ni(II) ion is a rather weak acid in aqueous solution. The hydrolysis reactions of Ni(II) may be defined by the following generalised equilibrium process  

The formation of five water soluble hydroxo complexes is generally recognised NiOH Ni(OH)2(aq), Ni(OH)-, NijOH Ni,(OH) . 

Only a few compilations and reviews have been published on the thermodynamic data of Ni(II) hydrolysis. Baes and Mesmer [76BAE/MES] have critically reviewed the available experimental data, and provided recommended values for / = 0, 0.1, 1.0 and 3.0 mol kg. A slightly different set of stability constants has been recommended by Smith and Martell [76SMI/MAR] in their compilation. In a recent review of Plyasunova et al. [98PLY/ZHA], critical evaluations, and in some cases re-evaluations, of literature data have been carried out, using the SIT approach. The values selected by these authors are given in Appendix A. 

The hydrolysis of Ni(II) has mainly been studied by potentiometric titrations, although some solubility, kinetic and calorimetric studies have also been published. 

In all cases when suffieient experimental data were reported, a re-evaluation of data was performed for the purposes of this review (see discussions in Appendix A). Such re-assessments were required for several reasons  

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