Solubility product of a sparingly soluble salt is an important parameter in both pure and applied physical chemistry such as determination of values of thermodynamic functions or environmental implications of components of the substance. This paper presents the determination of the solubility product of ammonium nickel phosphate hexahydrate (Ni-struvite) at 25°C by analysis of equilibria attained from both supersaturated and undersaturated solutions, i.e. precipitation and dissolution, respectively. Writing the dissolution process as NH4NiPO4·6H2O → NH3 + Ni2+ ++ 6 H2O, the value pKsp = 11.03 ± 0.03 is found for both precipitation and dissolution. The solubility is a little lower than that of the isomorphous Mg salt. This is to be expected from the lattice dimensions of the two phases, the crystals of Ni-struvite being slightly more compact.
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