Ksp of NiS=4.7×10−5 Ksp of ZnS=1.6×10−24 Ksp of CdS=8×10−27 Ksp of HgS=4×10−53 Since, the solubility products of NiS is highest and that of HgS is lowest, it implies that Hg2+ will start precipitating quickly than any other salt on introduction of H2S (sulphide ion source). On the same account Ni2+ will precipitate last.
