Ksp{Fe(OH)3}=[Fe3+ concentration ]×[OH- concentration] 3, and Ksp{Ni(OH)2}=[Ni2+ concentration ]×[OH- concentration] 2.

When the concentration of Ni2+ ion is 0. 1mol/L, the concentration of OH- ion is X, 0.1× x2 = 2.0×10-15, and the concentration of OH- ion of Ni(OH)2 that began to precipitate is 65438. The corresponding pH =14-POH =14+LG [OH-] = 7.15,

To completely precipitate Fe3+, it is usually defined as 10-6 mol/L,

When the Fe3+ ion in the solution is ≤ 10-6mol/ (i.e. the precipitation is completed), it is10-6× y 3 = 4.0×10-38.

Y = 3.42×10-1,and the corresponding PH =14-poh =14+LG [OH-] = 3.53, so the pH value of complete separation is 3.53-7.