Three metal ions—Ag+, Ni2+, and Zn2+—are present in a single 250.0 mL aqueous sample at initial concentrations of 0.010 M each. The analyst has access to solid NaOH, solid NH3·H2O (which supplies both NH3 and NH4+), and deionized water. At 25 °C the following equilibrium data apply:
• Ksp(Ag2O)=2.0×10−8 for the reaction Ag2O(s)+H2O(l)⇌2Ag++2OH−
• Ksp(Ni(OH)2)=1.6×10−16
• Ksp(Zn(OH)2)=3.0×10−17
• Kf[Ag(NH3)2+]=1.6×107 (formation constant for Ag++2NH3⇌Ag(NH3)2+)
Assume that complex formation with Ni2+ or Zn2+ by NH3 is negligible at the concentrations used.
(a) Show quantitatively which metal ion will precipitate first as NaOH is added to the initially neutral solution. Clearly present all calculations that compare ion product Q with Ksp for each metal.
(b) After the first precipitate is removed by filtration, the analyst adds concentrated ammonia until the solution reaches pH 11.00 and an excess of 0.010 M free NH3 is established. Calculate whether any of the remaining metal ions will precipitate or dissolve under these new conditions, taking complex formation into account.
(c) Propose a complete sequential procedure—using only NaOH, NH3·H2O, and filtration steps—that will allow separate recovery of each metal ion as a solid. Specify the pH (or reagent conditions) at each stage and justify each decision with equilibrium calculations or arguments.
(d) Evaluate the feasibility of extending your procedure to a fourth ion, Fe3+, under the same reagent constraints, discussing one chemical factor that could complicate the sequence.