The Chemistry of the Group II Analysis

A. The addition of sodium hydroxide does two things: (a) It precipitates the hydroxides of manganese,
nickel, and iron. (b) It forms the hydroxide complexes of aluminum and zinc, since these hydroxides are
amphoteric.
Mn2+(aq) + 2OH–(aq) → Mn(OH)2(s)
Ni2+(aq) + 2OH–(aq) → Ni(OH)2(s)
Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s)
Al3+(aq) + 4OH–(aq) → [Al(OH)4]–(aq) (aluminate ion)
Zn2+(aq) + 4OH–(aq) → [Zn(OH)4]2–(aq) (zincate ion)

B. The addition of nitric acid dissolves the hydroxide precipitate. When sodium bismuthate, a strong
oxidizing agent, is then added, the Mn2+ ion is oxidized to the permanganate ion.
Mn(OH)2(s) → Mn2+(aq) + 2OH–(aq)
Ni(OH)2(s) → Ni2+(aq) + 2OH–(aq)
Fe(OH)3(s) → Fe3+(aq) + 3OH–(aq)
14H+(aq) + 2Mn2+(aq) + 5BiO3–(aq) → 2MnO4–(aq) + 5Bi3+(aq) + 7H2O(l)
                                                                   purple

C. The addition of ammonia forms a precipitate of iron(III) hydroxide and the hexaamminenickel(II)
complex ion. Upon the addition of hydrochloric acid to the precipitate, it dissolves giving Fe3+ ions in
solution. Then treating this solution with ammonium thiocyanate forms a blood-red precipitate of the
hexaisothiocyanatoferrate(III) complex ion.
Fe3+(aq) + 3OH–(aq) → Fe(OH)3(s)
Ni2+(aq) + 6NH3(aq) → [Ni(NH3)6]2+(aq)
Fe(OH)3(s) + 3H+(aq) → Fe3+(aq) + 3HOH(l)
Fe3+(aq) + 6SCN–(aq) → [Fe(NCS)6]3–(aq)
                                            blood-red

D. Adding dimethylglyoxime, H2DMG, to the solution of hexaamminenickel(II) ion gives another
complex ion commonly called nickel dimethylglyoxime. [Note: Dimethylglyoxime (H2DMG, 2,3-
butanedionedioxime, C4H8N2O2) is an organic complexing agent that causes a colorful precipitation of
nickel ions.]
[Ni(NH3)6]2+(aq) + 2H2DMG → [Ni(HDMG)2](s) + 2NH4+(aq) + 4NH3(aq)
                                                                                      pink-red

E. The addition of nitric acid will break up the hydroxide complexes of aluminum and zinc. Then the
addition of ammonia to the resulting solution of Al3+ and Zn2+ ions will do two things: (a) the aluminum
ion will precipitate as aluminum hydroxide and (b) the zinc ion will form the tetraamminezinc(II)
complex ion. Now nitric acid is added to the aluminum hydroxide precipitate causing it to dissolve.
Then the addition of ammonia and aluminon reagent will cause a reddish-pink precipitate of
[Al(OH)3•aluminon] complex to form. (Note: The aluminon reagent is the ammonium salt of aurin
tricarboxylic acid, C22H23N3O9, a red dye.)
[Al(OH)4]–(aq) + 4H+(aq) → Al3+(aq) + 4HOH(l)
[Zn(OH)4]2–(aq) + 4H+(aq) → Zn2+(aq) + 4HOH(l)
Al3+(aq) + 3OH–(aq) → Al(OH)3(s)
Zn2+(aq) + 4NH3(aq) → [Zn(NH3)4]2+(aq)
Al(OH)3(s) + 3H+(aq) → Al3+(aq) + 3HOH(l)
Al3+(aq) + 3NH3(g) + 3H2O(l) + aluminon → [Al(OH)3•aluminon](s) + 3NH4+(aq)
                                                                              reddish-pink

F. The addition of HCl breaks up the tetraamminezinc(II) complex ion. Then the addition of potassium
hexacyanoferrate(II) forms a precipitate of K2Zn3[Fe(CN)6]. These two reactions are shown as one step
below.
[Zn(NH3)4]2+(aq) + 2K4[Fe(CN)6](aq) + 4H+(aq) → K2Zn3[Fe(CN)6]2(s) + 4NH4+(aq) + 6K+(aq)
                                                                                      cream to blue-grey