Complex Ions and Precipitation Reactions

Among the most common reactions in qualitative analysis are those involving the formation or decomposition of complex ions and precipitation reactions. These reactions may be performed directly by adding the appropriate anion, or a reagent such as H₂S or NH₃ may dissociate in water to furnish the anion. Strong acid may be used to dissolve precipitates containing a basic anion. Ammonia or sodium hydroxide may be used to bring a solid into solution if the cation in the precipitate forms a stable complex with NH₃ or OH^-.

A cation is usually present as a single principal species, which may be a complex ion, free ion, or precipitate. If the reaction goes to completion the principal species is a complex ion. The precipitate is the principal species if most of the precipitate remains undissolved. If a cation forms a stable complex, addition of a complexing agent at 1 M or greater generally will convert the free ion to complex ion.

The dissociation constant K_d can be used to determine the extent to which a cation is converted to a complex ion. The solubility product constant K_sp can be used to determine the fraction of cation remaining in a solution after precipitation. K_d and K_sp are both required to calculate the equilibrium constant for dissolving a precipitate in a complexing agent.

Complexes of Cations with NH3 and OH-

Cation	NH3 Complex	OH- Complex
Ag+	Ag(NH3)2+	--
Al3+	--	Al(OH)4-
Cd2+	Cd(NH3)42+	--
Cu2+	Cu(NH3)42+ (blue)	--
Ni2+	Ni(NH3)62+ (blue)	--
Pb2+	--	Pb(OH)3-
Sb3+	--	Sb(OH)4-
Sn4+	--	Sn(OH)62-
Zn2+	Zn(NH3)42+	Zn(OH)42-