13. Influence of Complex Formation on the Solubility

Following Le Chatelier’s principle, the solubility of a slightly soluble salt will increase when a ligand is added to the solution that preferentially forms a complex ion with the cation.

Example

AgCl is a slightly soluble salt. In the presence of NH₃, the Ag⁺-ion can form the complex ion Ag(NH₃)²⁺. The following equilibriums need to be considered:

(a)	AgCl(s) Ag+(aq) + Cl-(aq)
(b)	Ag+(aq) + 2 NH3(aq) Ag(NH3)2+(aq)

When NH₃ is added, reaction (b) will occur and [Ag⁺] will decrease. As a consequence equilibrium (a) will shift to the right: AgCl(s) dissolves.

For the global reaction

AgCl(s) + 2 NH₃(aq) Ag(NH₃)²⁺(aq) + Cl^-

we then have:

Summary