Solubility and Complex Ions

Learning Goals

You will learn how the solubility of a cation can be increased over the solubility of its salt by forming a complex ion.

Synopsis

By a complex ion, we mean an ion that forms a complex of the ion surrounded by ligand molecules. Attractive intramolecular forces stabilize the complex. An example is the Ni²⁺ ion that forms the water-soluble complex Ni(H₂O)₆²⁺.

We can also form water-soluble complexes of ions from an insoluble salt. For example, AgCl is not very soluble in water. But if we add ammonia to the AgCl solution, two simultaneous reactions will occur:

AgCl(s) Ag+(aq) + Cl-(aq) Ksp = 1.8 x 10-10,

Ag+(aq) + 2 NH₃(aq) Ag(NH₃)²⁺(aq) K = 1.6 x 10⁷.

The overall reaction is a sum of these two reactions:

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

and the final equilibrium constant is the product of the two equilibrium constants. The equilibrium constant, called the formation constant, would have the value K_formation = 2.9 x 10^-3.

The Ag(NH₃)⁺ ion is stabilized by ion-dipole interactions with water so that the solubility of AgCl has been greatly increased by the addition of ammonia.

Formation constants for a number of other complex ions have been determined and your text lists a few of them. From one of these, AgCl, you can see that if the salt is formed in the presence of excess Cl^- in solution, the AgCl₂^- ion, which is considerably more soluble than AgCl, will be formed. Thus, it is very important to consider the possibility of complex ion formation when studying salt solubilities.

Review Question

Work Exercise 19.16 on page 900 of your text.

Discussion Question for Bulletin Board Activity

From Appendix J, pick out another salt that is very insoluble and plan how to make it into a complex ion that would be considerably more soluble. Share your result with the class using the WebCT bulletin board.