Topic B. Factors Affecting Solubility
Le Châtelier's principle governs
the response of a system in equilibrium when a stress is introduced. Some “stressors”
that affect solubility equilibria are presence of common ion, pH
and presence of a complexing agent.
Subtopic B.4. Presence of Complexing Agent
Le Châtelier's principle governs the response of a system in equilibrium when a stress is introduced. Some “stressors” that affect solubility equilibria are presence of common ion, pH and presence of a complexing agent.
Molar solubility of precipitates significantly increases in the presence of complexing agents that form complexes with either ions. Their presence shifts the equilibrium forward due to a decrease in free ions.
Solubility
calculation in the presence of complexing agents may be done using metal
distribution (α):
Let’s
take AgSCN in a solution with 0.0150 M free ammonia. To simplify this problem,
neglect all acid/base properties of Ag+ and SCN– and only
account for the presence of NH3 as the complexing agent.
AgSCN: Ksp=1.1×10-12; Ammonia complexes: Kf1=2.04×103 Kf2=8.13×103
Solubility
of AgSCN is 1.0x10-6 M
in water (that is, without the auxiliary complexing agent). Observe that the solubility
increased in the presence of ammonia.