7.2.14 Common Ion Effect

Definition

Definition: Common Ion Effect

The Common Ion Effect is the reduction in the solubility of a sparingly soluble salt when a soluble compound containing one of the ions of the salt is added to the solution.

Mechanism

Consider a saturated solution of a sparingly soluble salt, such as Silver Chloride ():

If Sodium Chloride () is added to this solution:

1. is a strong electrolyte and dissociates completely:

2. The concentration of in significantly increases.
3. According to Le Chatelier’s Principle, the position of equilibrium shifts to the left to oppose the change (reduce concentration of ).
4. More precipitates out of solution.
5. The concentration of dissolved decreases.