The common ion effect is a phenomenon in chemistry that occurs when the addition of an ion already present in a solution affects the solubility or the equilibrium of a sparingly soluble salt. This effect can lead to the reduction in the solubility of the salt and, consequently, the precipitation of the salt from the solution.
Here’s how the common ion effect works:
Sparingly Soluble Salt: Consider a sparingly soluble salt that only partially dissolves in water, creating an equilibrium between the solid salt and its ions in solution. For example, let’s take the example of calcium sulfate (CaSO₄), which is sparingly soluble.
Equilibrium: In a solution of calcium sulfate, there is an equilibrium established between the solid CaSO₄ and the ions in solution:
CaSO₄ (s) ⇌ Ca²⁺ (aq) + SO₄²⁻ (aq)
Common Ion: Now, if you add a soluble compound that contains one of the ions already present in the equilibrium (in this case, Ca²⁺), the concentration of that ion in the solution increases due to the common ion. For example, you add calcium chloride (CaCl₂) to the solution.
Le Chatelier’s Principle: According to Le Chatelier’s Principle, when you increase the concentration of one of the ions (Ca²⁺ in this case), the equilibrium shifts to the left to relieve the stress. This means that more of the sparingly soluble salt (CaSO₄) will precipitate out of the solution, reducing its solubility.
Effect: The result of the common ion effect is a decrease in the solubility of the sparingly soluble salt in the presence of the common ion. This can lead to the formation of a precipitate.
The common ion effect is commonly observed in various chemical processes, including the precipitation of salts, the behavior of buffers, and the control of solubility in analytical chemistry. It is essential to consider the common ion effect when predicting the behavior of solutions with sparingly soluble salts or when trying to control the concentration of ions in a solution.