Common-Ion Effect

Understanding the Common-Ion Effect

• The Common-Ion Effect refers to the change in equilibrium occurring when an ion common to the equilibrium mixture is added.
• Le Chatelier’s Principle explains the common-ion effect. It states that if a system at equilibrium is disturbed, it will adjust in order to restore equilibrium.
• Adding a common ion to a solution shifts the equilibrium in the direction that reduces its concentration, thereby decreasing the solubility of the ionic compound.
• The common ion effect is a practical application of Le Chatelier’s principle.

Impact on Solubility Equilibrium

• The common-ion effect is often used to reduce the solubility of a particular component in a solution.
• This results in the less soluble compound precipitating from the solution.
• The effect is used in many real-world applications, such as the process of salting out, where the addition of salt to a solution lowers the solubility of proteins, causing them to precipitate.

Common-Ion Effect on pH

• The common-ion effect can also apply to the ionization of acids and bases.
• The addition of a strong acid to a weak acid solution causes a shift in the ionization equilibrium of the weak acid, decreasing the concentration of its ionization and therefore increasing the pH (making it less acidic).
• Similarly, adding a strong base to a weak base solution can shift the ionization equilibrium of the weak base, decreasing the concentration of its ionization and therefore decreasing the pH (making it more basic).