The common ion effect describes a phenomenon in chemistry where the presence of an ion already existing in a solution influences the behavior of another dissolved substance. This occurs when a second compound is added that shares an ion with the first compound. This effect governs how substances, particularly those that only partially dissolve or ionize, interact within a liquid medium. It dictates that the solubility of a salt or the ionization of an acid or base will be reduced when a source of the common ion is introduced.
The Underlying Principle of Equilibrium Shift
The mechanism behind the common ion effect is a direct consequence of Le Chatelier’s Principle. This principle states that when a system at chemical equilibrium experiences a change in concentration, it will shift its balance to counteract that change.
When a common ion is introduced, it acts as a stress by increasing the concentration of one of the product ions. To relieve this stress, the system consumes the excess added ion by shifting the reaction backward, favoring the formation of the original, undissociated compound.
Consider a reaction where compound AB dissolves into ions A+ and B-. If a source of B- is added, the concentration of B- increases instantly. This shift suppresses the original compound’s ability to dissolve or ionize.
How the Effect Reduces Compound Solubility
The common ion effect is illustrated by its impact on the solubility of sparingly soluble ionic salts. These salts dissolve only slightly in water, forming a saturated solution in equilibrium with its dissolved ions. The extent of this solubility is defined by the solubility product constant (Ksp), which remains constant at a given temperature.
Consider silver chloride (AgCl) in equilibrium with its ions (Ag+ and Cl-). If a highly soluble salt like sodium chloride (NaCl) is added, it completely dissociates and introduces a large amount of the common ion, Cl-, into the solution.
To maintain Ksp, the system shifts to the left, causing the excess Ag+ and Cl- ions to combine and precipitate as solid AgCl. This principle is utilized in industrial processes, such as the purification of table salt, where a common ion is added to force the target compound out.
The Common Ion Effect in Acid-Base Systems
The common ion effect controls the ionization of weak acids and bases. Adding a salt containing the conjugate base (or conjugate acid) suppresses the ionization of the weak species. This suppression is the core principle enabling the creation of buffer solutions.
Consider a weak acid like acetic acid (CH3COOH), which slightly dissociates into hydrogen ions (H+) and acetate ions (CH3COO-). If a salt containing the acetate ion, such as sodium acetate (CH3COONa), is dissolved, the concentration of the common ion (CH3COO-) increases substantially.
This added product ion pushes the acid’s equilibrium back toward the undissociated CH3COOH molecules. The resulting reduction in H+ concentration increases the solution’s pH. This mechanism allows the solution to resist dramatic changes in pH when small amounts of strong acid or base are added.