7.12 Common-Ion Effect

What is the common-ion effect?

The common-ion effect occurs when a solution's equilibrium shifts because an ion already present is added from another source. Adding a common ion shifts the equilibrium away from producing more of that ion, per Le Châtelier's principle. This decreases the solubility of a slightly soluble salt or changes the dissociation of a weak acid/base.

Effect on solubility equilibria

For a salt like AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq), adding NaCl introduces extra Cl⁻, a common ion. The reaction shifts left (Q > Ksp momentarily), precipitating more AgCl and lowering its molar solubility compared to pure water. The Ksp value itself doesn't change—only the equilibrium position and solubility do.

Effect on weak acid/base equilibria

Adding a common ion to a weak acid or base equilibrium suppresses ionization. For HA ⇌ H⁺ + A⁻, adding extra A⁻ (e.g., from a salt like NaA) shifts equilibrium left, decreasing [H⁺] and raising pH compared to the acid alone. This principle underlies buffer solutions, which resist pH change using conjugate acid-base pairs.

Key terms

Common-ion effect

A shift in equilibrium caused by adding an ion already present in the solution, per Le Châtelier's principle.

Effect on solubility

Adding a common ion decreases the solubility of a slightly soluble salt by shifting equilibrium toward the solid.

Effect on weak acid ionization

Adding the conjugate base (common ion) decreases ionization, lowering [H⁺] and raising pH.

Does Ksp change with common-ion effect?

No—Ksp stays constant; only the position of equilibrium and solubility change.

Connection to buffers

Buffers use the common-ion effect: adding a conjugate ion suppresses shifts in ionization, stabilizing pH.

Try an AP-style question

Why is AgCl less soluble in 0.10 M NaCl than in pure water?

1. The added Cl⁻ shifts the dissolution equilibrium left
2. NaCl reacts with AgCl to form a new solid
3. Na⁺ ions block AgCl from dissolving
4. NaCl changes the value of Ksp