Common Ion Effect: Definition, Examples and Sample Questions

The common Ion effect refers to the phenomenon wherein the ionization of the first electrolyte is suppressed after an electrolyte is added to another electrolyte having a common ion. The effect is a result of Le Chatelier’s Principle, commonly known as the Equilibrium Law. 

• A common ion is an ion that is present in both the ionic compound and the solution.
• Le Chatelier's Principle states that if a dynamic equilibrium is disturbed, the system responds by counteracting the change.
• When a common ion is introduced, it reduces the ionization of another electrolyte containing the same ion.
• This shift in equilibrium helps maintain a stable condition in the solution.
• The common ion effect is commonly observed in solutions involving weak acids or bases and their salts.
• Here, the addition of a common ion inhibits the ionization of the weak acid or base.

Key Terms: Common Ion Effect, Le Chatelier’s principle, Ionic Equilibrium, Transition Metal, Compound, Electrolyte

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What is the Common Ion Effect?

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The common ion effect reduces the solubility of a precipitate by adding a soluble compound with the same ions.

• In a solution with multiple species in chemical equilibrium, adding another species with the same ion increases the concentration of that ion.
• This increase in concentration boosts ion association.
• This is because the addition of the common ion shifts the equilibrium to the left, causing more of the precipitate to form.

The common ion effect can be used to control the precipitation of compounds. It can be used to desalinate seawater or to prevent kidney stones from forming.

Common Ion Effect

When gaseous hydrogen chloride (HCl) is passed through a sodium chloride (NaCl) solution, the concentration of chloride ions (Cl^-) increases. This is called the common ion effect.

• The excess chloride ions cause NaCl to precipitate out of the solution.
• This contributes to the dissociation of ions in HCl.
• This is because the solution wants to reach equilibrium again.
• To reach equilibrium, the chloride ions combine with sodium ions (Na+) that are already in the solution.
• This forms NaCl, which precipitates out of the solution.

The following equation shows this reaction:​

NaCl (aq) + HCl (g) ⇔ NaCl (s) + HCl (aq)

Not all compounds exhibit the common ion effect. Compounds made by transition metals are an exception.

• These compounds can form complex ions, which are soluble in water.
• For example, cuprous chloride (CuCl) is insoluble in water, but it becomes soluble in water when chloride ions are added.
• This is because the chloride ions form a complex ion with CuCl, called CuCl₂^-, which is soluble in water. 

CuCl (s) + Cl^- (aq) ⇔ CuCl₂- (aq)

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Effect on Solubility

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The solubility of a salt in a solution is affected by the presence of a common ion. This effect on solubility is used for a variety of purposes in daily life. 

• Drinking water purification: Sodium carbonate is added to hard water containing limestone or chalk to precipitate calcium carbonate, making the water softer.
• Soap production: Sodium chloride is added to soap solution to precipitate sodium salts of fatty acids, forming solid soap.
• Salt purification: A common ion is added to a saturated salt solution to precipitate more salt, purifying the existing salt

Common Ion Effect and Effect on Solubility

When a common ion is added to a weak acid or a weak base, it prevents the solution from ionizing as it would have done otherwise. The effect suppresses the ionization of a weak acid or base by adding more common ions. 

Read More: Planck's Quantum Theory and Black Body Radiation

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pH and the Common Ion Effect

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A buffer solution is a solution containing a base and its conjugate acid or an acid with its conjugate base. Adding a conjugate ion to a buffer solution changes its pH through the common ion effect.

• For example, acetate ions are generated when sodium acetate and acetic acid are dissolved in a solution.
• Sodium acetate completely dissociates, while acetic acid only partially ionizes because it is a weak acid.
• According to Le Chatelier's principle, the addition of acetate ions from sodium acetate suppresses the ionization of acetic acid.
• This shifts the equilibrium to the left.
• This reduces the dissociation of acetic acid and increases the pH of the solution.

Therefore, the common ion solution containing acetic acid and sodium acetate will have an increased pH. Hence, it will be less acidic when compared to an acetic acid solution.

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Things to Remember

• The common ion effect refers to the decrease in solubility of a salt in a solution when a common ion is added.
• Le Chatelier's principle states that when a change is applied to a system at equilibrium, the system will shift to counteract the change.
• Examples of common ion effects include adding sodium chloride to seawater to make it drinkable.
• The real-life applications of the common ion effect include purifying water, controlling pH and producing soap.