Common Ion Effect

How shared ions affect equilibrium and solubility

Principle

Adding an ion already present in equilibrium decreases solubility of a salt

Applies Le Chatelier's Principle: equilibrium shifts to consume added ion

Example: AgCl Solubility

AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq)

Ksp = [Ag⁺][Cl⁻] = 1.8 × 10⁻¹⁰

Scenario 1: AgCl in pure water

Let s = solubility → [Ag⁺] = [Cl⁻] = s

s² = 1.8 × 10⁻¹⁰ → s = 1.3 × 10⁻⁵ M

Scenario 2: AgCl in 0.1 M NaCl solution

[Cl⁻] ≈ 0.1 M (common ion from NaCl)

[Ag⁺] × 0.1 = 1.8 × 10⁻¹⁰ → [Ag⁺] = 1.8 × 10⁻⁹ M

Result: Solubility decreases from 1.3 × 10⁻⁵ M to 1.8 × 10⁻⁹ M

~10,000 times less soluble!

Applications

  • Buffer solutions: Common ion suppresses ionization of weak acid/base
  • Precipitation: Add common ion to force precipitation
  • Qualitative analysis: Separate ions by selective precipitation

Key Points

  • Common ion must be from strong electrolyte (fully dissociated)
  • Effect is most dramatic when common ion concentration is much larger than solubility
  • Ksp value doesn't change; only solubility decreases
  • Equilibrium shifts left (toward solid) when common ion added

Related Calculators

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Equilibrium Constant Calculator

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Solubility Product Ksp Formula

Ksp equilibrium expression

Le Chatelier's Principle

Equilibrium shifts and stress

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