The common ion effect happens when an ion already present in a solution is added from another source, causing an equilibrium to shift. It matters because it controls how much of a salt dissolves, how much a weak acid ionizes, and how buffers resist pH changes. The effect is a direct application of Le Châtelier's principle, where a system responds to added particles by reducing their impact.
Chemists use it in precipitation, qualitative analysis, medicine, and biological pH control.
For a slightly soluble salt such as AgCl, adding Cl- from NaCl makes the equilibrium AgCl(s) ⇌ Ag+(aq) + Cl-(aq) shift left, so less AgCl dissolves. For a weak acid such as acetic acid, adding acetate ion from sodium acetate shifts HC2H3O2 ⇌ H+ + C2H3O2- left, lowering ionization and helping stabilize pH. In buffers, the weak acid and its conjugate base share a common ion pair that can absorb added acid or base.
A worked solubility example shows the size of the effect: if Ksp for AgCl is 1.8 x 10^-10 and [Cl-] = 0.10 M, then [Ag+] = Ksp/[Cl-] = 1.8 x 10^-9 M.
Key Facts
- Common ion effect: adding an ion that is already in an equilibrium mixture shifts the equilibrium away from producing more of that ion.
- Le Châtelier's principle predicts the shift: added product ion drives the equilibrium toward reactants.
- For a salt AB(s) ⇌ A+(aq) + B-(aq), Ksp = [A+][B-].
- If AgCl(s) ⇌ Ag+ + Cl- and Ksp = 1.8 x 10^-10, then in 0.10 M NaCl, [Ag+] = 1.8 x 10^-9 M.
- For a weak acid HA ⇌ H+ + A-, Ka = [H+][A-]/[HA], so adding A- reduces acid ionization.
- Buffer equation: pH = pKa + log([A-]/[HA]), showing how a common ion controls pH.
Vocabulary
- Common ion
- A common ion is an ion that appears in two dissolved substances or in both sides of a solution equilibrium.
- Common ion effect
- The common ion effect is the decrease in solubility or ionization caused by adding an ion already present in an equilibrium.
- Solubility product
- The solubility product, Ksp, is the equilibrium constant for a slightly soluble ionic solid dissolving into its ions.
- Weak acid
- A weak acid is an acid that only partially ionizes in water, forming an equilibrium between HA, H+, and A-.
- Buffer
- A buffer is a solution containing a weak acid and its conjugate base that resists large changes in pH.
Common Mistakes to Avoid
- Ignoring the added ion concentration is wrong because the common ion often dominates the equilibrium concentration and greatly reduces solubility or ionization.
- Treating a common ion as a spectator ion is wrong because it participates in the equilibrium expression even if it comes from a soluble salt.
- Using Ksp = molar solubility for every salt is wrong because Ksp depends on ion powers and any ions already present in solution.
- Assuming adding a common ion changes the value of Ksp or Ka is wrong because equilibrium constants stay fixed at a given temperature while concentrations shift.
Practice Questions
- 1 AgCl has Ksp = 1.8 x 10^-10. Calculate the molar solubility of AgCl in 0.050 M NaCl, assuming the chloride from AgCl is small compared with 0.050 M.
- 2 A weak acid HA has Ka = 1.0 x 10^-5. A buffer contains 0.20 M HA and 0.10 M A-. Calculate the pH using pH = pKa + log([A-]/[HA]).
- 3 Explain why adding sodium acetate to acetic acid lowers the percent ionization of acetic acid but helps the solution resist a drop in pH when a small amount of strong acid is added.