The common ion effect is Le Chatelier’s principle applied to solubility. It explains why a salt is less soluble in a solution that already contains one of its ions.
How It Works
Consider dissolving AgCl in a solution of 0.10 M NaCl:
AgCl(s) ⇌ Ag+(aq) + Cl-(aq), Ksp=1.8×10−10
In pure water, s≈1.3×10−5 M (as we calculated in the previous section).
But in 0.10 M NaCl, there is already 0.10 M Cl- in solution. This is the “common ion” - the Cl- is shared between NaCl and AgCl.
Step 1: Set up the Ksp expression with the common ion already present.
[Ag+] = s (from dissolved AgCl)
[Cl-] = 0.10 + s ≈ 0.10 (since s will be tiny compared to 0.10)
Step 2: Solve for s. Ksp=[Ag+][Cl−]=(s)(0.10)=1.8×10−10
s=1.8×10−10/0.10=1.8×10−9 M
Result: AgCl is about 7,000 times LESS soluble in 0.10 M NaCl than in pure water. The common ion (Cl-) dramatically suppressed the solubility.
Why Does This Happen? (Q vs. Ksp)
Adding a common ion increases the ion product Qsp above Ksp. Since Qsp > Ksp, the system is supersaturated and shifts left (toward solid formation). Precipitation continues until the ion concentrations drop enough that Qsp = Ksp again.
Applications
Laboratory separations: The common ion effect is used to selectively precipitate compounds. If you have a solution containing both Ag+ and Cu²+ and you add NaCl, the AgCl (very low Ksp) precipitates while CuCl₂ (high solubility) stays in solution. Adding excess Cl- drives the precipitation of AgCl even further.
Buffer solutions: Buffers contain a weak acid and its conjugate base (or a weak base and its conjugate acid). The common ion effect explains why a buffer resists pH changes - the common ion from the salt suppresses the dissociation of the weak acid or base.
Common Ion vs. Non-Common Ion
Adding NaCl to a saturated AgCl solution decreases solubility (common ion: Cl-).
Adding NaNO₃ to a saturated AgCl solution has essentially no effect - Na+ and NO₃- are not part of the AgCl equilibrium, so they do not change Qsp.
Will PbCl₂ be more soluble in pure water or in 0.1 M NaCl solution?
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More soluble in pure water. The NaCl solution already contains Cl- ions (the common ion). This shifts the dissolution equilibrium of PbCl₂ to the left, reducing its solubility. The common ion effect suppresses solubility.
Does the common ion effect change the value of Ksp?
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No. Ksp is a constant at a given temperature - it does not change when you add a common ion. The common ion changes Q (making Q > Ksp), which causes the equilibrium to shift left. The molar solubility decreases, but Ksp itself is unchanged.
