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Common Ion Effect Calculator

Work out common ion effect instantly with clear inputs, formula shown and shareable results.

About Common Ion Effect Calculator

Adding an ion the salt already contains pushes the dissolution equilibrium backwards. For a 1:1 salt the ion product must still equal Ksp, so with the common ion fixed at C the solubility collapses to Ksp/C instead of the square root of Ksp. With Ksp = 1.8e-9 and 0.1 M of the common ion, solubility falls from about 4.2e-5 to 1.8e-8 mol/L - roughly a 2000-fold suppression.

Formulas

Suppressed solubility

s = Ksp / [common ion]

Unsuppressed solubility

s0 = sqrt(Ksp) for a 1:1 salt

Worked example

With the default values below, Common Ion Effect Calculator returns Solubility With The Common Ion of 1.8000 × 10⁻⁸. Change any input above and every figure here is recalculated in your browser.

Inputs used

Solubility product Kspminimum 0
0
Concentration of the common ion already presentminimum 0 mol/L
0.1 mol/L

Results

Solubility With The Common Ion
1.8000 × 10⁻⁸
Solubility In Pure Water
4.2426 × 10⁻⁵
Solubility Reduced By A Factor Of
2,357.02
Reduction In Solubility
99.958%

Step by step

  1. 1Values used
  2. 2Suppressed solubility
  3. 3Unsuppressed solubility
  4. 4Solubility With The Common Ion= 0.0000
  5. 5Solubility In Pure Water= 0.0000
  6. 6Solubility Reduced By A Factor Of= 2,357.02
  7. 7Reduction In Solubility= 99.958

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is the common ion concentration not increased by dissolution?

Because the salt is so much less soluble than the added ion is concentrated, the contribution is negligible - the standard simplifying assumption.

Is this why salt is added when recrystallising?

Yes. Saturating the solvent with a shared ion forces more of the product out of solution and improves recovery.

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