Common Ion Effect Calculator
When a sparingly soluble salt dissolves in water that already contains one of its ions, less of the salt can dissolve. This is the common ion effect, a direct consequence of Le Chatelier's principle acting on the dissolution equilibrium. For a 1:1 salt with solubility product Ksp in a solution holding concentration C of the common ion, the extra molar solubility s satisfies s times (s plus C) equals Ksp. This calculator solves that quadratic exactly and also reports the solubility in pure water for comparison. Supply the Ksp and the common ion concentration.
Common ion effect formula
pure water (1:1 salt): s0 = sqrt(Ksp)
with common ion: s(s + C) = Ksp
s = (-C + sqrt(C^2 + 4*Ksp)) / 2
suppression factor = s0 / s
Ksp must be greater than zero and the common ion concentration must be zero or greater. The quadratic is solved exactly, so the result holds even when C is comparable to the solubility.
Common ion context
- Adding a common ion shifts the dissolution equilibrium back toward the solid.
- For a 1:1 salt in pure water, solubility equals the square root of Ksp.
- The larger the common ion concentration, the greater the suppression.
- The effect is used to control precipitation and to purify products by washing.
- This solver assumes a 1:1 salt such as silver chloride.
Common ion effect: frequently asked questions
What is the common ion effect?
The common ion effect is the reduction in solubility or ionization of a substance when the solution already contains one of its ions. Adding a common ion shifts the dissolution equilibrium back toward the solid, lowering how much further can dissolve.
How do I calculate solubility with a common ion?
For a 1:1 salt MX with solubility product Ksp dissolving in a solution that already has a concentration C of the common ion, the additional molar solubility s satisfies s times (s + C) = Ksp. When C is much larger than s, s is approximately Ksp divided by C.
Why does a common ion lower solubility?
By Le Chatelier's principle, adding a product ion to the dissolution equilibrium pushes it back toward the undissolved solid. The ion product must still equal Ksp, so the concentration of the other ion, and thus the amount that dissolves, must decrease.
What is Ksp?
Ksp is the solubility product constant, the equilibrium constant for a sparingly soluble salt dissolving into its ions. For a 1:1 salt MX it equals the product of the molar concentrations of M and X at saturation. Smaller Ksp means lower solubility.
Does this calculator handle salts that are not 1:1?
This calculator solves the 1:1 case (one cation and one anion per formula unit, such as AgCl). Salts with different stoichiometry, like CaF2, follow a different cubic relationship, so use a 1:1 salt or adjust the formula manually for other ratios.
Official sources
- IUPAC Gold Book: solubility product and equilibrium constant terminology.
- NIST Chemistry WebBook: solubility and equilibrium reference data.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.