Common Ion Effect Calculator

How it works

For AB with a common ion at C: s² + C·s − Ksp = 0 → s = ( −C + √(C² + 4·Ksp) ) / 2

The common ion effect is the drop in a salt’s solubility when the solution already contains one of its ions. By Le Chatelier’s principle the added ion pushes the dissolution equilibrium backward, so less solid dissolves. For a 1:1 salt the exact solubility solves the quadratic Ksp = s(s + C); when C is much larger than s this reduces to the familiar s ≈ Ksp / C.

Worked example

AgCl (Ksp = 1.8 × 10⁻¹⁰) in pure water dissolves to √Ksp = 1.34 × 10⁻⁵ M. In 0.10 M NaCl the chloride is a common ion: s = (−0.10 + √(0.01 + 7.2 × 10⁻¹⁰)) / 2 ≈ 1.8 × 10⁻⁹ M — about 7,400 times less soluble.

Sources: Common-ion effect — Wikipedia

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