The common-ion effect is the effect of decreased solubility of one salt when another salt that has an ion in common with it is also present. For example, the solubility of silver chloride, AgCl, is lowered when sodium chloride, a source of the common ion chloride, is added to a suspension of AgCl in water.
The solubility, S, in the absence of a common ion can be calculated as follows. The concentrations [Ag] and [Cl] are equal because one mole of AgCl would dissociate into one mole of Ag and one mole of Cl. Let the concentration of [Ag(aq)] be denoted by x. Then
K for AgCl is equal to  at 25 °C, so the solubility is .
Now suppose that sodium chloride is also present, at a concentration of 0.01 mol dm = 0.01 M. The solubility, ignoring any possible effect of the sodium ions, is now calculated by
This is a quadratic equation in x, which is also equal to the solubility.
In the case of silver chloride, x is very much smaller than 0.01 M x, so the first term can be ignored. Therefore
a considerable reduction from . In gravimetric analysis for silver, the reduction in solubility due to the common ion effect is used to ensure "complete" precipitation of AgCl.