Common Ion Effect
The suppression of the ionisation or dissolution of a substance by adding a second substance that supplies one of the same ions.
Unit
mol/L
In depth
The added ion increases the ionic product, so by Le Chatelier's principle the equilibrium shifts back towards the undissociated or undissolved form. The effect is the basis of buffer action and is exploited to complete precipitations by washing with a solution containing a common ion.
Examples in the real world
Adding concentrated HCl to saturated NaCl solution precipitates salt; adding acetate ion suppresses acetic acid ionisation.