common-ion effect

common-ion effect, decrease in solubility of an ionic salt, i.e., one that dissociates in solution into its ions, caused by the presence in solution of another solute that contains one of the same ions as the salt. The common-ion effect is an example of chemical equilibrium. For example, silver chloride, AgCl, is a slightly soluble salt that in solution dissociates into the ions Ag⁺ and Cl^-, the equilibrium state being represented by the equation AgCl_solid ⇌Ag⁺+Cl^-. According to Le Châtelier's principle, when a stress is placed on a system in equilibrium, the system responds by tending to reduce that stress. In the system taken as an example, if another solute containing one of those ions is added, e.g., sodium chloride, NaCl, which supplies Cl^- ions, the solubility equilibrium of the solution will be shifted to remove more Cl^- from the solution, so that at the new equilibrium point there will be fewer Ag⁺ ions in solution and more AgCl precipitated out as a solid.

The common-ion effect is a term used to describe the effect on a solution of two dissolved solutes that contain the same ion.

The presence of a common ion suppresses the ionization of a weak acid or a weak base.

Examples

• If both sodium acetate and acetic acid are dissolved in the same solution they both dissociate and ionize to produce acetate ions. Sodium acetate is a strong electrolyte so it dissociates completely in solution. Acetic acid is a weak acid so it only ionizes slightly. According to Le Chatelier's principle, the addition of acetate ions from sodium acetate will suppress the ionization of acetic acid and shift its equilibrium to the left. Thus the percent dissociation of the acetic acid will decrease and the pH of the solution will increase.

NaC₂H₃O_2(s) → Na⁺_(aq) + C₂H₃O₂^-_(aq)

HC₂H₃O_2(l) ↔ H⁺_(aq) + C₂H₃O₂^-_(aq)

This will decrease the hydrogen ion concentration and thus the common-ion solution will be less acidic than a solution containing only acetic acid.

"A salt will be less soluble if one of its constituent ions is already present in the solution"

See also

• Buffer solution
• Solubility
• Solubility equilibrium