What is common ion effect ? Give 3 examples of salt that xan affect the ionization of HNO2 solution?

1 Answer
Feb 27, 2017


The common ion effect refers to adding to a solution at equilibrium, a salt which contains an ion in common with one of the products of that equilibrium. The effect is to shift the equilibrium toward the reactant side of the equation.


The common ion effect is used to reduce the concentration of one of the products in an aqueous equilibrium. This may mean reducing the concentration of a toxic metal ion, or controlling the pH of a solution. The latter case is known as buffering.

Here are two examples:

Barium sulfate is given to a patient prior to abdominal x-rays, as it blocks the rays, enabling the image of the gut to be seen clearly.

#BaSO_4 (s) rightleftharpoons Ba^(2+) + SO_4^(2-)#

Since #BaSO_4# is insoluble in water, the concentration of #Ba^(2+)# is very low.

But barium ion is highly toxic, and so, any amount is hard for the body to tolerate. To reduce the concentration of this as much as possible, lab techs add a small amount of soluble #Na_2SO_4# as a source of additional #SO_4^(2-)#. Because this solute also contains sulfate ion, the effect is known as the common ion effect.

The added sulfate ion shifts the above equilibrium to the left, and the barium ion concentration of the new equilibrium is greatly reduced.

As a second example, consider the weak acid #HNO_2# in aqueous equilibrium:

#HNO_2 + H_2O rightleftharpoons H_3O^+ + NO_2^-#

The solution will have a certain #[H_3O^+]# and pH. By adding a soluble salt which also contains the ion #NO_2^-# (such as #LiNO_2#, #NaNO_2# or #KNO_2#) we again shift the equilibrium to the left, which lowers the #[H_3O^+]# and moves the pH closer to neutral.

This process, known as buffering is an example of the common ion effect as well. It is used in a variety of systems when a particular pH must be achieved (and maintained), such as the pH of the blood stream.