What is meant by common ion effect?
What is meant by common ion effect?
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.
Explanation: 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.
BaSO4(s)⇌Ba2++SO2−4
Since BaSO4 is insoluble in water, the concentration of Ba2+ 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 Na2SO4 as a source of additional SO2−4. 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 HNO2 in aqueous equilibrium:
HNO2+H2O⇌H3O++NO−2
The solution will have a certain [H3O+]and pH. By adding a soluble salt which also contains the ion NO−2 (such as LiNO2, NaNO2 or KNO2) we again shift the equilibrium to the left, which lowers the [H3O+] 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.
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.
Explanation: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.
BaSO4(s)⇌Ba2++SO2−4
Since BaSO4 is insoluble in water, the concentration of Ba2+ 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 Na2SO4 as a source of additional SO2−4. 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 HNO2 in aqueous equilibrium:
HNO2+H2O⇌H3O++NO−2
The solution will have a certain [H3O+]and pH. By adding a soluble salt which also contains the ion NO−2 (such as LiNO2, NaNO2 or KNO2) we again shift the equilibrium to the left, which lowers the [H3O+] 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.
