Common ion effect: Important Applications

Common ion effect
Common ion effect

Common ion effect refers to the suppression of a weak electrolyte’s degree of ionization by the addition of a strong electrolyte that shares one ion with it.   Therefore, the common ion effect is used whenever it is necessary to prevent the ionization of weak electrolytes.

Let the electrolyte XY dissociates to give X+ and Y as

XY  ⇌ X+ + Y

Here exist an equilibrium between XY and X+ and Y- ions which can be termed an ionic equilibrium.

If another salt or strong electrolyte having a common ion to electrolyte XY is added to the solution of XY electrolyte, the degree of ionization of XY electrolyte( weak electrolyte) decreases. This can be explained as:

XY ⇌ X+             +         Y

XZ ⇌ X+        +          Z

                    Common ion

Le-Chatelier’s principle states that an equilibrium will adjust to reestablish itself if it differs from its balanced state. An equilibrium between a weak acid and a weak base will change in favor of the reactants, in this case, the weak acid or base, if a common ion is supplied.

The solubility reduces when a common ion is added because the reaction moves to the left to relieve the pressure of the extra product. As a result, the equilibrium shifts towards the reactants, resulting in precipitation, when a common ion is added to a reacting solution.

The common-ion effect is used to describe the effect on an equilibrium involving a substance that adds an ion that is a part of the equilibrium. Adding a common ion prevents the weak acid or weak base from ionizing as much as it would without the added common ion.


Hydrogen sulfide, which acts as a weak electrolyte, ionizes partially in water and establishes the equilibrium as,

H2S ⇌ 2H+ + S

When HCl (strong electrolyte) is added to the solution, HCl completely ionizes in the solution as:

HCl → 2H+ + S

Now according to Le-Chatelier’s principle, the addition of HCl into the solution of H2S, shifts the equilibrium very much to the left. As a result, H+ and S2- ions combine to give undissociated H2S molecules. Hence the addition of HCl into the solution of H2S suppresses the ionization of H2S in the solution.

Common ion effect on ionization of weak acid and weak base

 I.  Effect on weak acid

The presence of a strong electrolyte, sodium ethanoate (CH3COONa), which shares the ethanoate ion (CH3COO-), suppresses the ionization of a weak electrolyte, ethanoic acid (CH3COOH).



II. Effect on weak base

When strong electrolyte having NH4 + common ion  is added the ionization of  aweak electrolyte, ammonium hydroxide (NH4OH) is suppressed. So on the addition of NH4Cl ionization of NH4OH decreases. Their dissociation occurs as:

NH4Cl NH4+ + Cl

NH4OH ⇌  NH4+ + OH

Since ammonium ions (NH4 +) are common in this condition, the concentration of these ions in the solution will rise. According to Le-Chatelier’s principle, equilibrium will shift towards the left until Ksp = Qsp. The concentration of ammonium hydroxide (NH4OH) molecules is increased in order to maintain the value of the dissociation constant of the base (Kb). As a result, adding ammonium chloride (NH4Cl) to the reacting solution supress the ionization of ammonium hydroxide (NH4OH), causing a decrease in the pH of the solution.

Effect of common ion effect in pH

The common ion effect causes the pH of the buffer solution to alter when the conjugate ion of a buffer solution (solution comprising a base and its conjugate acid, or acid and its conjugate base) is introduced to it.

when acetic acid and sodium acetate are both dissolved in a particular solution, produce acetate ions. Acetic acid only partially ionizes, while sodium acetate completely dissociates. This is because sodium acetate is a strong electrolyte while acetic acid is a weak electrolyte.  As the dissociation of acetic acid is suppressed, the pH of the solution rises.  

As a result, the common ion solution containing sodium acetate and acetic acid will have a higher pH and will be less acidic than an acetic acid solution.

Effect of common ion effect in solubility

The solubility reduces when a common ion is added because the reaction moves to the left to relieve the pressure of the extra product. As a result, the equilibrium shifts towards the reactants, resulting in precipitation, when a common ion is added to a reacting solution.

The solid calcium sulfate and the solution’s constituent ions are in balance in a saturated calcium sulfate solution.

CaSO4(s)⇌Ca2+ + (aq) + SO2 – 

Ca(NO3)2(s)→Ca2+  (aq) + 2NO3 – 

The concentration of the calcium ion  would rise with the addition of calcium nitrate in this saturated solution.  So the ionic product Qsp would subsequently rise and exceed the solubility product Ksp.

The ionization equilibrium of calcium sulfate would move to the left, in accordance with LeChâtelier’s principle, to relieve the stress of the additional calcium ion.

As the equilibrium direction changed, the ions bonded to form solid calcium sulfate, which precipitated out of the solution.

The solubility of CaSO4 is reduced when calcium nitrate, which contains calcium ions, is added to a saturated solution of calcium sulfate. As a result, more CaSO4 precipitates out of the solution, and its solubility decreases.

Applications of common ion effect

  1. Drinking water can be obtained from aquifers containing chalk or limestone by using the common ion effect. Water is treated with sodium carbonate, which has the chemical formula Na2CO3, to decrease its hardness.So, highly soluble sodium carbonate is added during the water treatment process in order to precipitate out the sparingly soluble calcium carbonate from the water.
  2. In the salting-out procedure, which is used in the production of soap, the common ion effect is used. The soaps are precipitated out by decreasing the solubility of the soap solution using sodium chloride.
  3. The common ion effect is used in gravimetric measurements to totally precipitate one of the ions as a sparingly soluble salt with a very low solubility product value. For instance, silver ions precipitate as silver chloride.
  4. It is used for quantitative and qualitative analysis.
  5. It is used in the manufacturing of sodium bicarbonate by using solvay process.
  6. The idea of the common ion effect can be utilized to purify sodium chloride with the impurities like sodium and magnesium sulfates.


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About Author

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Kabita Sharma

Kabita Sharma is a graduate student from the central department of chemistry, Tribhuvan University. She has been actively involved in research related to natural products, computational chemistry, and nanochemistry. She is currently working on enzyme assay, molecular docking, and molecular dynamic simulation.

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