5.5.8 The Common Ion Effect

• A saturated solution is a solution that contains the maximum amount of dissolved salt
• If a second compound, which has an ion in common with the dissolved salt, is added to the saturated solution, the solubility of the salt reduces and a solid precipitate will be formed
• This is also known as the common ion effect
• For example, if a solution of potassium chloride (KCl) is added to a saturated solution of silver chloride (AgCl) a precipitate of silver chloride will be formed
  • The chloride ion is the common ion

• The solubility product can be used to predict whether a precipitate will actually form or not
  • A precipitate will form if the product of the ion concentrations is greater than the solubility product (K_sp)

Common ion effect in silver chloride

• When a KCl solution is added to a saturated solution of AgCl, an AgCl precipitate forms
• In a saturated AgCl solution, the silver chloride is in equilibrium with its ions

AgCl (s) ⇌ Ag⁺ (aq) + Cl^- (aq)

• When a solution of potassium chloride is added:
  • Both KCl and AgCl have the common Cl^- ion
  • There is an increased Cl^- concentration so the equilibrium position shifts to the left
  • The increase in Cl^- concentration also means that [Ag⁺ (aq)] [Cl^-(aq)] is greater than the K_sp for AgCl
  • As a result, the AgCl is precipitated

The addition of potassium chloride to a saturated solution of silver chloride results in the precipitate of silver chloride

Worked Example: Calculations using the K_sp values and the concentration of the common ion

Answer

• Step 1: Determine the equilibrium reaction of CaSO₄

CaSO₄ (s) ⇌ Ca²⁺ (aq) + SO₄^2- (aq)

• Step 2: Write down the equilibrium expression for K_sp

K_sp = [Ca²⁺ (aq)] [SO₄^2- (aq)]

• Step 3: Determine the concentrations of the ions

There are equal volumes of each solution

This means that the total solution was diluted by a factor of 2

The new concentrations of the Ca²⁺ ion is halved 

= 5.0 x 10^-4 mol dm^-3

The sulfate ion concentration remains the same as it is a common ion and its concentration is the same in both solutions

• Step 4: Substitute the values into the expression

Product of the ion concentrations = [Ca²⁺ (aq)] x [SO₄^2- (aq)]

= (5.0 x 10^-4) x (1.0 x 10^-3)

= 5.0 x 10^-7 mol² dm^-6

• Step 5: Determine if a precipitate will form

As the product of the ion concentration (5.0 x 10^-7 mol dm^-3 ) is smaller than the K_sp value (2.0 x 10^-5 mol² dm^-6), the CaSO₄ precipitate will not be formed