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pH and Solubility

Chemistry: Cambridge International A Level, topic 1.7.10

pH and Solubility

The solubility of a salt whose constituent ions are composed of a weak acid or base is pH sensitive. This can be understood by applying Le Châtelier’s principle.

Salts with acidic ions - example

Salts with acidic ions - example

  • When NH4Cl is dissolved in solution, NH4+ and Cl- ions are released:
  • NH4Cl(s) ⇌ NH4+(aq) + Cl-(aq)
  • The ammonium ion is a weak acid. So once in water, the NH4+ ionizies water:
  • NH4+(aq) + H2O(l) ⇌ H3O+(aq) + NH3(aq)
Salts with acidic ions - adding OH-

Salts with acidic ions - adding OH-

  • If OH- ions are added to the solution the following reaction occurs:
  • NH4+ + OH- → NH4OH
  • The removal of NH4+ ions will cause the equilibrium for the dissolution of NH4Cl (below) to shift to the right to replace the NH4+ ions:
  • NH4Cl(s) ⇌ NH4+(aq) + Cl-(aq)
  • So, increasing the pH of the solution by adding OH- ions increases the solubility of NH4Cl.
Salts with acidic ions - adding H+

Salts with acidic ions - adding H+

  • If H+ ions are added to a solution the following reaction occurs:
  • H+ + H2O → H3O+
  • This causes the equilibrium in the below reaction to shift to the left to remove the excess H3O+ ions, increasing [NH4+]
  • NH4+(aq) + H2O(l) ⇌ H3O+(aq) + NH3(aq)
  • Increasing [NH4+] will cause the below equilibrium to shift to the left:
  • NH4Cl(s) ⇌ NH4+(aq) + Cl-(aq)
  • So, lowering the pH by adding H+ ions decreases the solubility of NH4Cl.
Salts with basic ions - example

Salts with basic ions - example

  • Mg(OH)2 liberates Mg2+ and OH- ions during dissolution:
  • Mg(OH)2(s) ⇌ Mg2+(aq) + 2OH-(aq)
  • OH- ions are basic, so altering the pH of the solution affects the solubility of Mg(OH)2.
Salts with basic ions - adding OH-

Salts with basic ions - adding OH-

  • Adding OH- ions causes the below equilibrium to shift to the left to counteract the change:
  • Mg(OH)2(s) ⇌ Mg2+(aq) + 2OH-(aq)
  • So, increasing the pH by adding OH- ions reduces the solubility of Mg(OH)2.
Salts with basic ions - adding H+

Salts with basic ions - adding H+

  • As before, adding H+ ions to solution generates H3O+ ions. H3O+ reacts with OH- according to the below reaction:
  • H3O+ + OH- → 2H2O
  • Removal of OH- ions causes the equilibrium for the dissolution of Mg(OH)2 (below) to shift to the right to replace the OH- ions:
  • Mg(OH)2(s) ⇌ Mg2+(aq) + 2OH-(aq)
  • So, reducing the pH by adding H+ ions increases the solubility of Mg(OH)2.

Test yourself on pH and Solubility

  • NH4Br is dissolved in solution and KOH is subsequently added. What would happen to the solubility of NH4Br?

  • NH4Br is dissolved in a saturated solution. What would happen to the solubility of NH4Br if more NH4Br was added?

  • NH4Br is dissolved in solution and HCl is subsequently added. What would happen to the solubility of NH4Br?

  • Al(OH)3is dissolved in a neutral solution. A buffer is added and the pH of the solution is maintained at 10.

    What effect would this have on the solubility of Al(OH)3?

  • Al(OH)3is dissolved in a neutral solution. An aqueous solution of NH4Cl is then added.

    What effect would this have on the solubility of Al(OH)3?

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