pH & Solubility (College Board AP® Chemistry): Revision Note

pH & solubility

• The pH of some salts is affected by the pH of the environment

• Salts can be more soluble in acidic solutions, while others may be more soluble in basic solutions

  • If the salt contains a weak conjugate base (A^–) that will react with H₃O⁺, pH will change the solubility

  • If the salt contains a weakly acidic cation, such as NH₄⁺ or a small, highly charged metal ion like Al³⁺ or Fe³⁺, that will react with OH^-, pH will change the solubility

HA + H₂O ⇌ H₃O⁺ + A^–

acid + base ⇌ conjugate acid + conjugate base

• Using Le Châtelier’s principle, we can see how the solubility can change

• Not all conjugate acids and conjugate bases have the potential to react with H₃O⁺ or OH^– ions (i.e. react with an acid or a base)

  • If they do not have the potential to react with acids or bases they are negligible

• Some cations are weakly acidic, such as NH₄⁺ and small, highly charged metal ions (for example Al³⁺ and Fe³⁺)

  • Group 1 and group 2 metal ions are negligible: they do not react with OH^-

• Most anions (negatively charged ions) tend to be basic (except for the anion HSO₄^–, which is acidic)

  • Other exceptions include the conjugate bases of the strong acids, which are all mostly negligible

Fe(OH)₃ as an example

Fe(OH)₃ (s) ⇌ Fe³⁺ (aq) + 3OH^– (aq)

Decreasing pH (addition of H₃O⁺)

• H₃O⁺ ions will react with OH^- ions

H₃O⁺ (aq) + OH^– (aq) ⇌ 2H₂O (l)

• This removes OH^– ions so the equilibrium will shift to the right-hand side to oppose the change

• Decreasing the pH therefore increases the solubility of Fe(OH)₃

Increasing pH (addition of OH^–)

• There will be an excess of OH^– ions in solution

  • This is the common-ion effect

• This will cause the equilibrium to shift to the left-hand side

• Therefore increasing the pH by adding OH^– ions decreases the solubility of Fe(OH)₃

AgCl as an example

AgCl (s) ⇌ Ag⁺ (aq) + Cl^- (aq)

Decreasing pH (addition of H₃O⁺)

• Cl^- is the conjugate base of a strong acid (HCl), so it is negligible and does not react with H₃O⁺

• Therefore adding acid does not change the solubility of AgCl

BaF₂ as an example

BaF₂ (s) ⇌ Ba²⁺ (aq) + 2F^– (aq)

Decreasing pH (addition of H₃O⁺)

• F^– ions will react with H₃O⁺

• This will cause the equilibrium to shift to the right-hand side

• Therefore lowering the pH will increase the solubility of BaF₂

Increasing pH (addition of OH^–)

• Ba²⁺ is considered a negligible ion so will not react with OH^– ions

• Therefore increasing the pH by adding OH^- does not influence the solubility of BaF₂