pH & Solubility (College Board AP Chemistry)

• 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 weak conjugate acid (H₃O⁺) that will react with OH^–, pH will change the solubility

HA	+	H2O		H3O+	+	A–
ACID		BASE		CONJ. ACID		CONJ. BASE

• Using Le Chatelier'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
• Most cations (positively charged ions) are said to be acidic if:
  • Group 1 and 2 metal ions (or any transition metal with a charge of +1) are considered negligible
• 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) + OH^– (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

Al(NO₃)₃ as an example

Al(NO₃)₃ (s)  Al³⁺ (aq) + 3NO₃^– (aq)

• Decreasing pH (addition of H₃O⁺)
  • NO₃^– ions are negligible as it is a conjugate base of a strong acid (HNO₃)
  • Therefore the addition of H₃O⁺ does not influence the solubility

• Increasing pH (addition of OH^–)
  • Al³⁺ is an acidic ion so will react with OH^– ions
  • This will cause the equilibrium to shift to the right hand side 
  • Therefore increasing the pH by adding OH^– ions increases the solubility of Al(NO₃)₃

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 H₃O⁺ does not influence the solubility