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 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
- H3O+ ions will react with OH– ions
- 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
- There will be an excess of OH– ions in solution
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
Worked example
A strong acid is added to a saturated solution of lead oxalate, PbC2O4. Explain the effect on the solubility of the salt.
Answer:
- Equation: PbC2O4 (s) Pb2+ (aq) + C2O42- (aq)
- H3O+ ions will react with C2O42- (aq) in two steps:
- C2O42- (aq) + H3O+ (aq) HC2O4- (aq) + H2O (l)
- HC2O4- (aq) + H3O+ (aq) H2C2O4 (aq) + H2O (l)
- Therefore, the equilibrium will shift to the right-hand side and the solubility of PbC2O4 will increase