pH and Solubility
Quick Notes
- The solubility of a salt can change depending on pH, especially if one of the ions is a weak acid or base.
- This can be explained using Le Châtelier’s Principle.
- Lowering or raising the pH shifts the dissolution equilibrium and affects solubility.
Full Notes
The pH of a solution can influence the solubility a salt if the salt contains:
- A basic anion (conjugate base of a weak acid, for example a carbonate ion)
- A cation that acts as a weak acid (for example, ammonium NH4+)
- Hydroxide ions (OH−) directly
If the cation or anion in the salt do not react with H3O+ or OH−, then pH will not affect solubility of the salt.
Le Châtelier’s Principle Explanation
We can explain such changes in solubility in terms of Le Châtelier’s Principle (see Topic 7.9).
Le Châtelier’s Principle states that a system at equilibrium will adjust to minimize the impact of a disturbance or ‘stress’.
Example 1: Basic Anion – Sodium Fluoride (NaF)
Dissolution Equation: NaF(s) ⇌ Na+(aq) + F−(aq)
F− is a basic anion (conjugate base of the weak acid HF). It can react with hydronium ions: F−(aq) + H3O+(aq) ⇌ HF(aq) + H2O(l)
Effect of pH: Lowering pH (Adding Acid):
- Increases [H3O+], which reacts with F− to form HF.
- This removes F− from the solution.
- The equilibrium shifts to the right to replace F−, causing more NaF to dissolve.
- Result: Solubility increases in acidic conditions.
Effect of pH: Raising pH (Adding Base):
![AP Chemistry diagram showing that adding base to a NaF solution lowers [H3O+] so less F− converts to HF; equilibrium shifts left and NaF dissolves less, decreasing solubility.](images/nafoh-.png)
- Reduces [H3O+], so less F− is converted to HF.
- The equilibrium shifts to the left.
- Result: Solubility decreases in basic conditions.
Example 2: Metal Hydroxides (e.g., Fe(OH)3)
Fe(OH)3(s) ⇌ Fe3+(aq) + 3OH−(aq)
Lowering pH (adding H3O+):
- H3O+ reacts with OH−, decreasing [OH−]. To counter this decrease, equilibrium shifts right. This means more Fe(OH)3(s) dissolves — solubility increased.
Raising pH (adding OH-):
- increases [OH−] To counter this increase, equilibrium shifts left. This means more OH− combines with Fe3+ and more solid Fe(OH)3 is made — solubility decreased.
pH Has No Effect When...
pH doesn't affect solubility if the ions in the salt are from strong acids or strong bases, and do not react further.
For Example: NaCl (Na+ and Cl− are spectator ions). pH change does not affect solubility.
Summary
- pH can influence solubility when an ion in the salt is a weak acid or base.
- Acidic pH (low pH) generally increases the solubility of salts containing basic anions (e.g., carbonate, fluoride, hydroxide).
- Basic pH (high pH) can decrease solubility by shifting equilibrium toward the solid.
- Use Le Châtelier’s Principle to predict how solubility will respond to pH changes.