How does pH affect solubility?

1 Answer
Ernest Z.
Jul 19, 2014

Decreasing the pH increases the solubility of sparingly soluble bases and basic salts.

Explanation:

Increasing the pH has the opposite effect.

Many sparingly soluble compounds have solubilities that depend on pH.

It all involves the application of Le Châtelier's Principle.

Here are two common examples.

Bases

Zn(OH)2 is a sparingly soluble base.

Zn(OH)2(s)⇌Zn2+(aq)+2OH-(aq)

If you increase the pH by adding OH- ions, Le Châtelier's Principle says that the position of equilibrium will move to the left.

The solubility of the Zn(OH)2 decreases.

If you decrease the pH by adding H3O+ ions, the added H3O+ ions will react with the OH- ions and form water.

OH- ions are removed from solution.

According to Le Châtelier's Principle, the position of equilibrium moves to the right to replace the OH- ions.

The solubility of the Zn(OH)2 increases.

Salts of Strong Bases and Weak Acids

CaCO3 is the sparingly soluble salt of the strong base Ca(OH)2 and the weak acid H2CO3.

CaCO3(s)⇌Ca2+(aq)+CO2-3(aq)

CO2-3 is a base.

CO2-3(aq)+H2O(l)⇌HCO-3(aq)+OH-(aq)

The overall equilibrium is

CaCO3(s)+H2O(l)⇌Ca2+(aq)+HCO-3(aq)+OH-(aq)

If you increase the pH by adding OH- ions, Le Châtelier's Principle says that the position of equilibrium will move to the left.

The solubility of the CaCO3 decreases.

If you decrease the pH by adding H3O+ ions, the added H3O+ ions will react with the OH- ions and form water.

OH- ions are removed from solution.

According to Le Châtelier's Principle, the position of equilibrium moves to the right to replace the OH- ions.

The solubility of the CaCO3 increases.

Here's a video that discusses the effect of pH on solubility.

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