7.13 pH and Solubility

pH affects solubility

For salts containing a basic anion (conjugate base of a weak acid, like OH⁻, CO₃²⁻, S²⁻, or F⁻), solubility increases as pH decreases (more acidic). The H⁺ ions react with the anion, shifting the dissolution equilibrium forward via Le Chatelier's principle, removing anion and dissolving more solid.

Salts with nonbasic anions

Salts whose anion is the conjugate base of a strong acid (like Cl⁻, Br⁻, NO₃⁻, SO₄²⁻) show no significant pH dependence in their solubility, since these anions don't react appreciably with H⁺ or OH⁻ in solution.

Writing the coupled equilibria

To analyze pH effects, write both the dissolution (Ksp) equilibrium and the acid-base equilibrium of the anion with water or H⁺. Adding acid consumes the basic anion (e.g., CO₃²⁻ + 2H⁺ → H₂CO₃), lowering its concentration and driving the Ksp reaction to produce more dissolved ions, increasing solubility.

Key terms

Why does pH affect solubility of some salts?

If the anion is a conjugate base of a weak acid, it reacts with H⁺, removing anion and shifting the Ksp equilibrium to dissolve more solid.

Salts unaffected by pH

Salts with anions that are conjugate bases of strong acids (e.g., Cl⁻, NO₃⁻, SO₄²⁻) — these don't react with H⁺.

Effect of decreasing pH on CaCO₃ solubility

Solubility increases — H⁺ reacts with CO₃²⁻, removing it and shifting equilibrium to dissolve more CaCO₃.

Effect of increasing pH on Mg(OH)₂ solubility

Solubility decreases — more OH⁻ present shifts equilibrium left (common ion effect), keeping Mg(OH)₂ undissolved.

Examples of basic anions sensitive to pH

OH⁻, CO₃²⁻, S²⁻, F⁻, PO₄³⁻ — conjugate bases of weak acids that react with added H⁺.

Try an AP-style question

Why is Mg(OH)₂ more soluble in acidic solution?

1. H⁺ removes OH⁻, shifting dissolution right
2. Acid adds more Mg²⁺ ions to the solution
3. Acid increases the value of Ksp for Mg(OH)₂
4. Mg(OH)₂ turns into a gas that escapes