8.11 pH and Solubility

A slightly soluble salt is in equilibrium with its ions (topic 7.11). If something in the solution reacts with one of those ions and removes it, the dissolving equilibrium shifts to replace it, and more solid dissolves.

H₃O⁺ and OH⁻ are often that 'something'. The question to ask is: does either ion of the salt react with acid or base?

A useful shortcut: if a salt's anion is OH⁻ or the conjugate base of a weak acid (such as F⁻, CO₃²⁻, PO₄³⁻ or CH₃COO⁻), the salt dissolves more as the pH is lowered.

Hydroxides: Mg(OH)₂(s) ⇌ Mg²⁺(aq) + 2OH⁻(aq). Adding acid removes OH⁻ (H₃O⁺ + OH⁻ → 2H₂O), so the equilibrium shifts right and more Mg(OH)₂ dissolves. That's how milk of magnesia neutralizes stomach acid.

Salts of weak acids: the anion is a weak base. In CaF₂(s) ⇌ Ca²⁺(aq) + 2F⁻(aq), adding acid converts F⁻ to HF (F⁻ + H₃O⁺ → HF + H₂O), removing F⁻ and shifting the equilibrium right. Carbonates work the same way: CO₃²⁻ reacts with acid, which is why acid rain dissolves limestone and marble (CaCO₃).

Raising the pH (adding base) does the opposite for a metal hydroxide: extra OH⁻ is a common ion, so Mg(OH)₂ becomes less soluble.

If the anion is the conjugate base of a strong acid, such as Cl⁻, Br⁻, I⁻ or NO₃⁻, it is too weak a base to react with H₃O⁺. So the solubility of AgCl, AgBr or PbI₂ is essentially independent of pH. (Adding dilute HCl to AgCl does lower its solubility, but that's the common-ion effect of the Cl⁻, not a pH effect.)

The rule also works in reverse for a salt containing a weakly acidic ion, such as NH₄⁺. Adding base removes NH₄⁺ (NH₄⁺ + OH⁻ → NH₃ + H₂O), so the dissolving equilibrium of a salt containing that ion would shift toward more dissolved ions.

Ammonium salts count as soluble in this course (topic 4.7), so you'll mostly meet this idea with basic anions and hydroxides. Either way, the exam wants qualitative reasoning only: you won't be asked to calculate solubility at a given pH.

Tooth enamel is mostly hydroxyapatite, Ca₅(PO₄)₃OH. Acids made by mouth bacteria react with its OH⁻ and PO₄³⁻ ions, so enamel slowly dissolves in an acidic environment. Limestone statues and marble buildings erode in acid rain because CO₃²⁻ reacts with H₃O⁺. Caves form the same way, as slightly acidic groundwater dissolves limestone.