How does pH influence dissolution and solubility of weak acids and bases?

• A. Solubility increases when the drug is more ionized; weak acids solubilize more at higher pH, weak bases solubilize more at lower pH; ionization state controls solubility.
• B. Solubility increases when the drug is non-ionized; weak acids solubilize more at lower pH; weak bases solubilize more at higher pH.
• C. pH has no effect on solubility.
• D. Ionization state does not affect solubility.

Answer: (A)

The key idea is that solubility of weak electrolytes depends on their ionization state, which is controlled by pH. Ionized (charged) forms interact strongly with water via ion-dipole forces, making them more soluble in aqueous media; non-ionized forms are more lipophilic and less soluble.

For a weak acid, raising the pH shifts the equilibrium toward the deprotonated form (A−). This charged species dissolves more readily in water, so solubility increases at higher pH.

For a weak base, lowering the pH shifts the equilibrium toward the protonated form (BH+). This charged species is more water-soluble, so solubility increases at lower pH.

Thus, ionization state, governed by pH relative to the compound’s pKa, controls solubility: acids solubilize more at high pH, bases solubilize more at low pH, aligning with the statement that solubility increases when the molecule is more ionized.