- SO(OH)₂ (Sulfurous acid, H₂SO₃) and SO₂(OH)₂ (Sulfuric acid, H₂SO₄) are typical protonic acids, donating H⁺ from their O-H bonds. - B(OH)₃ (Boric acid, H₃BO₃) is an acid, but it acts as a Lewis acid by accepting a hydroxide ion (OH⁻) from water, which causes the water molecule to release a proton (H⁺). It does not donate its own proton. The reaction is B(OH)₃ + H₂O ⇌ B(OH)₄⁻ + H⁺. - Po(OH)₃ (Polonium(III) hydroxide) and Po(OH)₄ (Polonium(IV) hydroxide) can exhibit acidic character, especially Po(IV) hydroxide which is amphoteric, suggesting it can donate a proton. However, B(OH)₃ is the classical example of a compound containing -OH groups that is explicitly classified as a Lewis acid rather than a protonic acid in the Brønsted sense due to its mechanism of increasing [H⁺] in water.