What substance has the lowest H concentration but is still considered an acid?

The substance with the lowest hydrogen ion (H⁺) concentration that is still considered an acid is carbonic acid (H₂CO₃). It is a weak acid formed in solution when carbon dioxide (CO₂) dissolves in water. Despite having a lower concentration of H⁺ ions compared to strong acids like hydrochloric acid, it can still donate protons in a solution, thus retaining its classification as an acid.