Carbonic Acid

Carbonic acid (H₂CO₃) is a weak acid formed when carbon dioxide dissolves in water. It is present in rain, rivers, the ocean and blood, where it acts as a buffer, and it is the link between the gas in the air and the carbonate minerals in rocks.

CO₂ reacts with water in two steps: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. Only a small fraction of dissolved CO₂ is actually present as H₂CO₃, so chemists often report the combined effective constant, with a first acid dissociation constant (pKa) of about 6.3. The bicarbonate ion that forms can lose a second proton to become carbonate. The reaction is slow without a catalyst, which in living cells is the enzyme carbonic anhydrase.

Rain is slightly acidic, with a pH of about 5.6, because of dissolved CO₂, and this carbonic acid dissolves rocks, driving silicate weathering and the Urey reaction and carving limestone caves. In the ocean, the added CO₂ from human activity lowers the pH, as described in ocean acidification, and it reduces the carbonate ions that shell-building animals need.

Blood pH is kept at about 7.4 largely by the carbonic acid-bicarbonate system. CO₂ made by cellular respiration enters red blood cells and is converted to bicarbonate, which is carried in the plasma, and the lungs control pH by changing how much CO₂ is exhaled. The effect is linked to the way hemoglobin releases oxygen, the Bohr effect.

• Carbonic Anhydrase — the enzyme that speeds its formation
• Bicarbonate — the ion it produces
• Ocean Acidification — the result of excess CO₂ in the sea