H2CO3

H2CO3, commonly called carbonic acid, is a weak diprotic acid that exists predominantly in aqueous solutions as dissolved CO2 in equilibrium with hydrated carbonic species. When CO2 dissolves in water, it reacts to form H2CO3, but the molecule is transient and most of the dissolved inorganic carbon is present as CO2(aq) or as the bicarbonate and carbonate species in equilibrium with H+. The acid dissociation equilibria in water are: H2CO3 ⇌ H+ + HCO3- (pKa1 ≈ 6.35 at 25°C) and HCO3- ⇌ H+ + CO3^2- (pKa2 ≈ 10.33). Temperature affects these values.

In solution, carbonic acid decomposes to CO2 and H2O; the hydration step is often treated as CO2 + H2O ⇌ H2CO3, and many references use CO2(aq) as the effective form of carbonic acid. The second dissociation is less relevant at neutral pH due to bicarbonate buffering.

Carbonic acid plays a key role in the carbonate buffering system that maintains pH in natural waters and in blood. In physiology, carbonic anhydrase catalyzes the rapid interconversion of CO2 and HCO3-, enabling CO2 transport as bicarbonate. In the oceans, the dissolution of atmospheric CO2 forms carbonic acid, lowering pH and reducing carbonate ion availability for calcifying organisms, a process known as ocean acidification. In industry, carbonic acid contributes to the acidity of carbonated beverages and forms part of processes to capture and release CO2.

H2CO3 is, however, not typically isolated as a pure compound; it is represented by dissolved CO2 and the equilibrium species in aqueous solution.