kolénsav

Kolénsav, also known as carbonic acid (H2CO3), is a weak aqueous acid that forms when carbon dioxide dissolves in water. In solution, CO2(g) + H2O ⇌ H2CO3; the true concentration of H2CO3 is low, but it represents the hydrated form of CO2.

In water, H2CO3 dissociates in two steps: H2CO3 ⇌ H+ + HCO3− (pKa1 ≈ 6.35) and HCO3− ⇌ H+ + CO3^2− (pKa2 ≈ 10.33). At typical environmental pH, bicarbonate dominates; carbonate is more common at high pH.

Natural waters absorb CO2 from the atmosphere and minerals dissolve via carbonic acid, producing bicarbonate and carbonate. A common overall weathering reaction is CaCO3 + CO2 + H2O ⇌ Ca(HCO3)2. This process moderates geological dissolved solids and acidity.

Biologically, the carbonic acid/bicarbonate system helps regulate pH. In blood, CO2 reacts with water to form H2CO3, which dissociates to H+ and HCO3−. The enzyme carbonic anhydrase accelerates this process; kidneys maintain bicarbonate to keep pH near 7.40.

Kolénsav occurs in carbonated beverages where CO2 dissolves to form carbonic acid, imparting fizz and acidity. It is also central to the oceanic and freshwater carbon cycles, buffering pH changes from respiration and acid rain.

Although weak, H2CO3 is unstable and tends to decompose to CO2 and water, especially on warming or dilution. In practice, it is discussed in equilibrium with dissolved CO2 rather than as a separate concentrated acid.