Carbonic Acid - Acidity of Carbonic Acid

Acidity of Carbonic Acid

Carbonic acid is one of the polyprotic acids: It is diprotic - it has two protons, which may dissociate from the parent molecule. Thus, there are two dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion HCO₃−:

H₂CO₃ HCO₃− + H+

K_a1 = 2.5×10−4 mol/litre; pK_a1 = 3.6 at 25 °C.

Care must be taken when quoting and using the first dissociation constant of carbonic acid. In aqueous solution, carbonic acid exists in equilibrium with carbon dioxide, and the concentration of H₂CO₃ is much lower than the concentration of CO₂. In many analyses, H₂CO₃ includes dissolved CO₂ (referred to as CO₂(aq)), H₂CO₃* is used to represent the two species when writing the aqueous chemical equilibrium equation. The equation may be rewritten as follows:

H₂CO₃* HCO₃− + H+

K_a(app) = 4.6×10−7 mol/litre ; pK(app) = 6.3 at 25 °C and Ionic Strength = 0.0

Whereas this apparent pK_a is quoted as the dissociation constant of carbonic acid, it is ambiguous: it might better be referred to as the acidity constant of dissolved carbon dioxide, as it is particularly useful for calculating the pH of CO₂-containing solutions. A similar situation applies to sulfurous acid (H₂SO₃), which exists in equilibrium with substantial amounts of unhydrated sulfur dioxide.

The second constant is for the dissociation of the bicarbonate ion into the carbonate ion CO₃2−:

HCO₃− CO₃2− + H+

K_a2 = 4.69×10−11 mol/litre ; pK_a2 = 10.329 at 25 °C and Ionic Strength = 0.0