<P> At a given temperature, the composition of a pure carbonic acid solution (or of a pure CO solution) is completely determined by the partial pressure p CO 2 (\ displaystyle p_ ((\ text (CO)) _ (2))) of carbon dioxide above the solution . To calculate this composition, account must be taken of the above equilibria between the three different carbonate forms (H CO, HCO and CO), as well as of the hydration equilibrium between dissolved CO and H CO with constant K h = (H 2 CO 3) (CO 2) (\ displaystyle K_ (h) = (\ tfrac (((\ text (H)) _ (2) (\ text (CO)) _ (3))) (((\ text (CO)) _ (2))))) (see above) and of the following equilibrium between the dissolved CO and the gaseous CO above the solution: </P> <Dl> <Dd> CO (gas) ⇌ CO (dissolved) with (CO 2) p CO 2 = 1 k H, (\ displaystyle \ textstyle (\ frac (((\ text (CO)) _ (2))) (p_ ((\ text (CO)) _ (2)))) = (\ frac (1) (k_ (\ text (H)))),) where k = 29.76 atm / (mol / L) (Henry constant) at 25 ° C . </Dd> </Dl> <Dd> CO (gas) ⇌ CO (dissolved) with (CO 2) p CO 2 = 1 k H, (\ displaystyle \ textstyle (\ frac (((\ text (CO)) _ (2))) (p_ ((\ text (CO)) _ (2)))) = (\ frac (1) (k_ (\ text (H)))),) where k = 29.76 atm / (mol / L) (Henry constant) at 25 ° C . </Dd> <P> The corresponding equilibrium equations together with the (H +) (OH −) = 10 − 14 (\ displaystyle ((\ text (H)) ^ (+)) ((\ text (OH)) ^ (-)) = 10 ^ (- 14)) relation and the charge neutrality condition (H +) = (OH −) + (HCO 3 −) + 2 (CO 3 2 −) (\ displaystyle ((\ text (H)) ^ (+)) = ((\ text (OH)) ^ (-)) + ((\ text (HCO)) _ (3) ^ (-)) + 2 ((\ text (CO)) _ (3) ^ (2 -))) result in six equations for the six unknowns (CO), (H CO), (H), (OH), (HCO) and (CO), showing that the composition of the solution is fully determined by p CO 2 (\ displaystyle p_ ((\ text (CO)) _ (2))). The equation obtained for (H) is a cubic, whose numerical solution yields the following values for the pH and the different species concentrations: </P>

Aqueous carbonic acid is obtained by the reaction of carbon dioxide gas and liquid water