carbonic acid

H₂CO₃; the hydrated form of carbon dioxide (CO₂). It is a weak acid, dissociating in two stages to give hydrogen carbonate (HCO₃^?) and carbonate (CO₃^2?) ions. Natural waters are generally saturated with carbon dioxide from the atmosphere, and their pH is generally determined by the amount of bicarbonate and carbonate ions present. Rain water containing pure carbonic acid has a pH of about 5.

Carbonic acid
Other names	Carbon dioxide solution
Molecular formula	H2CO3
SMILES	C(=O)(O)O
Molar mass	62.03 g/mol
CAS number	463-79-6
Density	1.0 g/cm3 (dilute solution)
Solubility (water)	exists only in solution
Acidity (pKa)	6.36 (see text) 10.25
Disclaimer and references

Carbonic acid (ancient name acid of air or aerial acid) has the formula H2CO3. The equation obtained for [H+] is a cubic whose numerical solution yields the following values for the pH and the different species concentrations:

(atm)	pH	[CO2] (mol/L)	[H2CO3] (mol/L)	[HCO3−] (mol/L)	[CO32−] (mol/L)
10−8	7.00	3.36 x 10−10	5.71 x 10−13	1.42 x 10−9	7.90 x 10−13
10−6	6.81	3.36 x 10−8	5.71 x 10−11	9.16 x 10−8	3.30 x 10−11
10−4	5.92	3.36 x 10−6	5.71 x 10−9	1.19 x 10−6	5.57 x 10−11
3.5 x 10−4	5.65	1.18 x 10−5	2.00 x 10−8	2.23 x 10−6	5.60 x 10−11
10−3	5.42	3.36 x 10−5	5.71 x 10−8	3.78 x 10−6	5.61 x 10−11
10−2	4.92	3.36 x 10−4	5.71 x 10−7	1.19 x 10−5	5.61 x 10−11
10−1	4.42	3.36 x 10−3	5.71 x 10−6	3.78 x 10−5	5.61 x 10−11
1	3.92	3.36 x 10−2	5.71 x 10−5	1.20 x 10−4	5.61 x 10−11
2.5	3.72	8.40 x 10−2	1.43 x 10−4	1.89 x 10−4	5.61 x 10−11
10	3.42	0.336	5.71 x 10−4	3.78 x 10−4	5.61 x 10−11

We see that in the total range of pressure, the pH is always largely lower than pKa2 so that the CO3 concentration.