Carbonic Acid

When carbon dioxide dissolves in water it produces a feeble acid which turns the colour of ordinary blue litmus a port-wine red and discharges the crimson colour of phenolphthalein, but scarcely affects methyl orange. This acid is (meta) carbonic acid, H₂CO₃, formed by the hydroxylation of carbon dioxide, thus:

CO₂ + H₂O ⇔ CO(OH)₂.

When an aqueous solution of carbonic acid is boiled the acid is completely decomposed, with the escape of carbon dioxide, so that the original colour of an indicator, which had been changed by the carbonic acid, is restored.

The acidity of carbonic acid is due to its dissociation into the ions H^• and HCO₃', thus:

H₂CO₃ ⇔ H^• + HCO₃';

and the extent to which this reaction has taken place when equilibrium is reached is indicated by the electric conductivity of the solution. This conductivity was measured, first by Pfeiffer, later by Knox, and also by Walker and Cormack.

The following results were obtained by Walker and Cormack at a temperature of 18° C.:

Carbonic Acid, H₂CO₃

From Marble.		From solid CO2		From Marble.		From solid CO2
v	λv	y	K	v	λv	y	K
31.25	1.038	0.00309	0.06306	27.5	0.972	0.00289	0.06305
62.5	1.475	0.00439	0.06309	55.0	1.368	0.00407	0.06303
93.7	1.800	0.00536	0.06308	82.5	1.679	0.00500	0.06304
125.0	2.083	0.00620	0.06309	110.0	1.930	0.00575	0.06302
			Mean 0.06308				Mean 0.06304

v = volume in litres containing 1 gram-molecule of CO₂

λ_v = molecular conductivity determined by experiment

γ = proportion of acid dissociated = λ_v/λ_∞ = λ_v/336, where 336 = molecular conductivity at infinite dilution

K = dissociation constant γ²/(1-γ)v (Ostwald's dilution law).

The figures under γ convey no information as to the degree of hydroxylation of the carbon dioxide in solution; that is as to what proportion has been converted into H₂CO₃, some of which is subsequently dissociated, and what proportion remains as CO₂; they show, however, the ratio of the concentration of H^• ions to the possible concentration if all the carbon dioxide were present as dissociated carbonic acid.

Since the strength of an acid is indicated by the relative magnitude of its dissociation constant K it is useful to compare the relative strengths of carbonic and other acids.

In the table on the opposite page are given the dissociation constants of the more common weak inorganic acids, together with their percentage ionisation in decinormal and centinormal solutions, as calculated by the aid of Ostwald's dilution formula (vide supra). The values for hydrochloric acid are calculated direct from the conductivity at the above-mentioned concentrations and at infinite dilution. These figures are of practical utility. Thus, if equivalent quantities of acetic and carbonic acids compete for an equivalent of a base in decinormal solution, we are able to say that the base will be shared between the acids in the ratio 1.3:0.25, the acetic acid thus taking about five times as much of the base as the carbonic acid.

Acid.	Temp. ° C.	K	100 γ
			N/100 Soln	N/10 Soln
Hydrochloric, Cl-H	18	-	97.1	92.6
Nitrous, NO2-H	25	5×10-4	20.0	5.0
Acetic, C2H3O2-H	18	1.8×10-5	4.2	1.30
Carbonic, HCO3-H	18	3×10-7	0.774	0.25
Carbonic, HCO3-H	4	5×10-4	29.0	9.5
Sulphydric, HS-H	18	9×10-8	0.42	0.13
Boric, H2BO3-H	18	5.8×10-10	0.042	0.013
Hydrocyanic, CN-H	18	1.3×10-9	0.036	0.011
Water, HO-H	18	0.71×10-14	-	-

The degree of ionisation of the second hydrogen atom of H₂CO₃ has been estimated by several observers, the most reliable value being:

K = = 6×10^-11

The "true strength" of carbonic acid has been estimated by Thiel and Strohecker. By the "true strength" is meant the degree of ionisation of the H₂CO₃ actually formed rather than the ionisation with reference to CO₂ dissolved. It has been found that in a 0.00812M solution of carbon dioxide at 4° C. only 0.67 per cent, is present as carbonic acid. The "true strength" of carbonic acid is therefore 100/0.67 times its strength found from conductivity measurements. Thus the true dissociation constant is 5×10^-4 instead of 3×10^-7; and the acid is found to be twice as " strong " as formic acid. Such a conclusion might be anticipated, since carbonic acid is hydroxyformic acid.