What is Carbonic Acid?

Carbonic acid can be defined as a carbon-containing compound having the chemical formula H2CO3. The carbon dioxide solutions in water comprise small amounts of this compound. The carbonic acid's chemical formula can also be written as OC(OH)2 because, in this compound, there is one carbon-oxygen double bond.


Carbonic acid can often be described as a respiratory acid due to the reason it is the only acid exhaled by the human lungs in the gaseous state. It forms bicarbonate and carbonate salts, and it is a weak acid.


Note: H2CO3 is capable of dissolving limestone, which leads to the calcium bicarbonate (Ca(HCO3)2 formation. This is the primary reason for various features of limestone, including stalactites and stalagmites.


H2CO3 Structure

The carbonic acid or H2CO3 structure can be illustrated below.

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From the illustration above, we can understand that the carbonic acid structure consists of one carbon-oxygen double bond and two carbon-oxygen single bonds. The oxygen atoms that participate in a single bond with the carbon each contain one hydrogen atom attached to them.

Carbonic acid, formed by the hydrolysis and dissolution of CO2 in water, is the primary natural leaching agent in various temperate ecosystems. Carbonic acid is both unstable, weak and dissociates quickly into the bicarbonate ions (HCO3) and hydrogen ions (H+).

When CO2 is dissolved in water, it participates in the chemical equilibrium as given below.

CO2 + H2O ⇌ H2CO3

However, in the chemical equilibrium described above, only a small amount of carbon dioxide is converted into carbonic acid.


Properties of H2CO3

A few major physical and chemical properties of carbonic acid are listed in this subsection:


Physical Properties

  • The carbonic acid molar mass can be given as 62.024 grams per mole.

  • The H2CO3 compound holds a pKa value of 3.6.

  • It has a density of 1.668 grams per cubic centimetre in a standard state.

  • Generally, this compound exists as a solution. But, there has been reported that NASA scientists have prepared solid H2CO3 samples.

  • The conjugate base corresponding to the carbonic acid is bicarbonate.

Chemical Properties 

  • H2CO3 is unstable in nature, and it is a weak acid.

  • In the presence of water, it undergoes partial dissociation to yield HCO3 (bicarbonate) and H+ ions.

  • Carbonic acid is a diprotic acid, and hence, it can form two types of salts: carbonates and bicarbonates.

  • Adding a small quantity of base to H2CO3 yields bicarbonate salts, whereas the addition of a base in excess amount yields carbonate salts.

  • It should also note that carbonic acid can be obtained as the burning of fossil fuels at an industrial scale or a by-product of industrial fermentation processes.

Carbonic Acid Uses

H2CO3 is an essential compound with a wide range of applications. A few of the carbonic acid uses are listed below:

  • The preparation of sparkling wine, carbonated water, and other aerated drinks involves carbonic acid use.

  • H2CO3 can be used in the precipitation of various ammonium salts like ammonium persulfate.

  • It helps in carbon dioxide transportation out of the body.

  • H2CO3 protonates many bases that contain nitrogen in blood serum

  • Ringworm and other dermatitides are treated through the carbonic acid application over the affected area.

  • Solutions that contain this compound are much effective in contact lens cleaning.

  • It can be orally consumed to induce vomiting whenever required (such as, in the case of a drug overdose).

Carbonic Acid Importance in Blood

The bicarbonate ion or molecule is an intermediate for the carbon dioxide transportation out of the human body through the respiratory gas exchange process. The hydration reactions undergone by carbon dioxide are very slow, especially in the absence of a suitable catalyst. The presence of the enzyme family called carbonic anhydrases in the red blood cells will result in an increase in reaction rate. 


The carbonic anhydrase enzymes' enzymes work to catalyze the conversion of water and carbon dioxide into dissociated carbonic acid ions. This forms the bicarbonate anions that get dissolved in the plasma of the blood. This catalyzed reaction will get reversed in the lungs, resulting in CO2 formation, and then exhaled.


A Comment on the Carbonic Acid's Acidity

Carbonic acid is described as a carboxylic acid which holds a substituted hydroxyl group. It is also called polyprotic acid. Actually, this compound is diprotic and, hence, it has two protons which dissociate from the primary parent molecule. Thus, two dissociation constants exist, where the first one is for the dissociation into the bicarbonate ion.

FAQs (Frequently Asked Questions)

1. What is the Carbonic Acid Importance in Oceans?

Ans: The absorption of the excess amount of carbon dioxide in the atmosphere (primarily because of human activities) by oceans is believed to have caused a pH shift of the ocean's water approximately by -0.1. Now, the absorbed carbon dioxide reacts with ocean water and produces H₂CO₃. Commonly, this process is referred to as ocean acidification.

2. Mention the Role of Carbonic Acid in the Blood?

Ans: Bicarbonate acts as an intermediate in the respiratory gas exchange for conveying CO⁻₂ out of the body. Generally, the hydration reaction of CO₂ is too slow in the catalyst absence, but the red blood cells hold a substance called carbonic anhydrase, which increases the rate of reaction by creating dissolved bicarbonate (HCO₃⁻) in the blood plasma.

3. Can Carbonic Acid be Said as a Strong Acid?

Ans: No, H₂CO₃ does not belong to a strong acid. It is also a weak acid that dissociates into a bicarbonate ion (which is HCO₃⁻ anion) and proton (H⁺ cation). This compound dissociates only in aqueous solutions partly. Moreover, the conjugate base of the carbonic acid - the bicarbonate ion, is relatively a good base. 

4. Mention if Carbonic Acid is Dangerous?

Ans: Carbonic acid is not considered dangerous or toxic to human health because it is naturally present in the human body. But, it is important to make a note that exposure to high concentrations of H₂CO₃ may irritate the eyes and respiratory tract.