Carbonic Acid

Carbonic acid is an inorganic compound as it lacks a bond between hydrogen and carbon atoms. Its chemical formula is H₂CO₃. However, it can be written as OC(OH)₂ owing to the presence of one carbon-oxygen double bond in its molecule. It is also called acid of air, aerial acid or dihydrogen carbonate.

It is a weak acid, which produces carbonate and bicarbonate salts. Further, it is also referred to as a respiratory acid as it is the only acid that is excreted in the gaseous state by the lungs in our body.

Carbonic acid is a very common acid owing to the abundance of carbon dioxide and water. It is found only in the form of its salts like carbonates; acid salts like hydrogen carbonates; amines like carbamic acid and as acid chlorides such as carbonyl chloride. Carbonic acid also dissolves limestone to form calcium bicarbonate Ca(HCO₃)₂.

Carbonic Acid Structure

The molecule of carbonic acid (CH₂O₃) is made of one carboxyl group and two hydroxyl groups. Being a diprotic acid, it can donate two protons.

Carbonic Acid Occurrence

It is found in the blood of humans. Its formation takes place when water gets dissolved with carbon dioxide. The lungs move it out of the body. Besides this, it is also present in coal, meteors, rainwater, calcite, erythrocytes, volcanoes, proteins, plants, and more.

Physical Properties of Carbonic Acid

• It appears as white solid.
• It is odourless and tastes alkaline.
• Its molar mass is 62.024 gm per mole.
• In a standard state, its density is 1.668 gm per cubic centimetre.
• Its pKa value is 6.35.
• Its corresponding conjugate base is bicarbonate.
• It is generally found as a solution. But, it is believed that NASA scientists have prepared some samples of H₂CO₃ in solid-state.

Chemical Properties of Carbonic Acid

• It is a weak and unstable acid.
• It dissociates partially in the presence of water to give H⁺ and HCO₃^-.
• It is a diprotic acid and thus can form two types of salts: bicarbonates and carbonates.
• With a small amount of base it forms bicarbonates, and with a large amount of base it forms carbonate salt.
• Carbonated water (carbon dioxide in water) can be referred to as carbonic acid.

Preparation of Carbonic Acid

It is prepared by the dissolution or hydrolysis of CO₂ in water. For example, carbon dioxide, when dissolved in water results in the following chemical equilibrium and forms carbonic acid.

CO₂ + H₂O ⇌ H₂CO₃

However, a small amount of carbonic acid is formed as less carbon dioxide changes into carbonic acid.

In industries on a large scale, it is obtained as the by-product of fermentation, fossil fuel burning and other processes.

It is also prepared by making strong acid act on calcium carbonate and by allowing the carbon dioxide to pass into the water. The experiment is described below:

Reagents required:

• Calcium carbonate
• Hydrochloric acid solution
• Litmus
• Water

Steps:

• Take some water in a beaker.
• Add 5 to 6 drops of litmus into the beaker.
• Take the required amount of calcium carbonate into a test tube.
• Add a small amount of hydrochloric acid into the test tube and close it with a stopper having an outlet tube.
• Now place the end of the outlet tube into the litmus solution. The gas produced in the test tube will produce bubbles in the litmus solution and the colour of this solution changes
• Now test the pH of this solution. The pH shows it is a mild acid, not water and thus carbonic acid has been prepared.

The chemical reactions that take place in the above method are as follows:

i) CaCO₃+ HCl → CaCl₂+ CO₂ ↑+ H₂O

ii) CO₂+ H₂O → H₂CO₃

Uses of Carbonic Acid

Carbonic acid offers a wide range of uses. Some of which are as follows:

• It is used in the preparation of carbonated water, fizzy wine and other such aerated drinks.
• It is required in the precipitation of various ammonium salts such as ammonium persulfate.
• It makes possible the movement of carbon dioxide out of the body.
• It donates protons to various nitrogen-containing bases present in the blood serum.
• It is also used to treat ringworm and various other dermatitides.
• It is also used in the solutions that are used to clean the contact lenses.
• In case of a drug overdose, it can be taken orally to induce vomiting.
• It is used as a mouthwash and as a vaginal spray.
• It is required in the hydrolysis of starch.
• It is required as gas for welding.

Importance of Carbonic Acid in Blood

The bicarbonate ion acts as an intermediate for the removal of carbon dioxide from our bodies through the respiratory exchange of gases. The hydration of carbon dioxide is very slow in the absence of a catalyst. But, when carbonic anhydrases, present in the blood are used as catalysts, the reaction rate of hydration increases and thus the conversion of carbon dioxide and water to ions of carbonic acid increases. The bicarbonate anions produced in this reaction get dissolved in the blood plasma. This reaction is reversed in the lungs to form carbon dioxide, which is exhaled out of the body during expiration.

Importance of Carbonic Acid in Oceans

The excess of carbon dioxide that is released into the atmosphere is absorbed by the oceans. It is believed that due to this absorption the pH of water of oceans has changed by around -0.1. Further, the absorbed carbon dioxide reacts with ocean water to form H₂CO₃, this process is known as ocean acidification.

Health Hazards of Carbonic Acid

It is not toxic for human health as it is naturally found in the human body. However, if someone is exposed to a high concentration of H₂CO₃ for a longer duration, he or she may experience irritation in the eyes and respiratory tract.