Products
Carbonic acid is present in very small quantities in, for example, mineral water (sparkling water) and sodas.
Structure and properties
Carbonic acid (H
2
CO
3
, M
r
= 62.0 g/mol) is a weak, biprotonic acid that is counted among the inorganic compounds despite the carbon atom. It is very unstable in the presence of water and therefore it was not clear in the past whether it exists at all. Under special conditions, however, it can be isolated and crystallized as a pure substance and is remarkably stable in the absence of water. The salts of carbonic acid are known as carbonates (CO
3
2-
, without hydrogen atoms) and hydrogen carbonates (HCO
3
–
, bicarbonates, with 1 hydrogen atom). These include, for example, calcium carbonate (lime), sodium carbonate (soda ash), potassium carbonate (potash), and sodium hydrogen carbonate (sodium bicarbonate).
Effects
Carbonic acid is formed when carbon dioxide (CO
2
) is dissolved in water. The following equilibrium is formed:
- CO
2
(carbon dioxide) + H
2
O (water) ⇌ H
2
CO
3
(carbonic acid)
The solution of carbon dioxide is favored by increased pressure and low temperature. The reaction leads to a slight acidification of the water due to deprotonation:
- H
2
CO
3
⇌ HCO
3
–
+ H
+
⇌ CO
3
2-
+ H
+
Carbonic acid is very unstable in water and decomposes rapidly. Water “with carbonic acid” (in German: “Sprudel”) is produced by introducing carbon dioxide. When the bottle is opened, the pressure drops and it begins to bubble, i.e. the gas is released again. The pH of ordinary effervescence is about 5.5, with a lower CO
2
-content, it rises to values between 6 and 7. In addition to the reduction in pressure, an increase in temperature or shaking of the bottle also causes the carbon dioxide to be released. The addition of acids or bases can shift the equilibrium. The carbon dioxide content in sparkling water is very low. So in a restaurant, you should really order “water with carbon dioxide.” This is because well over 99% of the carbon dioxide is dissolved in the water. Carbonic acid is important for buffer systems in the human body, for the acid-base balance and for adjusting the pH of the blood. It also plays a central role in transporting carbon dioxide from the periphery to the lungs. CO
2
is formed during ATP production in the mitochrondria. Enzymes, carbonic anhydrases, exist in the human body to accelerate the reaction between carbon dioxide, carbonic acid and bicarbonate.
Applications
In pharmacy, carbonates and hydrogen carbonates in particular play an important role, for example, as excipients, for the treatment of stomach burns and for the production of active ingredient salts.