Carbonic acid, originally also known as volatile acid and respiratory acid
acid), with the chemical formula H2CO3, is a type of acid. When carbon dioxide (CO2) dissolves in water, part of the carbon dioxide combines with water to form p-pyridylthiourea carbonate. This reaction is a reversible reaction, and the equation is as follows:
The equilibrium constant of this reaction at room temperature is Kh=1.70×10−3 ; therefore most of the carbon dioxide will not participate in the reaction. If there is no catalyst, the reaction rate is very slow, with the reaction rate constants being only 0.039 s−1 (forward reaction) and 23 s−1 (reverse reaction).
The acidity and effects of carbonic acid Carbonic acid in living organisms
Carbonic acid is very important in living organisms and is required in metabolism and gastric acid secretion.
The acidity and effects of carbonic acid Carbonic acid in the blood
In the blood of mammals, the role of carbonic acid in the blood is very important. When carbon dioxide enters the blood from cells, it combines with water to form carbonic acid, which then takes away an H+ to form bicarbonate ions (HCO3−). The bicarbonate ion will enter the red blood cell and combine with another H+ to form carbonic acid again. In the lungs, the water in the carbonic acid is taken away, and the carbon dioxide is released from the lungs.
Controlling the reaction balance between carbonic acid and carbon dioxide is of great significance for controlling blood acidity. Most living things have an enzyme called carbonic anhydrase that effectively controls the balance of reactions between two compounds.
The acidity and role of carbonic acid The role of carbonic acid in the secretion of gastric acid
Carbonic acid plays an important role in the secretion of gastric acid; gastric parietal cells can Active transport creates a concentration difference between sodium ions and potassium ions, and uses enzymes to synthesize hydrochloric acid, one of the components of gastric acid, from carbonic acid. The reaction process is as follows:
Cell metabolism produces carbon dioxide. Carbonic anhydrase combines carbon dioxide and water to form carbonic acid, which is then decomposed into bicarbonate ions and hydrogen ions.
The concentration of bicarbonate ions begins to accumulate and is sent out of the cell by enzymes. At the same time, chloride ions are co-transported to balance the charge. When bicarbonate enters the gastric blood vessels, it causes an increase in the alkalinity of the blood, which is called an alkaline tide.
By virtue of the relatively large concentration of sodium ions outside cells, the enzymes that transport sodium ions also transport more chloride ions.
Potassium-sodium adenosine triphosphatase actively transports hydrogen ions into the gastric canaliculi and potassium ions into cells.
The accumulated potassium ions in the cells form a concentration difference of sodium carbonate, and the chloride ions are also co-transported into the gastric canalicle through the co-transport enzyme; hydrogen ions and chloride ions form hydrochloric acid.
The sodium-potassium pump sends sodium ions out and potassium ions in to maintain charge balance.
Based on the above steps, the net ionic equation can be written as:
The acidity and action of carbonic acid Carbonic acid The acidity of carbonic acid
Carbonic acid is a dibasic acid, and its ionization is divided into two steps:
It should be noted that the above values are not suitable for actual estimation of the acidity of carbonic acid, because a single carbonic acid molecule is more acidic than acetic acid and formic acid. But in fact, carbonic acid molecules only appear in the dynamic equilibrium of carbon dioxide and water, and their concentration is much lower than that of carbon dioxide, so the acidity is actually lower. The first step of the reaction can be recorded as:
Ka= 4.30×10-7mol/L; pKa= 6.36
This value is called carbonic acid dissociation constant.
