Carbonic acid

• Carbon acid
• Dihydrogencarbonate
• Hydroxyameisensäure
• Hydroxymethansäure

Exists in the presence of water only solved

• 6.35
• 10.33
• 3.45 ± 0.15 ( calculated by kinetic model )

Template: Infobox chemical / molecular formula search is not possible

Carbonic acid ( H2CO3 ) is an inorganic acid and the reaction product of its acid anhydride carbon dioxide ( CO2) with water. The salts of the dibasic acid, the carbonates and bicarbonates. Your esters are also called carbonates, or carbonic acid esters. Technical meaning of the polyester, which are called polycarbonates.

The gas is CO2 compared to O2 and N2 relatively soluble in water and reacts in a small proportion ( about 0.2 %, depending on temperature) to form carbonic acid:

The gas CO2 is colloquially often inaccurate as carbonic acid. In fact, in the water chemistry of dissolved CO2 with the actual acid H2CO3 is usually summarized as free carbonic acid. She is compared to the sum of carbonate and bicarbonate as the bound carbonic acid.

Carbonic acid plays an important role in both the water and the blood and body fluids, acid -base balance.

History

Carbonic acid was first known in mineral waters because of the sour taste. Andreas Libavius ​​reported in 1597 in his treatise de judico aquarum Mineralium of the carbonic acid. More in-depth knowledge has been transmitted by Johan Baptista van Helmont concerning the fermentation and the action of mineral acids on carbonated rocks.

Dissociation equilibrium

Dissolved carbon dioxide in aqueous solution is in equilibrium with carbonic acid:

The Erlenmeyer rule describes the instability of molecules having two hydroxyl groups on the same carbon atom. Therefore, the equilibrium lies far on the side of the anhydride; the proportion of the acid molecule is in aqueous solution at only around 0.2 %. This proportion is overly dependent on the temperature. In organisms, the reaction by the enzyme carbonic anhydrase is accelerated. Carbonic acid is a dibasic acid. Therefore you are their protons in two dissociation of water or other bases from:

The pKa of the first acidity constant is ( with temperature-dependent deviations) actually at about 3.88, carbon dioxide is thus a medium strong acid comparable with acetic acid and citric acid. However, since the proportion of carbonic acid according to equation ( 1) is difficult to determine the reactions (1) and (2) are combined to (3):

And yield the (almost always referred to ) value of about 6.5 for the pKa value. Free carbon dioxide is thus a weak acid. Reaction product of the bicarbonate ion HCO3-.

The pK a value for the second acidity constant is 10.5. Reaction product is the carbonate ion CO32-.

The concentrations of the three (actually four) carbonic acid species, ie the free carbonic acid ( H2CO3 and dissolved CO2 ), the bicarbonate and carbonate and the oxonium ions with each other through the law of mass action in a calculable connection. The concentration of the hydronium ions is expressed by pH. At a given pH therefore the proportion of the species is determined:

At pH 4, more than 99 % are present as free carbon dioxide. At a pH value of 6.5, which therefore equal to the pKa of the first acidity constant, are equal before much free carbonic acid and bicarbonate; the proportion of the carbonate is far below 1%. At about pH 8.3, the maximum amount of hydrogen is approximately 98 % before; each just under 1% are free carbonic acid and carbonate. At a pH equal to the pKa of the second acidity constant of 10.5 are equal amounts of bicarbonate and carbonate and a vanishingly small amount of free carbon dioxide. At pH 12.5, the carbonate has a share of 99 %, hydrogen is just under 1% and free carbonic acid only occurs in trace amounts.

Carbonic acid as a pure substance

In the laboratory, it has been possible to produce carbonic acid (in the narrow sense) as a pure substance. However, that is - in practical terms - more of a scientific curiosity. At low temperatures and absolute absence of water or metal ions ( both strongly catalyze the decomposition reaction to carbon dioxide and water) carbonic acid H2CO3 are shown as a clear, colorless liquid.

Aggressive carbonation and Related

Denote Another group of trivial names that are chemically different species, but proportions, originates from the field of water chemistry for limy water. It should be noted that the following terms, respectively proportions of the so-called free carbon dioxide affect, in which no distinction is made between carbon dioxide and the carbon dioxide in the strict sense.

According to the lime-carbonic acid equilibrium, the concentrations of calcium and carbonic acid are interdependent. We distinguish the amount of carbon dioxide associated on the amount of the excess and the ( lime ) aggressive carbonic acid. Associated acid keeps the equilibrium quantity of carbonic acid species the pH just so low that, dependent on the concentration of the carbonate multiplied just not exceed that of the calcium, the solubility of calcium carbonate. Moreover, any free carbonic acid is regarded as excessive. Of this, a portion could further bring lime in solution is therefore ( lime ) are aggressive; the rest of the surplus would then require an additional associated carbonic acid.

With increasing values ​​for the carbonate hardness of the proportion of the corresponding free carbonic acid increases disproportionately. For example, this value was 5.1 ° dH 1.83 mg / l CO2 and 10.2 ° dH 11.67 mg / l CO2. This results in the mixing of waters to a mixed water problem. The mixing of waters with different carbonate yields mixed water with aggressive carbonation, even if the output water in the lime - carbonic acid equilibrium were.

The mathematical relationships are summarized in the Tillmansschen equation, with the corresponding " free carbonic acid " for each content can be calculated to calcium. Below the summary of this equation:

The elements of the equation mean:

• = Concentration of the respective calculated free carbon dioxide (CO2) in mol / kg
• = Tillmanssche constant
• = Square of the concentration of bicarbonates ( HCO3) in mol / kg
• = Concentration of calcium in mol / kg

More detailed report Tillmanssche equation.

For the preparation and decarbonization of waters precise knowledge of this equilibrium and its adjustment speed is of great importance. Thus, in the treatment of drinking water, the raw water through semi- burnt dolomite ( calcium magnesium carbonate, CaMg ( CO3) 2 ) is directed so that it does not contain excess " free carbonic acid ", as iron or other metals react with this and so, for example, pipes would corrode steel. These reactions are dependent on concentration in equilibrium with the corresponding carbonates. Therefore one speaks, for example, of " iron - aggressive carbonic acid ." Dolomite is used because in the presence of magnesium ions, the adjustment speed of the Tillmans'schen equilibrium is greatly accelerated, which would take with pure calcium carbonate for far too long.

For some user groups, eg in fisheries, the amount mentioned terms are often so misunderstood, as if, for example, the " aggressive carbon dioxide " would be particularly harmful, as for the fish. However, since fish do not consist of lime, the aggressive carbonic acid not otherwise worked against them than the rest of carbonic acid. For the respiration of fish rather the total dissolved CO2 concentration is essential for a potential acidic burns only the pH of the water. The " associated carbonic acid " is there so misunderstood, as if it were bound in a special way to the bicarbonate and therefore does not expel water through aeration or consume through photosynthesis by algae. In fact, both processes, the entire " free carbonic acid " is available, making it thereby eventually comes to an increase of the pH value, thereby a shift in the amount of equilibrium towards more carbonate and to exceed the solubility of lime, ie to a lime precipitation.

Derivatives of carbonic acid

In addition, organic derivatives of carbonic acid are known, various carbonic acid ester. They are easily available by the reaction of phosgene with alcohols. Of industrial significance are the amides of carbonic acid. Your parent compound is urea, a diamide of carbonic acid; as an example may be mentioned (of urea, urea), the urethanes. There are substituted esters of mono-amide of carbonic acid, carbamic acid; these are the parent compounds extremely important plastics, polyurethanes.

Use

Carbon dioxide is used for countless manufacturing processes worldwide, and they probably should be known from soft drinks to the end user best. Johann Jacob Schweppe developed in the late 18th century, a process by which water can be offset with carbon dioxide. In the 19th century, they began to mix bottled water carbon dioxide, to make it durable.