Carbonic acid Totally Explained

Everything about Carbonic Acid totally explained

| Section2 = | Section3 = }} Carbonic acid (ancient name acid of air or aerial acid) has the formula H₂CO₃. It is also a name sometimes given to solutions of carbon dioxide in water, which contain small amounts of H₂CO₃. The salts of carbonic acids are called bicarbonates (or hydrogencarbonates) and carbonates. It is a weak acid. Carbonic acid shouldn't be confused with carbolic acid, an antiquated name for phenol.
Carbon dioxide dissolved in water is in equilibrium with carbonic acid: ť :CO₂ + H₂O ⇌ H₂CO₃

The hydration equilibrium constant at 25°C is K_h= 1.70×10⁻³: hence, the majority of the carbon dioxide isn't converted into carbonic acid and stays as CO₂ molecules. In the absence of a catalyst, the equilibrium is reached quite slowly. The rate constants are 0.039 s⁻¹ for the forward reaction (CO₂ + H₂O → H₂CO₃) and 23 s⁻¹ for the reverse reaction (H₂CO₃ → CO₂ + H₂O).

Role of carbonic acid in blood

Carbonic acid plays a very important role as a buffer in mammalian blood. When CO₂ enters the blood from various cells, it's combined with water to produce carbonic acid. It then has a H⁺ taken away from it to become bicarbonate (HCO₃^-). In order to transport the bicarbonate that's in the blood stream out of the body, it enters another red blood cell, has H⁺ attached to it to form carbonic acid once again, then has H₂O taken away from it and is expelled from the red blood cell as CO₂. Then the carbon dioxide is permitted to be expelled out of capillaries and into the lungs.
The equilibrium between carbon dioxide and carbonic acid is very important for controlling the acidity of body fluids, and almost all living organisms have an enzyme, carbonic anhydrase, which catalyzes the conversion between the two compounds, increasing the reaction rate by a factor of nearly a billion.

Acidity of carbonic acid

Carbonic acid is diprotic, that's it has two hydrogens which dissociate from the parent molecule, and thus there are two dissociation constants:
ť :H₂CO₃ ⇌ HCO₃⁻ + H⁺

   ::K_a1 = 2.5×10⁻⁴; pK_a1 = 3.60 at 25 °C, for -log (2.5×10⁻⁴) = 3.60. ť :HCO₃⁻ ⇌ CO₃²⁻ + H⁺

   ::K_a2 = 5.61×10⁻¹¹; pK_a2 = 10.25 at 25 °C.
   Care must be taken when quoting and using the first dissociation constant of carbonic acid. The value given above is correct for the H₂CO₃ molecule, and shows that it's a stronger acid than acetic acid or formic acid: this might be expected from the influence of the electronegative oxygen substituent. However, in aqueous solution carbonic acid only exists in equilibrium with carbon dioxide, and the concentration of H₂CO₃ there's much lower than the CO₂ concentration, reducing the measured acidity. The equation may be rewritten as follows (c.f. sulfurous acid):
ť :CO₂ + H₂O ⇌ HCO₃⁻ + H⁺

   ::K_a = 4.30×10⁻⁷; pK_a = 6.36.
   This figure is quoted as the dissociation constant of carbonic acid, although this is ambiguous: it might better be referred to as the acidity constant of carbon dioxide, as it is particularly useful for calculating the pH of CO₂ solutions.

pH and composition of a carbonic acid solution

At a given temperature, the composition of a pure carbonic acid solution (or of a pure CO₂ solution) is completely determined by the partial pressure

scriptstyle p_

Instability of carbonic acid

It has long been recognized that it's impossible to obtain pure hydrogen bicarbonate at room temperatures (about 20 °C or about 70 °F). However, in 1991 scientists at NASA's Goddard Space Flight Center (USA) succeeded in making the first pure H₂CO₃ samples. They did so by exposing a frozen mixture of water and carbon dioxide to high-energy radiation, and then warming to remove the excess water. The carbonic acid that remained was characterized by infrared spectroscopy. The fact that the carbonic acid was prepared by irradiating a solid H₂O + CO₂ mixture has given rise to suggestions that H₂CO₃ might be found in outer space, where frozen ices of H₂O and CO₂ are common, as are cosmic rays and ultraviolet light, to help them react. The same carbonic acid polymorph (denoted beta-carbonic acid) was prepared by a cryotechnique at the University of Innsbruck: alternating layers of glassy aqueous solutions of bicarbonate and acid were heated in vacuo, which causes protonation of bicarbonate, and the solvent was subsequently removed. A second polymorph (denoted alpha-carbonic acid) was prepared by the same technique at the University of Innsbruck using methanol rather than water as a solvent.
It has since been shown, by theoretical calculations, that the presence of even a single molecule of water causes carbonic acid to revert to carbon dioxide and water fairly quickly. Pure carbonic acid is predicted to be stable in the gas phase, in the absence of water, with a calculated half-life of 180,000 years.
There is a hypothetical acid orthocarbonic acid which is even more hydrated, being H₄CO₄.

Further Information

Get more info on 'Carbonic Acid'.

External Link Exchanges

Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:

<a href="http://carbonic_acid.totallyexplained.com">Carbonic acid Totally Explained</a>

Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned.