Hydrogen carbonates

Structural formula of the hydrogen carbonate ion

Hydrogen carbonates , also acidic carbonates or outdated bicarbonates , are the salts of carbonic acid that are created by simply neutralizing this acid with a base . The anion of these salts, the hydrogen carbonate ion (HCO ₃ ^- ), is often referred to as hydrogen carbonate (or bicarbonate ) for short . If the second acid function ( carboxy group ) is also neutralized by adding more bases , carbonates are obtained .

As a result, dissolved hydrogen carbonate ions, due to their potential to both release a proton ( proton donor ) and take it up again ( proton acceptor ), have an important physiological importance as a component of the carbonic acid-bicarbonate system and thus the largest capacity of the blood buffer systems in regulation the acid-base balance of mammals.

Important hydrogen carbonates are sodium hydrogen carbonate (baking soda, Bullrich salt - NaHCO ₃ ), ammonium hydrogen carbonate ( staghorn salt - NH ₄ HCO ₃ ) and calcium hydrogen carbonate ("carbonate hardness" of water - Ca (HCO ₃ ) ₂ ).

Properties of hydrogen carbonates

The state of aggregation is determined as carbonates, because between the bicarbonate ions and cations ionic bonds are present and thereby form regular ionic lattice. Hydrogen carbonates are colorless and appear white in powder form, unless the cation adds a color. Hydrogen carbonates are odorless substances. The mostly soluble hydrogen carbonates form electrically conductive solutions with water because, in addition to the cations, they form freely mobile, hydrated hydrogen carbonate anions. Hydrogen carbonate is amphoteric .

Decomposition: Hydrogen carbonate decomposes above a temperature of 50 ° C; among other things, water and carbon dioxide are produced .

Although the hydrogen carbonates of the alkaline earth metals ( calcium , magnesium , barium , strontium ) are readily soluble in water, they are related to the corresponding carbonates, which are all sparingly soluble, via the dissociation equilibrium of the carbonic acid. In order not to exceed the solubility product of the carbonates, the pH value must be sufficiently low, which is ensured by the presence of a minimum concentration of free carbonic acid, thus of dissolved carbon dioxide. This is called the associated carbonic acid. If it escapes from the water or if it is consumed in lakes through photosynthesis , the carbonates partially separate in crystalline form ( scale , sea ​​chalk )

It is therefore not possible (under normal conditions) to produce the hydrogen carbonates of the alkaline earths as a solid. When the solutions are concentrated , the carbonates are always formed.

Reactions of hydrogen carbonates

Hydrogen carbonate ions react as a weak base with water to form hydroxide ions and unstable carbonic acid. A chemical equilibrium is established here that is predominantly on the left-hand side, since the p K _B value of hydrogen carbonate ions is relatively large at 7.48.

${\displaystyle {\rm {HCO_{3}^{-}+H_{2}O\rightleftarrows H_{2}CO_{3}+OH^{-}}}}$

When adding a stronger acid such as B. Hydrochloric acid to hydrogen carbonates, these react as a weak base to carbon dioxide and water:

${\displaystyle {\rm {HCO_{3}^{-}+H_{3}O^{+}\rightarrow CO_{2}+2\ H_{2}O}}}$

This reaction is important with organic acids for effervescent powder .

With calcium ions an equilibrium with poorly soluble calcium carbonate is established :

${\displaystyle {\rm {2\ HCO_{3}^{-}+Ca^{2+}\rightleftarrows CaCO_{3}+CO_{2}+H_{2}O}}}$

This equilibrium shifts strongly to the right side when heated and is important for the formation of scale . When carbon dioxide is introduced into a calcium carbonate suspension, the equilibrium shifts to the left with the dissolution of calcium carbonate.

Occurrence and use of sodium hydrogen carbonate

Sodium hydrogen carbonate comes as a mineral ( Nahcolith ) and a. in the US. It is used in food technology, for. B. as baking and effervescent powder use. In medicine it is used against heartburn ("Bullrichsalz" ^® ). Sodium hydrogen carbonate is also part of fire extinguishing powder .

Production of sodium hydrogen carbonate

${\displaystyle \mathrm {Na_{2}CO_{3}+CO_{2}+H_{2}O\longrightarrow 2\ NaHCO_{3}} }$

Sodium carbonate, carbon dioxide and water react to form sodium hydrogen carbonate.

This reaction must take place with cooling.

Use of ammonium hydrogen carbonate

Deer horn salt is used in the baking of gingerbread and breaks down into ammonia (NH ₃ ), water and carbon dioxide. The commercially available salt mostly consists of a mixture of ammonium hydrogen carbonate , ammonium carbonate and ammonium carbamate .

literature

• Arnold F Holleman; Egon Wiberg; Nils Wiberg, Textbook of Inorganic Chemistry , Walter de Gruyter & Co., Berlin, New York 2007, ed. 102, ISBN 978-3-11-017770-1