What is the valence of hydrogen carbonate




As Carbonates the salts of the completely dissociated carbon acid are called. The associated double negatively charged anion (“acid residue”) is the carbonate ion CO32−.

The esters of carbonic acid with the general structural formula R-O-C (= O) -O-R ', where R and R' are carbon-containing radicals, are also called carbonates. If R and R 'belong to a single molecule, a ring-shaped compound such as propylene carbonate is formed.

Properties of (inorganic) carbonates

Carbonates are ionic salts and therefore usually crystalline solids at room temperature. The carbonate anion does not introduce any color of its own to the compounds, so that their color may be determined by the relevant cation. Carbonates are odorless. With the exception of the alkali carbonates, they are poorly soluble in water.

Reactions of carbonates

  • Comproportionation between carbonic acid and carbonate:
Calcium carbonate reacts with water and carbon dioxide to form calcium hydrogen carbonate.

This reaction takes place when limestone dissolves in carbonated groundwater. It is the origin of water hardness.

Magnesium carbonate dissociates into a magnesium ion and a carbonate ion.
Calcium carbonate breaks down into calcium oxide and carbon dioxide.

This is the reaction in the manufacture of quicklime.

See also: Carbonate-silicate cycle

Detection of carbonates

As a preliminary test, solid material to be analyzed can be mixed with concentrated hydrochloric acid. A gas evolution (CO2) indicates carbonate. When performing this test in a test tube, the flame of an introduced burning chip should be smothered. The resulting gas can also be identified by a precipitation reaction: Barium hydroxide solution (baryta water) is placed in a fermentation tube (carbonate tube) that is inserted into a pierced stopper at one end. The sample is placed in the test tube, an acid (hydrochloric acid, sulfuric acid) is poured over it and it is quickly closed with the stopper on the carbonate tube. The mixture is then heated in the test tube in a water bath, and if carbonates are present in the sample, the barium hydroxide solution is gradually clouded because white barium carbonate (BaCO3) flocculates.

The quantitative detection of low carbonate concentrations in water is carried out together with the determination of hydrogen carbonates, often by a titration with hydrochloric acid ("SBV"): if the measured material has a pH value above 8.3 at the beginning, the consumption of hydrochloric acid is up to pH 8.3 equivalent to the carbonate concentration. The further acid consumption until a pH of 4.3 is reached corresponds to the sum of carbonate and hydrogen carbonate. For water with pH values ​​below 8.3, titrate only up to 4.3 (only hydrogen carbonate) and calculate the original, then very low proportion of carbonate ions from the dissociation equilibrium of the carbonic acid.

Carbonates and hydrogen carbonates can also be determined by ion HPLC or by capillary electrophoresis. In both cases, a "total carbonate" is determined and the components carbonate, hydrogen carbonate and "free carbonic acid" are calculated from the dissociation equilibrium of the carbonic acid, taking into account pH, ionic strength and temperature.

Basic reaction

The carbonate ions react with water to form hydrogen carbonate and hydroxide ions. It is the one basic reaction.

Occurrence and use of calcium carbonate ("lime")

Calcium carbonate occurs in pure form as limestone, marble and chalk. It is an essential organic mineral and is common in all living things. The shells and shells of mussels and snails as well as the coral sticks are made of this mineral.

It is used:

  • Construction: As aggregate cement, in concrete and is used for the construction of buildings, as stone and natural stone
  • Technology: So-called Nicol prisms for optical devices (polarization apparatus) are made from Icelandic double spar as whitewash chalk.
  • Art: Marble is one of the most important means of expression in sculpture and stonemasonry, calcium carbonate also serves as a white pigment
  • Agriculture: Found in fertilizers.
  • chalk (Blackboard chalk) also consists of calcium carbonate.
  • medicine: Calcium carbonate can be used as an acid-binding agent (antacid) against over-acidification of the stomach; however, the inevitably developing carbon dioxide (see formula above) leads to stomach gas and subsequent belching; in extreme cases, an existing gastric ulcer can break through. Through the CO2 and the carbonic acid formed and, through the neutralization of gastric juice, moreover, the hydrochloric acid secretion in the stomach is stimulated again. As a calcium carrier, the substance is important for bone formation.
  • Pharmaceutical technology: As an indifferent carrier, e.g. B. used in powders and ointments, furthermore as a coating aid.
  • Food technology: Food additive E 170, approved e.g. B. as a non-toxic coloring agent for coated tablets and for food decorations.
  • Cosmetics: Used externally as a mild, drying powder and in preparations for dental care.

Important carbonates

Organic carbonates

The covalent carbonates, which are formally derived from alcohols and carbonic acid, do not occur, of course. The simplest synthetically produced carbonic acid ester is dimethyl carbonate.

In recent years, organic carbonates in polymeric form have gained greater economic importance than polycarbonates, particularly in the production of CDs and DVDs.

Categories: Fabric Group | Carbonate