|Molecular formula||C 6 H 8 O 7|
colorless, odorless solid
|External identifiers / databases|
Decomposition: from 175 ° C
|pK s value||
|As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .|
Citric acid (alternative spelling citric acid ) is a colorless, water-soluble carboxylic acid that is one of the tricarboxylic acids and one of the fruit acids . In addition to the anhydrous variant, there is also citric acid monohydrate (C 6 H 8 O 7 · H 2 O), which contains one molecule of crystal water per molecule of citric acid . The salts and esters of citric acid are the citrates . A constitutional isomer of citric acid is isocitric acid .
Carl Wilhelm Scheele isolated citric acid from lemon juice for the first time in 1784 - hence the name. However, citric acid should have been known to the first alchemists , albeit under a different name. The Arab alchemist Jabir ibn Hayyān (Geber) is said to have discovered citric acid as early as the 9th century .
Citric acid is one of the most widespread acids in the plant kingdom and occurs as a metabolic product in all organisms. For example, lemon juice contains 5-7% citric acid. But it also occurs in apples, pears, sour cherries, raspberries, blackberries, currants, conifers, mushrooms, tobacco leaves, in wine and even in milk.
Citric acid is widespread because it (eponymous) occurs as an intermediate in the citric acid cycle (also tricarboxylic acid cycle, Krebs cycle). This process plays a key role in the carbohydrate and fatty acid metabolism of all oxygen-consuming organisms, including humans. This cycle also provides the basic molecular structures for the construction of most amino acids .
Extraction and presentation
Manufactured from citrus fruits
According to the original process, citric acid was obtained from citrus fruits . Lemon juice was mixed with concentrated ammonia solution, concentrated and filtered. The easily soluble ammonium citrate was converted into less soluble calcium citrate by precipitation with calcium chloride . The solution was filtered again and the filter cake was dissolved in 25% strength sulfuric acid, calcium sulfate ( gypsum ) which is even more difficult to dissolve precipitated. After another filtration, a citric acid solution is obtained. The pure citric acid is obtained by crystallization.
Citric acid is produced industrially by fermenting sugary raw materials such as molasses and corn. Aspergillus niger strains are used for fermentation . Especially in the USA and China, transgenic variants of the mold are often used; in Europe this is not permitted. In citric acid production, three conditions in particular must be met:
- High glucose and oxygen content in the nutrient medium
- Low pH (pH <3). On the one hand, this inhibits the secondary enzyme of citrate synthase in the citric acid cycle , aconitase . Such a low pH value is far from the pH optimum of the enzyme and greatly reduces its activity . This means that the fungi only metabolize the citric acid to a small extent . On the other hand, the outer membrane of the fungal cells becomes unstable and the citric acid is released into the external medium. In addition, the risk of contamination from undesired foreign organisms is low at such a low pH value.
- Low Fe 2+ concentration (<5 mg / l). As a result, aconitase lacks the cofactor . The Fe 2+ ions are bound by adding potassium hexacyanidoferrate (III) . Mn 2+ concentrations (<2 µg / l) also lead to high citrate yields.
The purity and tolerance of citric acid on the market show great differences. Different Aspergillus niger strains produce mycotoxins under certain growth conditions . The production of these substances does not take place under controlled conditions. There are no residues of mold in highly purified citric acid.
Citric acid forms rhombic crystals in the anhydrous state , which taste slightly sour. An aqueous solution of citric acid conducts the electrical current, as the carboxy groups split off protons and thus mobile charge carriers ( ions ) are present in the solution. The acid dissociation constants of citric acid are pK s 1 = 3.13, pK s 2 = 4.76 and pK s 3 = 6.4. The partially or completely dissociated acid residue of citric acid is called citrate. For the solubility in water, widely differing values are given in the literature.
Citric acid is one of the organic tricarboxylic acids due to its three carboxy groups (-COOH) . In addition, the hydroxyl group (-OH) at position 3 of the carbon backbone identifies it as a hydroxycarboxylic acid . Citric acid is a chelator for metal ions .
Citric acid undergoes many reactions typical of carboxylic acids:
- Deprotonation / salt formation
- Substitution reactions on the carboxyl carbon :
Citric acid can be oxidized with various oxidizing agents ( e.g. peroxides or hypochlorites ). Depending on the reaction conditions it can β -ketoglutaric acid , oxalic acid , carbon dioxide and water.
When taken in small amounts, citric acid indirectly promotes bone growth because it promotes the absorption of calcium . However, in larger quantities it has a toxic effect ( LD 50 in rats: 3 g / kg).
Citric acid is often found in detergent detergents because it is odorless. Due to the acidic effect, it automatically dissolves calcium carbonate into the water-soluble calcium dicitrato complex, a typical chelate complex [Ca (Cit) 2 ] 4− . The complex disintegrates as the temperature and pH increase, and water-insoluble calcium citrate Ca 3 (Cit) 2 precipitates . Calcium citrate dissolves again if there is an excess of citric acid. Due to the risk of clogging, it is not recommended to use citric acid solution as a decalcifier for heated, pump-operated piping systems such as coffee machines or heat exchangers.
For dissolving lime, iron and manganese compounds that are in the clogging of wells form, citric acid together with a small addition of ascorbic acid (alternatively, as a reducing agent sodium dithionite ) are used.
Citric acid and its salts are used for preservation and also for homogenization as melting salt and as acidulants or acid regulators of foods, for example in beverages. It is contained in effervescent powder and sticks together with sodium hydrogen carbonate . As a natural component of most fruits, it is contained in fruit juices ( fruit acids ). Citric acid has unlimited approval as a food additive in most foods in the EU under the number E 330 . Exceptions are chocolate products and fruit juices, for which there is only limited authorization, and some foods, such as honey, milk and butter, for which there is no authorization. Since it acts as a peptizer , it is used to produce stable suspensions . In fish dishes, it can convert biogenic amines into non-volatile salts and thus reduce the fish odor.
The multiple use of citric acid in food has been criticized because as a complexing agent it dissolves aluminum and increases its absorption. Citric acid dissolves calcium and other minerals from the tooth enamel , which favors the development of tooth decay or tooth erosion .
Citric acid and citrates prevent blood clotting. Therefore, donated blood is preserved in bags that contain a citrate buffer solution . For coagulation analyzes, citrated blood is used , in which blood is mixed with citrate in a ratio of 9: 1 (9 parts blood + 1 part 0.11 mol / l sodium citrate ). A special use is in cell separators. Blood is taken from a vein , the desired blood components ( e.g. platelets ) are separated in the device , and the remaining blood is returned to the vein. Citrate buffer is added to the blood in the device so that it does not form dangerous clots .
Further areas of application for citric acid:
- Citric acid is used as a water softener and alternative fabric softener.
- Citric acid is the passivation of stainless steel used. In this process, the free iron components are released from the surface. This influences the chromium-iron ratio, which leads to an improvement in the passive layer and thus to an improvement in the corrosion protection.
- Citric acid is used to adjust the pH of cosmetics, e.g. B. skin care lotion or cream used.
- Citric acid also serves as a rust remover in high doses .
- Citric acid is used as a catalyst in the production of biochar through hydrothermal carbonization .
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In an article by the so-called "Zentrum der Gesundheit" the following sources are mentioned:
- High levels of citric acid in confectionery and beverages increase the risk of tooth damage , opinion of the Federal Office for Risk Assessment dated February 24, 2005.
- C. Swain, GB Chainy: Effects of aluminum sulphate and citric acid ingestion on lipid peroxidation and on activities of superoxide dismutase and catalase in cerebral hemisphere and liver of developing young chicks. In: Molecular and cellular biochemistry . Volume 187, Number 1-2, October 1998, pp. 163-172, PMID 9788754 .
- P. Slanina, W. Frech, LG Ekström, L. Lööf, S. Slorach, A. Cedergren: Dietary citric acid enhances absorption of aluminum in antacids. In: Clinical chemistry . Volume 32, Number 3, March 1986, pp. 539-541, PMID 3948402 .
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- Peter Brandt: The "hydrothermal carbonization": a remarkable possibility to minimize or even avoid the formation of CO 2 ? , in: Journal for Consumer Protection and Food Safety , 2009, 4 (2), pp. 151–154; doi: 10.1007 / s00003-009-0472-7 .