What is the density of citric acid

Citric acid

Structural formula
Surname Citric acid
other names
  • citric acid
  • 2-hydroxypropane-1,2,3-tricarboxylic acid (IUPAC)
  • 3-carboxy-3-hydroxyglutaric acid
  • 2-hydroxy-1,2,3-propane tricarboxylic acid
  • 3-carboxy-3-hydroxypentanedioic acid
  • E 330
  • INCI name: Citric Acid
Molecular formula C.6H8O7
CAS number 77-92-9 (anhydrous)
5949-29-1 (monohydrate)
ATC code


Brief description

colorless, odorless solid[1]

Molar mass 192.13 g mol−1
Physical state



1.665 g cm−3 (18 ° C)[1]

Melting point

153 ° C (anhydrous)[2]
100 ° C (monohydrate)[2]

boiling point

Decomposition: from 175 ° C[1]


Easily soluble in water: 750 g · l−1 (20 ° C)[1]

safety instructions

3000 mg kg−1 (Rat, oral) [1]

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions.
Template: Infobox chemical / molecular formula search available

Citric acid[6] (also citric acid[7]) is a colorless, water-soluble carboxylic acid that is one of the fruit acids. In addition to the anhydrous variant, there is also citric acid monohydrate (C.6H8O7 · H2O), which contains one molecule of water of crystallization per molecule of citric acid. The salts and esters of citric acid are the Citrates.


Carl Wilhelm Scheele isolated citric acid from lemon juice for the first time in 1784 - hence the name. However, citric acid was probably already 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 living things, including humans. This cycle also provides the basic molecular structures for the construction of most of the amino acids.

Extraction and presentation

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Manufacture from citrus fruits

Citric acid was obtained from citrus fruits using the original method: concentrated ammonia solution is added to lemon juice, thickened and filtered. The easily soluble ammonium citrate is converted into less soluble calcium citrate by precipitation with calcium chloride. The solution is filtered again and the filter cake is dissolved in 25% sulfuric acid, with calcium sulphate (gypsum) that is even more difficult to dissolve precipitating. After another filtration, a citric acid solution is obtained. The pure citric acid is obtained by crystallization.

Biotechnical manufacturing

Citric acid is produced industrially by fermenting sugary raw materials such as molasses and corn. Be used for fermentation Aspergillus nigerStrains used. In the USA and China in particular, transgenic variants of the mold are often used, but this is not permitted in Europe. In the production of citric acid, three conditions in particular must be met:

  1. High glucose and oxygen content in the nutrient medium
  2. 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 mushrooms 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 unwanted foreign organisms is low at such a low pH value.
  3. Low Fe2+-Concentration (<5 mg / l). As a result, aconitase lacks the cofactor. The fairy2+-Ions are bound by adding potassium hexacyanidoferrate (III). Mn2+-Concentrations (<2 µg / l) also lead to high citrate yields.

The purity and tolerance of citric acid on the market show great differences. Various Aspergillus Niger strains produce mycotoxins (fungal toxins) under certain growth conditions. The production of these substances does not take place under controlled conditions. There are no mold residues in highly purified citric acid.


In the anhydrous state, citric acid forms rhombic crystals that taste slightly sour. Ingested 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 (LD50 in rats: 3 g / kg).

Citric acid is counted among 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.

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 pKs1 = 3.13, pKs2 = 4.76 and pKs3 = 6.4. The partially or completely dissociated acid residue of citric acid is called citrate.

Chemical properties

Citric acid undergoes many reactions typical of carboxylic acids, including

Citric acid can be oxidized with various oxidizing agents (e.g. peroxides or hypochlorites). Depending on the reaction conditions can β-Ketoglutaric acid, oxalic acid, carbon dioxide and water are formed.


Citric acid not only has a lime-dissolving effect due to its acidic effect, but also by forming a calcium complex. It is often used in cleaning products to avoid the unpleasant smell of vinegar cleaners. However, the complexes are not heat-resistant and disintegrate into relatively poorly soluble calcium citrate when heated. Citric acid can be used, for example, to decalcify kettles, immersion heaters, taps, shower heads, dishwashers and washing machines.

Citric acid and its salts are used to preserve and acidify food, for example in beverages. It is contained in effervescent powder and sticks together with sodium hydrogen carbonate. Citric acid is particularly used in lemonades and iced tea, but it also occurs naturally in fruit juices. Citric acid is in the EU as a food additive under the number E 330 Unlimited permitted in most foods. Exceptions are, for example, chocolate products and fruit juices, for which there is only limited approval, and some foods, such as honey, milk and butter, for which there is no approval.[8] Since it also 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.

Citric acid and citrates prevent blood clotting. That is why donated blood is preserved in bags containing 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 to prevent dangerous clots from forming.

Citric acid is also used as a rinsing solution for root canal treatments in dentistry.

Further areas of application for the salts of citric acid:

  • Citric acid is used as a water softener and alternative fabric softener.
  • Citric acid is used to passivate stainless steel. In this process, the free iron components are removed from the stainless steel surface. This has a positive influence on the chromium-iron ratio, which leads to an improvement in the passive layer and thus to an improvement in the corrosion protection of stainless steel.
  • Calcium citrate-containing preparations are used as dietary supplements in the wellness industry.
  • Similar preparations are also given to dogs to strengthen their teeth and bones.
  • Magnesium citrate is said to increase the magnesium level in the body as a preparation or as a dietary supplement and thus, for example, prevent leg cramps and increase general performance.
  • Citric acid is also used as a catalyst in biochar production through hydrothermal carbonization.[9]
  • The salts trisodium citrate and trilithium citrate are used in construction chemistry - depending on the amount added - as a retarder or accelerator for the hardening of cementitious compounds.
  • Many basic active pharmaceutical ingredients are given as citrates (for example, sildenafil citrate in Viagra).
  • Citric acid is used to adjust the pH of cosmetics, e.g. B. skin care lotion or cream used.[10][11]
  • Citric acid is also used in high doses as a rust remover.

Biological importance

Citrate is one of the most important inhibitors of urinary stone formation. Increased citrate levels in the urine therefore reduce the risk of urinary stones forming. The citrate excreted in the urine comes from the metabolism (citric acid cycle) on the one hand, and is absorbed with food on the other.

Individual evidence

  1. 1,01,11,21,31,41,51,61,7Entry to citric acid in the GESTIS substance database of the IFA, accessed on December 30, 2012 (JavaScript required).
  2. 2,02,1RÖMPP Online, Version 3.28, "Citric Acid", accessed on November 26, 2012.
  3. 3,03,13,2Christian Beyer: Quantitative inorganic analysis: a companion for theory and practice. Springer Science & Business, 1996, ISBN 3-528-06779-9, p. 96 (restricted preview in the Google book search).
  4. 4,04,14,2data sheet Citric acid from Sigma-Aldrich, accessed November 30, 2009.
  5. ↑ Since December 1, 2012, only GHS hazardous substance labeling has been permitted for substances. The R-phrases of this substance may still be used to classify preparations until June 1, 2015, after which the EU hazardous substance labeling is of purely historical interest.
  6. ↑ Karl-Friedrich Arndt, Axel Satzger, Technical University of Dresden (ed.): Langenscheidt specialist dictionary chemistry and chemical engineering English German-English. Langenscheidt, Berlin 2009, ISBN 9783861174769 (online specialist dictionaries from Langenscheidt Chemie: search for citric acid and citric acid).
  7. ↑ Duden | Citric acid | Spelling, meaning, definition
  8. ↑ Annex 4 (to Section 5, Paragraph 1 and Section 7) Limited additives.
  9. ↑ Peter Brandt: The "hydrothermal carbonization": a remarkable possibility to prevent the formation of CO2 to minimize or even to avoid?, in: Journal for consumer protection and food safety, 2009, 4 (2), pp. 151-154; doi: 10.1007 / s00003-009-0472-7.
  10. ↑ Vollrath, Hopp: Citric acid: example of a biotechnological product, in: Basics of the life sciences, Wiley-VCH Verlag, Weinheim 2000, ISBN 978-3-527-29560-9.
  11. ↑ Professor Blume's media offer: Citric acid in cosmetics.


  • Rolf D. Schmid: Pocket atlas of biotechnology and genetic engineering, 2nd edition, Wiley-VCH Verlag, Weinheim 2006, ISBN 3-527-31310-9.

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