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Citric acid
Identifiers
CAS number 77-92-9 Yes check.svgY
PubChem 311
ChemSpider 305
SMILES
InChI
InChI key KRKNYBCHXYNGOX-UHFFFAOYAM
Properties
Molecular formula C6H8O7
Molar mass 192.124 g/mol (anhydrous)
210.14 g/mol (monohydrate)
Appearance crystalline white solid
Density 1.665 g/cm3
Melting point

153 °C

Boiling point

decomposes at 175 °C

Solubility in water 133 g/100 ml (22°C)
Solubility in THF, ethanol, methanol anhydrous: THF 1.80 M, ethanol 1.6 M, methanol 3.08 M [1]
monohydrate: THF 1.52 M, ethanol 1.78 M, methanol 2.27 M [2]
Acidity (pKa) pKa1=3.15
pKa2=4.77
pKa3=6.40
Hazards
Main hazards skin and eye irritant
Flash point  ?°C
Related compounds
Related compounds sodium citrate, calcium citrate
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Citric acid is a weak organic acid, and it is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of virtually all living things. It can also be used as an environmentally benign cleaning agent.

Citric acid exists in greater than trace amounts in a variety of fruits and vegetables, most notably citrus fruits. Lemons and limes have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits (about 47 g/L in the juices[3]). The concentrations of citric acid in citrus fruits range from 0.005 mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes. Within species these values vary depending on the cultivar and the circumstances in which the fruit was grown.

Contents

Properties

Citric acid crystal under polarized light, enlarged 200x

At room temperature, citric acid is a white crystalline powder. It can exist either in an anhydrous (water-free) form or as a monohydrate. The anhydrous form crystallizes from hot water, where as the monohydrate forms when citric acid is crystallized from cold water. The monohydrate can be converted to the anhydrous form by heating above 78 °C. Citric acid also dissolves in absolute (anhydrous) ethanol (76 parts of citric acid per 100 parts of ethanol) at 15 degrees Celsius.

In chemical structure, citric acid shares the properties of other carboxylic acids. When heated above 175°C, it decomposes through the loss of carbon dioxide and water. Citric acid leaves a white crystalline precipitate.

Citric acid is a slightly stronger acid than typical carboxylic acids because the anion can be stabilised by intramolecular hydrogen-bonding from other protic groups on citric acid.

Measurement

Citric acid has been used as an additive to soft drinks, beer, and seltzer, and occurs naturally in many juices. This causes a problem in measurement because the standard measuring technique for sugar is refractive index. The refractive index of sugar and citric acid is almost identical. For soft drinks and orange juice the best measure of sweetness is the sugar/acid ratio. Recently, the use of infrared sensors has allowed measurement of both Brix (sugar content) and acidity by detecting sugars and citric acid through their characteristic molecular vibrations; this gives an accurate assessment of a drink's sweetness.

History

Lemons, oranges, and other citrus fruits contain high concentrations of citric acid

The discovery of citric acid has been credited to the 8th century Persian alchemist Jabir Ibn Hayyan (Geber).[4][5][6] Medieval scholars in Europe were aware of the acidic nature of lemon and lime juices; such knowledge is recorded in the 13th century encyclopedia Speculum Majus (The Great Mirror), compiled by Vincent of Beauvais.[citation needed] Citric acid was first isolated in 1784 by the Swedish chemist Carl Wilhelm Scheele, who crystallized it from lemon juice.[7][8] Industrial-scale citric acid production began in 1890 based on the Italian citrus fruit industry.

In 1893, C. Wehmer discovered that Penicillium mold could produce citric acid from sugar. However, microbial production of citric acid did not become industrially important until World War I disrupted Italian citrus exports. In 1917, the American food chemist James Currie discovered that certain strains of the mold Aspergillus niger could be efficient citric acid producers, and Pfizer began industrial-level production using this technique two years later, followed by Citrique Belge in 1929.

In this production technique, which is still the major industrial route to citric acid used today, cultures of Aspergillus niger are fed on a sucrose or glucose-containing medium to produce citric acid. The source of sugar is corn steep liquor, molasses, hydrolyzed corn starch or other inexpensive sugary solutions.[9] After the mould is filtered out of the resulting solution, citric acid is isolated by precipitating it with lime (calcium hydroxide) to yield calcium citrate salt, from which citric acid is regenerated by treatment with sulfuric acid.

Krebs cycle

Citric acid is one of a series of compounds involved in the physiological oxidation of fats, proteins, and carbohydrates to carbon dioxide and water.

This series of chemical reactions is central to nearly all metabolic reactions, and is the source of two-thirds of the food-derived energy in higher organisms. Hans Adolf Krebs received the 1953 Nobel Prize in Physiology or Medicine for the discovery. The series of reactions is known by various names, including the citric acid cycle, the Krebs cycle, and the tricarboxylic acid cycle (or TCA cycle).

Uses

In 2007, world wide annual production stands at approximately 1,700,000 MT. More than 50% of this volume is being produced in China. More than 50% is being used as acidulent in beverages and some 20% in other food applications. 20% is being used for detergent applications and 10% for other non-food related applications like cosmetics, pharma and in the chemical industry.

Food additive

As a food additive, citric acid is used as a flavoring and preservative in food and beverages, especially soft drinks. It is denoted by E number E330. Citrate salts of various metals are used to deliver those minerals in a biologically available form in many dietary supplements. The buffering properties of citrates are used to control pH in household cleaners and pharmaceuticals. In the United States the purity requirements for citric acid as a food additive are defined by the Food Chemical Codex (FCC), which is published by the United States Pharmacopoeia (USP). Can be used for cherry packing. The market name is color guard.

Water softening

Citric acid's ability to chelate metals makes it useful in soaps and laundry detergents. By chelating the metals in hard water, it lets these cleaners produce foam and work better without need for water softening. In a similar manner, citric acid is used to regenerate the ion exchange materials used in water softeners by stripping off the accumulated metal ions as citrate complexes.The saturation point for citric acid and water is 59%

Others

Citric acid is used in biotechnology and the pharmaceutical industry to passivate high-purity process piping (in lieu of using nitric acid). Nitric acid is considered hazardous to dispose once used for this purpose, while citric acid is not.[citation needed]

Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. In industry it is used to dissolve rust from steel.[10]

Citric acid is commonly used as a buffer to increase the solubility of brown heroin. Single-use citric acid sachets have been used as an inducement to get heroin users to exchange their dirty needles for clean needles in an attempt to decrease the spread of AIDS and hepatitis[11]. Other acidifiers used for brown heroin are ascorbic acid, acetic acid, and lactic acid; in their absence, a drug user will often substitute lemon juice or vinegar.

Citric acid is one of the chemicals required for the synthesis of HMTD, a highly heat-, friction-, and shock-sensitive explosive similar to acetone peroxide. For this reason, purchases of large quantities of citric acid may rouse suspicion of potential terrorist activity.[citation needed]

Citric acid can be added to ice cream to keep fat globules separate, and can be added to recipes in place of fresh lemon juice as well. Citric acid is used along with sodium bicarbonate in a wide range of effervescent formulae, both for ingestion (e.g., powders and tablets) and for personal care (e.g., bath salts, bath bombs, and cleaning of grease).

Citric acid is commonly employed in wine production as a substitute or improver where fruits containing little or no natural acidity are used. It is mostly used for inexpensive wines due to its low cost of production.[12]

Citric acid can be used in shampoo to wash out wax and coloring from the hair. It is notably used in the product "Sun-in" for bleaching, but is generally not recommended due to the amount of damage it causes.[citation needed]

Citric acid is also used as a stop bath as part of the process for developing photographic film. The developer is normally alkaline, so a mild acid will neutralize it, increasing the effectiveness of the stop bath when compared to plain water.[13]

Citric acid is used as one of the active ingredients in the production of anti-viral tissues.[14]

Citric acid can be used in food coloring to balance the pH level of the normally basic dye.

Citric acid may be used as the main ripening agent in the first steps of making mozzarella cheese.[15]

Citric acid was the first successful eluant used for total ion-exchange separation of the lanthanides, during the Manhattan Project in the 1940s. In the 1950s, it was replaced by the far more efficient EDTA.

Citric acid is used as a good alternative to nitric acid in the process of stainless steel passivation (ie "Citrisurf")

Citric acid can be used as a delay to prompt natural cement. It can delay the very rapid setting time substantially.

Citric acid is one of several acids that is used by home brewers to modify brewing water for making beer.

Safety

Contact with dry citric acid or with concentrated solutions can result in skin and eye irritation, so protective clothing should be worn when handling these materials.[16]

Excessive consumption is capable of eroding the tooth enamel.[17]

Contact to the eyes can cause a burning sensation, and may cause blindness with prolonged exposure in extremely high concentrations (as anything with low enough pH will).

Sometimes a high concentration of citric acid can damage hair and bleach it.

The leaflet of Villejuif

The leaflet of Villejuif (also known as the flyer of Villejuif or the list of Villejuif) was a scientifically inaccurate rumour, passed via a leaflet or flyer, that caused mass panic in Europe in the 1980s as it included common unharmful chemical substances such as citric acid (E330) in a list of 10 dangerous carcinogens.

Compendial status

See also

Notes

  1. ^ Solubility of citric acid anhydrous in non-aqueous solvents
  2. ^ Solubility of citric acid monohydrate in non-aqueous solvents
  3. ^ Penniston KL, Nakada SY, Holmes RP, Assimos DG (2008). "Quantitative Assessment of Citric Acid in Lemon Juice, Lime Juice, and Commercially-Available Fruit Juice Products" (PDF). Journal of Endourology 22 (3): 567. doi:10.1089/end.2007.0304+. PMID 18290732. http://www.liebertonline.com/doi/pdfplus/10.1089/end.2007.0304. 
  4. ^ http://www.islamicspain.tv/Arts-and-Science/The-Culture-of-Al-Andalus/Chemistry.htm
  5. ^ http://muslimmedianetwork.com/mmn/?p=1553
  6. ^ http://journals.iucr.org/a/issues/2008/01/00/sc5012/index.html
  7. ^ http://web1.caryacademy.org/chemistry/rushin/StudentProjects/CompoundWebSites/2001/Citric%20Acid/history.htm
  8. ^ http://books.google.com/books?id=OUXOm8bdG1UC&pg=PA944&dq=how+citric+acid+was+discovered
  9. ^ Citric acid production by a novel Aspergillus niger isolate: II. Optimization of process parameters through statistical experimental designs. Bioresource Technology 98(18) 3470-3477. [1]
  10. ^ Use of ammoniated citric acid for the chemical cleaning of high pressure boilers.
  11. ^ Garden, J., Roberts, K., Taylor, A., and Robinson, D. (2003). "Evaluation of the Provision of Single Use Citric Acid Sachets to Injecting Drug Users" (pdf). Scottish Center for Infection and Environmental Health.
  12. ^ J. Robinson (ed) "The Oxford Companion to Wine" Third Edition pg 171 Oxford University Press 2006 ISBN 0198609906
  13. ^ Stopbaths
  14. ^ "Tissues that fight germs". CNN. 2004-07-14. http://money.cnn.com/2004/07/14/news/fortune500/kleenex/. Retrieved 2008-05-08. 
  15. ^ [2]
  16. ^ http://msds.chem.ox.ac.uk/CI/citric_acid_monohydrate.html
  17. ^ http://www.ukfoodguide.net/e330.htm
  18. ^ British Pharmacopoeia Commission Secretariat (2009). "Index, BP 2009". http://www.pharmacopoeia.co.uk/pdf/2009_index.pdf. Retrieved 4 February 2010. 
  19. ^ "Japanese Pharmacopoeia, Fifteenth Edition". 2006. http://jpdb.nihs.go.jp/jp15e/JP15.pdf. Retrieved 4 Februally 2010. 

References

External links


Citric acid
File:Zitronensäure - Citric
Identifiers
CAS number 77-92-9 Y
PubChem 311
ChemSpider 305
UNII XF417D3PSL Y
SMILES
InChI
InChI key
Properties
Molecular formula C6H8O7
Molar mass 192.124 g/mol (anhydrous)
210.14 g/mol (monohydrate)
Appearance crystalline white solid
Density 1.665 g/cm3
Melting point

153 °C

Boiling point

decomposes at 175 °C

Solubility in water 133 g/100 ml (22°C)
Solubility in THF, ethanol, methanol anhydrous: THF 1.80 M, ethanol 1.6 M, methanol 3.08 M [1]
monohydrate: THF 1.52 M, ethanol 1.78 M, methanol 2.27 M [2]
Acidity (pKa) pKa1=3.15
pKa2=4.77
pKa3=6.4
Hazards
Main hazards skin and eye irritant
Flash point  ?°C
Related compounds
Related compounds sodium citrate, calcium citrate
 Y (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Citric acid is a weak organic acid. It is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of virtually all living things. It can also be used as an environmentally benign cleaning agent.

Citric acid exists in greater than trace amounts in a variety of fruits and vegetables, most notably citrus fruits. Lemons and limes have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits (about 47 g/L in the juices[3]). The concentrations of citric acid in citrus fruits range from 0.005 mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes. Within species these values vary depending on the cultivar and the circumstances in which the fruit was grown.

Contents

Properties

At room temperature, citric acid is a white crystalline powder. It can exist either in an anhydrous (water-free) form or as a monohydrate. The anhydrous form crystallizes from hot water, where as the monohydrate forms when citric acid is crystallized from cold water. The monohydrate can be converted to the anhydrous form by heating above 78 °C. Citric acid also dissolves in absolute (anhydrous) ethanol (76 parts of citric acid per 100 parts of ethanol) at 15 degrees Celsius.

In chemical structure, citric acid shares the properties of other carboxylic acids. When heated above 175°C, it decomposes through the loss of carbon dioxide and water. Citric acid leaves a white crystalline precipitate.

Citric acid is a slightly stronger acid than typical carboxylic acids because the anion can be stabilised by intramolecular hydrogen-bonding from other protic groups on citric acid.

Measurement

Citric acid has been used as an additive to soft drinks, beer, and seltzer, and occurs naturally in many juices. This causes a problem in measurement because the standard measuring technique for sugar is refractive index. The refractive index of sugar and citric acid is almost identical. For soft drinks and orange juice the best measure of sweetness is the sugar/acid ratio. Recently, the use of infrared sensors has allowed measurement of both Brix (sugar content) and acidity by detecting sugars and citric acid through their characteristic molecular vibrations; this gives an accurate assessment of a drink's sweetness.

History

, oranges, limes, and other citrus fruits contain high concentrations of citric acid]] The discovery of citric acid has been credited to the 8th century Islamic alchemist Jabir Ibn Hayyan (Geber).[4][5][6] Medieval scholars in Europe were aware of the acidic nature of lemon and lime juices; such knowledge is recorded in the 13th century encyclopedia Speculum Maius (The Great Mirror), compiled by Vincent of Beauvais.[citation needed] Citric acid was first isolated in 1784 by the Swedish chemist Carl Wilhelm Scheele, who crystallized it from lemon juice.[7][8] Industrial-scale citric acid production began in 1890 based on the Italian citrus fruit industry.

In 1893, C. Wehmer discovered that Penicillium mold could produce citric acid from sugar. However, microbial production of citric acid did not become industrially important until World War I disrupted Italian citrus exports. In 1917, the American food chemist James Currie discovered that certain strains of the mold Aspergillus niger could be efficient citric acid producers, and Pfizer began industrial-level production using this technique two years later, followed by Citrique Belge in 1929.

In this production technique, which is still the major industrial route to citric acid used today, cultures of Aspergillus niger are fed on a sucrose or glucose-containing medium to produce citric acid. The source of sugar is corn steep liquor, molasses, hydrolyzed corn starch or other inexpensive sugary solutions.[9] After the mould is filtered out of the resulting solution, citric acid is isolated by precipitating it with lime (calcium hydroxide) to yield calcium citrate salt, from which citric acid is regenerated by treatment with sulfuric acid.

Krebs cycle

Citric acid is one of a series of compounds involved in the physiological oxidation of fats, proteins, and carbohydrates to carbon dioxide and water.

This series of chemical reactions is central to nearly all metabolic reactions, and is the source of two-thirds of the food-derived energy in higher organisms. Hans Adolf Krebs received the 1953 Nobel Prize in Physiology or Medicine for the discovery. The series of reactions is known by various names, including the citric acid cycle, the Krebs cycle, and the tricarboxylic acid cycle (or TCA cycle). The cycle.

Interactive pathway map

Click on genes, proteins and metabolites below to link to respective articles. [10]

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Citric acid cycle [edit]

Uses

In 2007, world wide annual production stands at approximately 1,600,000 tonnes.[1] More than 50% of this volume is being produced in China. More than 50% is being used as acidulent in beverages and some 20% in other food applications. 20% is being used for detergent applications and 10% for other non-food related applications like cosmetics, pharma and in the chemical industry.

Food additive

As a food additive, citric acid is used as a flavoring and preservative in food and beverages, especially soft drinks. It is denoted by E number E330. Citrate salts of various metals are used to deliver those minerals in a biologically available form in many dietary supplements. The buffering properties of citrates are used to control pH in household cleaners and pharmaceuticals. In the United States the purity requirements for citric acid as a food additive are defined by the Food Chemical Codex, which is published by the United States Pharmacopoeia (USP).

Water softening

Citric acid's ability to chelate metals makes it useful in soaps and laundry detergents. By chelating the metals in hard water, it lets these cleaners produce foam and work better without need for water softening. In a similar manner, citric acid is used to regenerate the ion exchange materials used in water softeners by stripping off the accumulated metal ions as citrate complexes.The saturation point for citric acid and water is 59%.

Others

Ash rocks and Citric acid is used in biotechnology and the pharmaceutical industry to passivate high-purity process piping (in lieu of using nitric acid). Nitric acid is considered hazardous to dispose once used for this purpose, while citric acid is not.[citation needed]

Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. In industry it is used to dissolve rust from steel.[2]

Citric acid is commonly used as a buffer to increase the solubility of brown heroin. Single-use citric acid sachets have been used as an inducement to get heroin users to exchange their dirty needles for clean needles in an attempt to decrease the spread of AIDS and hepatitis[3]. Other acidifiers used for brown heroin are ascorbic acid, acetic acid, and lactic acid; in their absence, a drug user will often substitute lemon juice or vinegar.

Citric acid is one of the chemicals required for the synthesis of HMTD, a highly heat-, friction-, and shock-sensitive explosive similar to acetone peroxide. For this reason, purchases of large quantities of citric acid may rouse suspicion of potential terrorist activity.[citation needed]

Citric acid can be added to ice cream to keep fat globules separate, and can be added to recipes in place of fresh lemon juice as well. Citric acid is used along with sodium bicarbonate in a wide range of effervescent formulae, both for ingestion (e.g., powders and tablets) and for personal care (e.g., bath salts, bath bombs, and cleaning of grease).

Citric acid is commonly employed in wine production as a substitute or improver where fruits containing little or no natural acidity are used. It is mostly used for inexpensive wines due to its low cost of production.[4]

Citric acid can be used in shampoo to wash out wax and coloring from the hair. It is notably used in the product "Sun-in" for bleaching, but is generally not recommended due to the amount of damage it causes.[citation needed]

Citric acid is also used as a stop bath as part of the process for developing photographic film. The developer is normally alkaline, so a mild acid will neutralize it, increasing the effectiveness of the stop bath when compared to plain water.[5]

Citric acid is used as one of the active ingredients in the production of anti-viral tissues.[6]

Citric acid can be used in food coloring to balance the pH level of the normally basic dye.

Citric acid is used as an odorless alternative to white vinegar for home dyeing with acid dyes.

Citric acid may be used as the main ripening agent in the first steps of making mozzarella cheese.[7]

Citric acid was the first successful eluant used for total ion-exchange separation of the lanthanides, during the Manhattan Project in the 1940s. In the 1950s, it was replaced by the far more efficient EDTA.

Citric acid is used as a successful alternative to nitric acid in the process of stainless steel passivation (ie "Citrisurf")

Citric acid can be used as a delay to prompt natural cement. It can delay the very rapid setting time substantially.

Citric acid is one of several acids that is used by home brewers to modify brewing water for making beer.

Safety

Contact with dry citric acid or with concentrated solutions can result in skin and eye irritation, so protective clothing should be worn when handling these materials.[8]

Excessive consumption is capable of eroding the tooth enamel.[9]

Contact to the eyes can cause a burning sensation, and may cause blindness with prolonged exposure in extremely high concentrations (as anything with low enough pH will).

Sometimes a high concentration of citric acid can damage hair and bleach it.

The leaflet of Villejuif

The leaflet of Villejuif (also known as the flyer of Villejuif or the list of Villejuif) was a scientifically inaccurate rumor, passed via a leaflet or flyer, that caused mass panic in Europe in the 1980s as it included common unharmful chemical substances such as citric acid (E330) in a list of 10 dangerous carcinogens.

Compendial status

See also

Notes

  1. ^ "Citric Acid Production". http://www.ncbi.nlm.nih.gov/pubmed/17875481. 
  2. ^ Use of ammoniated citric acid for the chemical cleaning of high pressure boilers.
  3. ^ Garden, J., Roberts, K., Taylor, A., and Robinson, D. (2003). "Evaluation of the Provision of Single Use Citric Acid Sachets to Injecting Drug Users" (pdf). Scottish Center for Infection and Environmental Health.
  4. ^ J. Robinson, ed (2006). "The Oxford Companion to Wine" (Third ed.). Oxford University Press. p. 171. ISBN 0198609906. 
  5. ^ Stopbaths[dead link]
  6. ^ "Tissues that fight germs". CNN. 2004-07-14. http://money.cnn.com/2004/07/14/news/fortune500/kleenex/. Retrieved 2008-05-08. 
  7. ^ "Mozzarella Cheese Recipe". New England Cheesemaking Supply Company. p. 21. http://www.cheesemaking.com/store/pg/21.html. Retrieved 4 June 2010. 
  8. ^ Safety (MSDS) data for citric acid, monohydrate
  9. ^ E330 Citric acid
  10. ^ British Pharmacopoeia Commission Secretariat (2009). "Index, BP 2009". http://www.pharmacopoeia.co.uk/pdf/2009_index.pdf. Retrieved 4 February 2010. 
  11. ^ "Japanese Pharmacopoeia, Fifteenth Edition". 2006. http://jpdb.nihs.go.jp/jp15e/JP15.pdf. Retrieved 4 Februally 2010. 

References

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

'CITRIC ACID,' Acidum citricum, or Oxytricarballylic Acid, C 3 H 4 (Oh) (Co. Oh) 3, a tetrahydroxytribasic acid, first obtained in the solid state by Karl Wilhelm Scheele, in 1784, from the juice of lemons. It is present also in oranges, citrons, currants, gooseberries and many other fruits, and in several bulbs and tubers. It is made on a large scale from lime or lemon juice, and also by the fermentation of glucose under the influence of Citromycetes pfefferianus, C. glaber and other ferments. Lemon juice is fermented for some time to free it from mucilage, then boiled 2 Cf. the name Kathian in a Ramessid list of cities of Cyprus, Oberhummer, Die Inset Cypern (Munich, 1903), p. 4.

3 Gen. x. 4; Num. xxiv. 24; Is. xxiii. I, 12; Jer. ii. 10; Ezek. xxvii. 6.

4 Dan. xi. 30; 1 Macc. i. 1; viii. 5.

5 Schrader, "Die Sargonstele des Berliner Museums," in Abh. d. k. Preuss. Akad. Wiss. (1881); Zur Geogr. d. assyr. Reiches (Berlin, 1 89 0), pp. 337-344.

FIG. 5. - Asiatic Cithara in transition (or rotta). From a fresco at Beni-Hasan (c. 1700 B.C.).

FIG. 6. - Roman Cithara in transition, of the Lycian Apollo (Rome Mus. Capit.).

and filtered, and neutralized with powdered chalk and a little milk of lime; the precipitate of calcium citrate so obtained is decomposed with dilute sulphuric acid, the solution filtered, evaporated to remove calcium sulphate and concentrated, preferably in vacuum pans. The acid is thus obtained in colourless rhombic prisms of the composition C 6 H 8 0 7 +H 2 0. Crystals of a different form are deposited from a strong boiling solution of the acid. About 20 gallons of lemon juice should yield about 1 0 lb of crystallized citric acid. The acid may also be prepared from the juice of unripe gooseberries. Calcium citrate must be manufactured with care to avoid an excess of chalk or lime, which would precipitate constituents of the juice that cause the fermentation of the citrate and the production of calcium acetate and butyrate.

The synthesis of citric acid was accomplished by L. E. Grimaux and P. Adam in 1881. Glycerin when treated with hydrochloric acid gives propenyl dichlorhydrin, which may be oxidized to s-dichloracetone. This compound combines with hydrocyanic acid to form a nitrile which hydrolyses to dichlorhydroxy iso-butyric acid. Potassium cyanide reacts with this acid to form the corresponding dinitrile, which is converted by hydrochloric acid into citric acid. This series of operations proves the constitution of the acid. A. Haller and C. A. Held synthesized the acid from ethyl chlor-acetoacetate (from chlorine and acetoacetic ester) by heating with potassium cyanide and saponifying the resulting nitrile. The acetone dicarboxylic acid, CO(CH 2 CO 2 H) 2, so obtained combines with hydrocyanic acid, and this product yields citric acid on hydrolysis.

Citric acid has an agreeable sour taste. It is soluble in 4ths of its weight of cold, and in half its weight of boiling water, and dissolves in alcohol, but not in ether. At 150° C. it melts, and on the continued application of heat boils, giving off its water of crystallization. At 175° C. it is resolved into water and aconitic acid, C 6 H 6 0 6, a substance found in Equisetum fluviatile, monkshood and other plants. A higher temperature decomposes this body into carbon dioxide and itaconic acid, C 5 H 6 0 4, which, again, by the expulsion of a molecule of water, yields citraconic anhydride, C 5 H 4 0 3. Citric acid digested at a temperature below 40° C. with concentrated sulphuric acid gives off carbon monoxide and forms acetone dicarboxylic acid. With fused potash it forms potassium oxalate and acetate. It is a strong acid, and dissolved in water decomposes carbonates and attacks iron and zinc.

The citrates are a numerous class of salts, the most soluble of which are those of the alkaline metals; the citrates of the alkaline earth metals are insoluble. Citric acid, being tribasic, forms either acid monometallic, acid dimetallic or neutral trimetallic salts; thus, mono-, diand tri-potassium and sodium citrates are known. On warming citric acid with an excess of lime-water a precipitate of calcium citrate is obtained which is redissolved as the liquid cools.

The impurities occasionally present in commercial citric acid are salts of potassium and sodium, traces of iron, lead and copper derived from the vessels used for its evaporation and crystallization, and free sulphuric, tartaric and even oxalic acid. Tartaric acid, which is sometimes present in large quantities as an adulterant in commercial citric acid, may be detected in the presence of the latter, by the production of a precipitate of acid potassium tartrate when potassium acetate is added to a cold solution. Another mode of separating the two acids is to convert them into calcium salts, which are then treated with a perfectly neutral solution of cupric chloride, soluble cupric citrate and calcium chloride being formed, while cupric tartrate remains undissolved. Citric acid is also distinguished from tartaric acid by the fact that an ammonia solution of silver tartrate produces a brilliant silver mirror when boiled, whereas silver citrate is reduced only after prolonged ebullition.

Citric acid is used in calico printing, also in the preparation of effervescing draughts, as a refrigerant and sialogogue, and occasionally as an antiscorbutic, instead of fresh lemon juice. In the form of lime juice it has long been known as an antidote for scurvy. Several of the citrates are much employed as medicines, the most important being the scale preparations of iron. Of these iron and ammonium citrate is much used as a haematinic, and as it has hardly any tendency to cause gastric irritation or constipation it can be taken when the ordinary forms of iron are inadmissible. Iron and quinine citrate is used as a bitter stomachic and tonic. In the blood citrates are oxidized into carbonates; they therefore act as remote alkalis, increasing the alkalinity of the blood and thereby the general rate of chemical change within the body (see Acetic Acid).


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File:Zitronensäure 200fach
Citric acid crystals, magnified about 200 times, seen through a polarizing filter.

Citric acid is a weak organic acid. It can be found in citrus fruits. It acts like a preservative. It is also used to add a sour (acidic) taste to foods and soft drinks. In the European Union it is known as E 330, as a food additive.

Carl Wilhelm Scheele was the first who could extract citric acid from lemons, in 1782. The substance was probably known to alchemists, perhaps with a different name. The Arabian alchemist Geber is said to have discovered citric acid in the 9th century. Citric Acid contains 6 Carbon atoms, 8 Hydrogen atoms and 7 Oxygen atoms. Its chemical formula is C6H8O7.

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