View source for Citric acid

{{distinguish|Vitamin C}}
{{Merge from |Citrate |date=June 2010}}
{{Chembox
| Watchedfields = changed
| verifiedrevid = 329233107
|   Name = Citric acid
|   ImageFile = Zitronensäure - Citric acid.svg
|   ImageSize = 150px
|   ImageFile1 = Citric-acid-3D-balls.png
|   ImageSize1 = 150px
|   ImageName = Citric acid
|   IUPACName = 3-carboxy-3-hydroxypentanedioic acid
|   OtherNames = 2-hydroxypropane- 1,2,3- tricarboxylic acid <br> 3-hydroxypentanedioic acid-3-carboxylic acid<br />
Hydrogen citrate
| Section1 = {{Chembox Identifiers
|   PubChem = 311
|   InChI = 1/C6H8O7/c7-3(8)1-6(13,5(11)12)2-4(9)10/h13H,1-2H2,(H,7,8)(H,9,10)(H,11,12)
| SMILES = O=C(O)CC(O)(C(=O)O)CC(=O)O
| InChIKey = KRKNYBCHXYNGOX-UHFFFAOYAM
| CASNo = 77-92-9
|    CASNo_Ref = {{cascite}}
|   ChemSpiderID = 305
|   RTECS = 
  }}
| Section2 = {{Chembox Properties
|   Formula = C<sub>6</sub>H<sub>8</sub>O<sub>7</sub>
|   MolarMass = 192.124 g/mol (anhydrous)<br />210.14 g/mol (monohydrate)
|   Appearance = crystalline white solid
|   Density = 1.665 g/cm<sup>3</sup>
|   Solubility = 133 g/100 ml (22°C) 
| SolubleOther = anhydrous: THF 1.80 M, ethanol 1.6 M, methanol 3.08 M <ref>[http://oru.edu/cccda/sl/solubility/allsolvents.php?solute=citric%20acid%20anhydrous Solubility of citric acid anhydrous in non-aqueous solvents]</ref><br />monohydrate: THF 1.52 M, ethanol 1.78 M, methanol 2.27 M <ref>[http://oru.edu/cccda/sl/solubility/allsolvents.php?solute=citric%20acid%20monohydrate Solubility of citric acid monohydrate in non-aqueous solvents]</ref>
| Solvent = [[THF]], [[ethanol]], [[methanol]]
|   MeltingPt = 153 °C
|   BoilingPt = decomposes at 175 °C
|   pKa = pK<sub>a1</sub>=3.15<br />pK<sub>a2</sub>=4.77<br />pK<sub>a3</sub>=6.4
  }}
| Section7 = {{Chembox Hazards
|   MainHazards = skin and eye irritant
|   FlashPt = ?°C
|   RPhrases =  
|   SPhrases = 
  }}
| Section8 = {{Chembox Related
|   OtherCpds = [[sodium citrate]], [[calcium citrate]]
  }}
}}

'''Citric acid''' is a weak [[organic chemistry|organic]] [[acid]]. It is a natural [[preservative]] and is also used to add an acidic, or [[Taste#Sourness|sour]], taste to foods and [[soft drink]]s. In [[biochemistry]], it is important as an intermediate in the [[citric acid cycle]] and therefore occurs in the [[metabolism]] of virtually all [[Life|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 [[fruit]]s and vegetables, most notably [[Citrus|citrus fruit]]s. [[Lemon]]s and [[lime (fruit)|limes]] have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits (about 47&nbsp;[[gram per litre|g/L]] in the juices<ref>{{cite journal | author=Penniston KL, Nakada SY, Holmes RP, Assimos DG | title=Quantitative Assessment of Citric Acid in Lemon Juice, Lime Juice, and Commercially-Available Fruit Juice Products | journal=Journal of Endourology  | volume=22 | issue=3 | year=2008  |url=http://www.liebertonline.com/doi/pdfplus/10.1089/end.2007.0304 | format = [[PDF]] | pmid=18290732  | pages = 567 | doi = 10.1089/end.2007.0304 | pmc=2637791}}</ref>). The concentrations of citric acid in citrus fruits range from 0.005&nbsp;[[molarity|mol/L]] for oranges and grapefruits to 0.30&nbsp;mol/L in lemons and limes.  Within species these values vary depending on the [[cultivar]] and the circumstances in which the fruit was grown.

==Properties==
[[Image:Zitronensäure im Mikroskop mit Polfilter besser.jpg|thumb|left|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 [[hydrate|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&nbsp;°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 acid]]s.  When heated above 175°C, it decomposes through the loss of [[carbon dioxide]] and [[water (molecule)|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 [[carbonated water|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==
[[Image:Lemon-edit1.jpg|thumb|left|[[Lemons]], [[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 [[alchemy|alchemist]] [[Abu Musa Jabir Ibn Hayyan|Jabir Ibn Hayyan]] (Geber).<ref name="geber1">http://www.islamicspain.tv/Arts-and-Science/The-Culture-of-Al-Andalus/Chemistry.htm</ref><ref name="geber2">{{cite news|url=http://muslimmedianetwork.com/mmn/?p=1553|title=Muslim Scientists and Thinkers:  Abu Musa Jabir bin Hayyan|last=Aslam|first=Syed|date=December 6, 2007|work=[[The Muslim Observer]]|accessdate=4 June 2010}}</ref><ref name="geber3">{{cite journal|last=Derewenda|first=Zygmunt S.|year=2007|title=On wine, chirality and crystallography|journal=Acta Crystallographica Section A, Foundations of Crystallography|volume=64|issue=1|pages=246–258|issn=0108-7673|pmid=18156689|doi=10.1107/S0108767307054293|url=http://journals.iucr.org/a/issues/2008/01/00/sc5012/index.html}}</ref><!--How? Did he isolate it? More details would be helpful--> 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|date=September 2008}}  Citric acid was first isolated in 1784 by the Swedish chemist [[Carl Wilhelm Scheele]], who crystallized it from lemon juice.<ref name="history">http://web1.caryacademy.org/chemistry/rushin/StudentProjects/CompoundWebSites/2001/Citric%20Acid/history.htm</ref><ref name="book1">{{cite book|last=Graham|first=Thomas|title=Elements of chemistry, including the applications of the science in the arts|url=http://books.google.com/?id=OUXOm8bdG1UC&pg=PA944|accessdate=4 June 2010|year=1842|publisher=Hippolyte Baillière, foreign bookseller to the Royal College of Surgeons, and to the Royal Society, 219, Regent Street.|page=944}}</ref>  Industrial-scale citric acid production began in 1890 based on the [[Italy|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 [[United States|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.<ref>Citric acid production by a novel ''Aspergillus niger'' isolate: II. Optimization of process parameters through statistical experimental designs. Bioresource Technology 98(18) 3470-3477. [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V24-4N3GNR2-1&_user=10&_coverDate=12%2F31%2F2007&_rdoc=9&_fmt=high&_orig=browse&_srch=doc-info%28%23toc%235692%232007%23999019981%23667118%23FLA%23display%23Volume%29&_cdi=5692&_sort=d&_docanchor=&_ct=34&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=2acf7e2b5cd851f834e0e7a6ccbc4997]</ref> After the mould is filtered out of the resulting solution, citric acid is isolated by [[Precipitation (chemistry)|precipitating]] it with lime ([[calcium hydroxide]]) to yield calcium citrate salt, from which citric acid is regenerated by treatment with [[sulfuric acid]].

== Krebs cycle ==
{{Main|Citric acid cycle}}
Citric acid is one of a series of [[Compound (chemistry)|compound]]s involved in the [[physiological]] [[oxidation]] of fats, [[protein]]s, and [[carbohydrate]]s to carbon dioxide and water.

This series of chemical reactions is central to nearly all [[metabolic]] [[chemical reaction|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'').

== Interactive pathway map ==
{{TCACycle_WP78|highlight=Citric_acid}}

== Uses ==

In 2007, world wide annual production stands at approximately 1,600,000 [[tonnes]].<ref> {{ cite web
|url = http://www.ncbi.nlm.nih.gov/pubmed/17875481
|title=Citric Acid Production
}}</ref>
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 [[beverage]]s, especially soft drinks. It is denoted by [[E number]] '''E330'''. Citrate salts of various [[metal]]s are used to deliver those minerals in a biologically available form in many [[dietary supplement]]s.  The [[Buffering agent|buffering]] properties of citrates are used to control [[pH]] in household cleaners and [[pharmaceutical]]s. 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 [[chelation|chelate]] metals makes it useful in [[soap]]s and laundry [[detergent]]s.  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 softener]]s 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|date=September 2008}}

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.<ref>[http://www.angelfire.com/mt/chemicalcleaning/webboiler.html Use of ammoniated citric acid for the chemical cleaning of high pressure boilers].</ref>

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 [[needle exchange|exchange]] their dirty needles for clean needles in an attempt to decrease the spread of [[AIDS]] and [[hepatitis]]<ref>Garden, J., Roberts, K., Taylor, A., and Robinson, D. (2003).  "Evaluation of the Provision of Single Use Citric Acid Sachets to Injecting Drug Users" [http://www.drugmisuse.isdscotland.org/eiu/pdfs/citric_acid_full.pdf (pdf)].  Scottish Center for Infection and Environmental Health.</ref>.  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 [[Hexamethylene triperoxide diamine|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 [[Terrorism|terrorist]] activity.{{Citation needed|date=September 2008}}

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 [[effervescence|effervescent]] formulae, both for ingestion (''e.g.'', powders and tablets) and for personal care (''e.g.'', [[bath salts]], [[bath bomb]]s, and cleaning of [[Petroleum|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.<ref name="Oxford pg 171">{{cite book|editor=J. Robinson|title="The Oxford Companion to Wine"|edition=Third|year=2006|publisher=Oxford University Press|isbn=0198609906|page=171}}</ref> 

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|date=September 2008}}

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.<ref name="Silverprint-stop">[http://www.silverprint.co.uk/chem4.html Stopbaths<!-- Bot generated title -->]{{Dead link|date=June 2010}}</ref>

Citric acid is used as one of the active ingredients in the production of anti-viral tissues.<ref>{{cite news | url = http://money.cnn.com/2004/07/14/news/fortune500/kleenex/ | title = Tissues that fight germs  | publisher = CNN | date = 2004-07-14 | accessdate = 2008-05-08}}</ref>

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 dye]]s.

Citric acid may be used as the main ripening agent in the first steps of making [[mozzarella cheese]].<ref name="New England Cheesemaking Supply Company">{{cite web|url=http://www.cheesemaking.com/store/pg/21.html|title=Mozzarella Cheese Recipe|publisher=New England Cheesemaking Supply Company|page=21|accessdate=4 June 2010}}</ref>

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]] [http://www.citrisurf.com/ (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 [[homebrewing|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.<ref>[http://msds.chem.ox.ac.uk/CI/citric_acid_monohydrate.html Safety (MSDS) data for citric acid, monohydrate]</ref>

Excessive consumption is capable of eroding the [[tooth enamel]].<ref name="ukfood">[http://www.ukfoodguide.net/e330.htm E330 Citric acid]</ref>

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 ===
{{Main|Villejuif leaflet}}

'''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 (pamphlet)|flyer]], that caused [[Mass hysteria|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 [[carcinogen]]s.

== Compendial status ==
* [[British Pharmacopoeia]] <ref name=ib29>{{cite web
  | last = British Pharmacopoeia Commission Secretariat
  | first = 
  | authorlink = 
  | coauthors = 
  | title = Index, BP 2009
  | work =
  | publisher = 
  | year = 2009
  | url = http://www.pharmacopoeia.co.uk/pdf/2009_index.pdf
  | format =
  | doi =
  | accessdate = 4 February 2010
  }}</ref>
* [[Japanese Pharmacopoeia]] <ref name=jp15>{{cite web
  | last = 
  | first = 
  | authorlink = 
  | coauthors = 
  | title = Japanese Pharmacopoeia, Fifteenth Edition
  | work =
  | publisher = 
  | year = 2006
  | url = http://jpdb.nihs.go.jp/jp15e/JP15.pdf
  | format =
  | doi =
  | accessdate = 4 Februally 2010
  }}</ref> {{Clarify|date=January 2010}}

== See also ==
*[[Citric acid intolerance]]
*The closely related acids [[isocitric acid]], [[aconitic acid]], and [[propane-1,2,3-tricarboxylic acid]] (tricarballylic acid, carballylic acid)
*[[Acids in wine]]

== Notes ==
{{reflist}}

== References ==
*[http://www.coastalscents.com/cfwebstore/index.cfm?fuseaction=feature.display&feature_id=217 Citric Acid MSDS Sheet]

== External links ==
{{Commons category|Citric acid}}
* {{PubChemLink|311}}
*[http://www.ecama.org  The European Citric Acid Manufacturers Association]
*[http://www.nextbio.com/b/home/home.nb?q=Citric+acid NextBio Citric Acid Entry]
*[http://www2.iq.usp.br/docente/gutz/Curtipot_.html Citric acid analysis - free spreadsheet for titration of acids and pH calculation]
* [http://www.kasel.at/kasel-chemicals/products-applications/products/citric-acid-2013-applications-key-industrial-uses Applications of Citric Acid]
* [http://www.mistral.ie/details.php?code=CITRIC MSDS sheet for Citric Acid]

{{DEFAULTSORT:Citric Acid}}
[[Category:Hydroxy acids]]
[[Category:Citric acid cycle compounds]]
[[Category:Household chemicals]]
[[Category:Chelating agents]]
[[Category:Food acidity regulators]]
[[Category:Photographic chemicals]]
[[Category:Tricarboxylic acids]]

[[ar:حمض الليمون]]
[[az:Limon turşusu]]
[[zh-min-nan:Lê-bóng-sng]]
[[bg:Лимонена киселина]]
[[ca:Àcid cítric]]
[[cs:Kyselina citronová]]
[[cy:Asid sitrig]]
[[da:Citronsyre]]
[[de:Citronensäure]]
[[et:Sidrunhape]]
[[el:Κιτρικό οξύ]]
[[es:Ácido cítrico]]
[[eo:Citra acido]]
[[eu:Azido zitriko]]
[[fa:اسید سیتریک]]
[[fr:Acide citrique]]
[[ga:Aigéad citreach]]
[[gl:Ácido cítrico]]
[[ko:시트르산]]
[[hi:सिट्रिक अम्ल]]
[[hr:Limunska kiselina]]
[[id:Asam sitrat]]
[[it:Acido citrico]]
[[he:חומצה ציטרית]]
[[lv:Citronskābe]]
[[lb:Zitrounesaier]]
[[hu:Citromsav]]
[[nl:Citroenzuur]]
[[ja:クエン酸]]
[[no:Sitronsyre]]
[[nn:Sitronsyre]]
[[pl:Kwas cytrynowy]]
[[pt:Ácido cítrico]]
[[ro:Acid citric]]
[[ru:Лимонная кислота]]
[[simple:Citric acid]]
[[sl:Citronska kislina]]
[[sr:Limunska kiselina]]
[[fi:Sitruunahappo]]
[[sv:Citronsyra]]
[[ta:சிட்ரிக் அமிலம்]]
[[tr:Sitrik asit]]
[[uk:Лимонна кислота]]
[[ur:سٹرک تیزاب]]
[[vi:Axít citric]]
[[zh:檸檬酸]]
[[Category:2Fix]]