Citric acid

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Citric acid monohydrate
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Citric acid
Stereo skeletal formula of citric acid
Stereo skeletal formula of citric acid
Ball-and-stick model of citric acid
Ball-and-stick model of citric acid
Crystal sample from the saturated citric acid solution.
Names
IUPAC name
Citric acid[1]
Preferred IUPAC name
2-Hydroxypropane-1,2,3-tricarboxylic acid[1]
Identifiers
3D model (
JSmol
)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard
 100.000.973 Edit this at Wikidata
EC Number
  • 201-069-1
E number E330 (antioxidants, ...)
IUPHAR/BPS
KEGG
RTECS number
  • GE7350000
UNII
  • InChI=1S/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) checkY
    Key: KRKNYBCHXYNGOX-UHFFFAOYSA-N checkY
  • 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)
    Key: KRKNYBCHXYNGOX-UHFFFAOYAM
  • OC(=O)CC(O)(C(=O)O)CC(=O)O
Properties
HOC(CO2H)(CH2CO2H)2
Molar mass 192.123 g/mol (anhydrous), 210.14 g/mol (monohydrate)[2]
Appearance white solid
Odor Odorless
Density 1.665 g/cm3 (anhydrous)
1.542 g/cm3 (18 °C, monohydrate)
Melting point 156 °C (313 °F; 429 K)
Boiling point 310 °C (590 °F; 583 K) decomposes from 175 °C[3]
54% w/w (10 °C)
59.2% w/w (20 °C)
64.3% w/w (30 °C)
68.6% w/w (40 °C)
70.9% w/w (50 °C)
73.5% w/w (60 °C)
76.2% w/w (70 °C)
78.8% w/w (80 °C)
81.4% w/w (90 °C)
84% w/w (100 °C)[4]
Solubility Soluble in acetone, alcohol, ether, ethyl acetate, DMSO
Insoluble in C
6H
6, CHCl3, CS2, toluene[3]
Solubility in ethanol 62 g/100 g (25 °C)[3]
Solubility in amyl acetate 4.41 g/100 g (25 °C)[3]
Solubility in diethyl ether 1.05 g/100 g (25 °C)[3]
1,4-dioxane
 35.9 g/100 g (25 °C)[3]
log P −1.64
Acidity (pKa) pKa1 = 3.13[5]
pKa2 = 4.76[5]
pKa3 = 6.39,[6] 6.40[7]
pKa4 = 14.4[8]
1.493–1.509 (20 °C)[4]
1.46 (150 °C)[3]
Viscosity 6.5 cP (50% aq. sol.)[4]
Structure
Monoclinic
Thermochemistry
226.51 J/(mol·K) (26.85 °C)[9]
252.1 J/(mol·K)[9]
Std enthalpy of
formation
(ΔfH⦵298)
 −1543.8 kJ/mol[4]
1985.3 kJ/mol (474.5 kcal/mol, 2.47 kcal/g),[4] 1960.6 kJ/mol[9]
1972.34 kJ/mol (471.4 kcal/mol, 2.24 kcal/g) (monohydrate)[4]
Pharmacology
A09AB04 (WHO)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Skin and eye irritant
GHS labelling:
GHS05: CorrosiveGHS07: Exclamation mark[5]
Warning
H290, H319, H315[5]
P305+P351+P338[5]
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
2
1
0
Flash point 155 °C (311 °F; 428 K)
345 °C (653 °F; 618 K)
Explosive limits
 8%[5]
Lethal dose or concentration (LD, LC):
3000 mg/kg (rats, oral)
Safety data sheet (SDS) HMDB
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Citric acid is an

weak organic acid.[10] It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the metabolism of all aerobic organisms.[10]

More than two million tons of citric acid

chelating agent.[11]

A citrate is a derivative of citric acid; that is, the

trianion
is part of a salt, the formula of the citrate trianion is written as C
6H
5O3−
7 or C
3H
5O(COO)3−
3.

Natural occurrence and industrial production

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

Citric acid occurs in a variety of fruits and vegetables, most notably

mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes; these values vary within species depending upon the cultivar
and the circumstances under which the fruit was grown.

Citric acid was first isolated in 1784 by the chemist Carl Wilhelm Scheele, who crystallized it from lemon juice.[13][14]

Industrial-scale citric acid production first began in 1890 based on the Italian

mold could produce citric acid from sugar.[16]
However, microbial production of citric acid did not become industrially important until World War I disrupted Italian citrus exports.

In 1917, American food chemist James Currie discovered that certain strains of the mold Aspergillus niger could be efficient citric acid producers,[17] and the pharmaceutical company 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, carbohydrate solution.[18] After the mold is filtered out of the resulting suspension, citric acid is isolated by precipitating it with calcium hydroxide to yield calcium citrate salt, from which citric acid is regenerated by treatment with sulfuric acid, as in the direct extraction from citrus fruit juice.

In 1977, a patent was granted to

Krebs cycle reaction.[19]

Global production was in excess of 2,000,000 tons in 2018.[20] More than 50% of this volume was produced in China. More than 50% was used as an acidity regulator in beverages, some 20% in other food applications, 20% for detergent applications, and 10% for applications other than food, such as cosmetics, pharmaceuticals, and in the chemical industry.[15]

Chemical characteristics

Speciation diagram for a 10-millimolar solution of citric acid

Citric acid can be obtained as an anhydrous (water-free) form or as a monohydrate. The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid is crystallized from cold water. The monohydrate can be converted to the anhydrous form at about 78 °C. Citric acid also dissolves in absolute (anhydrous) ethanol (76 parts of citric acid per 100 parts of ethanol) at 15 °C. It decomposes with loss of carbon dioxide above about 175 °C.

Citric acid is a tribasic acid, with pKa values, extrapolated to zero ionic strength, of 3.128, 4.761, and 6.396 at 25 °C.[21] The pKa of the hydroxyl group has been found, by means of 13C NMR spectroscopy, to be 14.4.[22] The speciation diagram shows that solutions of citric acid are buffer solutions between about pH 2 and pH 8. In biological systems around pH 7, the two species present are the citrate ion and mono-hydrogen citrate ion. The SSC 20X hybridization buffer is an example in common use.[23][24] Tables compiled for biochemical studies are available.[25]

Conversely, the pH of a 1 mM solution of citric acid will be about 3.2. The pH of fruit juices from

citrus fruits
like oranges and lemons depends on the citric acid concentration, with a higher concentration of citric acid resulting in a lower pH.

Acid salts of citric acid can be prepared by careful adjustment of the pH before crystallizing the compound. See, for example, sodium citrate.

The citrate ion forms complexes with metallic cations. The

chelate effect. Consequently, it forms complexes even with alkali metal cations. However, when a chelate complex is formed using all three carboxylate groups, the chelate rings have 7 and 8 members, which are generally less stable thermodynamically than smaller chelate rings. In consequence, the hydroxyl group can be deprotonated, forming part of a more stable 5-membered ring, as in ammonium ferric citrate, [NH+4]5Fe3+(C6H4O4−7)2·2H2O.[26]

Citric acid can be esterified at one or more of its three carboxylic acid groups to form any of a variety of mono-, di-, tri-, and mixed esters.[27]

Biochemistry

Citric acid cycle

Main article: Citric acid cycle

Citrate is an intermediate in the

Hans Adolf Krebs received the 1953 Nobel Prize in Physiology or Medicine
for the discovery.

Some bacteria (notably

traits evolve. They found evidence that, in this case, the innovation was caused by a rare duplication mutation
due to the accumulation of several prior "potentiating" mutations, the identity and effects of which are still under study. The evolution of the Cit+ trait has been considered a notable example of the role of historical contingency in evolution.

Other biological roles

Citrate can be transported out of the

allosterically regulates the enzyme acetyl-CoA carboxylase, which is the regulating enzyme in the conversion of acetyl-CoA into malonyl-CoA
(the commitment step in fatty acid synthesis). In short, citrate is transported into the cytoplasm, converted into acetyl-CoA, which is then converted into malonyl-CoA by acetyl-CoA carboxylase, which is allosterically modulated by citrate.

High concentrations of cytosolic citrate can inhibit phosphofructokinase, the catalyst of a rate-limiting step of glycolysis. This effect is advantageous: high concentrations of citrate indicate that there is a large supply of biosynthetic precursor molecules, so there is no need for phosphofructokinase to continue to send molecules of its substrate, fructose 6-phosphate, into glycolysis. Citrate acts by augmenting the inhibitory effect of high concentrations of ATP, another sign that there is no need to carry out glycolysis.[30]

Citrate is a vital component of bone, helping to regulate the size of apatite crystals.[31]

Applications

Food and drink

Powdered citric acid being used to prepare lemon pepper seasoning

Because it is one of the stronger edible acids, the dominant use of citric acid is as a flavoring and preservative in food and beverages, especially soft drinks and candies.

United States Pharmacopoeia
(USP).

Citric acid can be added to ice cream as an emulsifying agent to keep fats from separating, to caramel to prevent sucrose crystallization, or in recipes in place of fresh lemon juice. Citric acid is used 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 sold in a dry powdered form is commonly sold in markets and groceries as "sour salt", due to its physical resemblance to table salt. It has use in culinary applications, as an alternative to vinegar or lemon juice, where a pure acid is needed. Citric acid can be used in food coloring to balance the pH level of a normally basic dye.[citation needed]

Cleaning and chelating agent

Structure of an iron(III) citrate complex[33][34]

Citric acid is an excellent

eluant used for total ion-exchange separation of the lanthanides, during the Manhattan Project in the 1940s.[35] In the 1950s, it was replaced by the far more efficient[36] EDTA
.

In industry, it is used to dissolve rust from steel, and to passivate stainless steels.[37]

Cosmetics, pharmaceuticals, dietary supplements, and foods

Citric acid is used as an acidulant in creams, gels, and liquids. Used in foods and dietary supplements, it may be classified as a processing aid if it was added for a technical or functional effect (e.g. acidulent, chelator, viscosifier, etc.). If it is still present in insignificant amounts, and the technical or functional effect is no longer present, it may be exempt from labeling <21 CFR §101.100(c)>.

Citric acid is an

alpha hydroxy acid and is an active ingredient in chemical skin peels.[38]

Citric acid is commonly used as a buffer to increase the solubility of brown heroin.[39]

Citric acid is used as one of the active ingredients in the production of facial tissues with antiviral properties.[40]

Other uses

The

pharmaceuticals
.

Citric acid is used as an odorless alternative to

white vinegar for fabric dyeing with acid dyes
.

Sodium citrate is a component of Benedict's reagent, used for both qualitative and quantitative identification of reducing sugars.[41]

Citric acid can be used as an alternative to nitric acid in passivation of stainless steel.[42]

Citric acid can be used as a lower-odor stop bath as part of the process for developing photographic film. Photographic developers are alkaline, so a mild acid is used to neutralize and stop their action quickly, but commonly used acetic acid leaves a strong vinegar odor in the darkroom.[43]

Citric acid/potassium-sodium citrate can be used as a blood acid regulator. The citric acid is included to improve palatability[44]

Citric acid is an excellent soldering flux,[45] either dry or as a concentrated solution in water. It should be removed after soldering, especially with fine wires, as it is mildly corrosive. It dissolves and rinses quickly in hot water.

Alkali citrate can be used as an inhibitor of kidney stones by increasing urine citrate levels, useful for prevention of calcium stones, and increasing urine pH, useful for preventing uric acid and cystine stones.[46]

Synthesis of other organic compounds

Citric acid is a versatile precursor to many other organic compounds. Dehydration routes give itaconic acid and its anhydride.[47] Citraconic acid can be produced via thermal isomerization of itaconic acid anhydride.[48] The required itaconic acid anhydride is obtained by dry distillation of citric acid. Aconitic acid can be synthesized by dehydration of citric acid using sulfuric acid:[49]

(HO2CCH2)2C(OH)CO2H → HO2CCH=C(CO2H)CH2CO2H + H2O

Acetonedicarboxylic acid can also be prepared by decarboxylation of citric acid in fuming sulfuric acid.[50]

Safety

Although a weak acid, exposure to pure citric acid can cause adverse effects. Inhalation may cause cough, shortness of breath, or sore throat. Over-ingestion may cause abdominal pain and sore throat. Exposure of concentrated solutions to skin and eyes can cause redness and pain.[51] Long-term or repeated consumption may cause erosion of tooth enamel.[51][52][53]

Compendial status

See also

Explanatory notes

  1. ^ This still does not make the lemon particularly strongly acidic. This is because, as a weak acid, most of the acid molecules are not dissociated so not contributing to acidity inside the lemon or its juice.

References

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    ISBN 978-0-85404-182-4
    .
  2. ^ CID 22230 from PubChem
  3. ^ a b c d e f g "citric acid". chemister.ru. Archived from the original on November 29, 2014. Retrieved June 1, 2014.
  4. ^ a b c d e f CID 311 from PubChem
  5. ^ a b c d e f Fisher Scientific, Citric acid. Retrieved on 2014-06-02.
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  7. ^ "Ionization Constants of Organic Acids". Michigan State University. Retrieved January 11, 2012.
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  9. ^ a b c Citric acid in Linstrom, Peter J.; Mallard, William G. (eds.); NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg (MD) (retrieved 2014-06-02)
  10. ^ a b c "Citric acid | C6H8O7 - PubChem". Archived from the original on January 19, 2022. Retrieved December 19, 2021.
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  13. ^ Scheele, Carl Wilhelm (1784). "Anmärkning om Citron-saft, samt sätt at crystallisera densamma" [Note about lemon juice, as well as ways to crystallize it]. Kungliga Vetenskaps Academiens Nya Handlingar [New Proceedings of the Royal Academy of Science]. 2nd series (in Swedish). 5: 105–109.
  14. ^ Graham, Thomas (1842). Elements of chemistry, including the applications of the science in the arts. Hippolyte Baillière, foreign bookseller to the Royal College of Surgeons, and to the Royal Society, 219, Regent Street. p. 944. Retrieved June 4, 2010.
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  19. ^ US 4056567-V.Lamberti and E.Gutierrez
  20. ^ "Global Citric Acid Markets Report, 2011-2018 & 2019-2024". prnewswire.com. March 19, 2019. Retrieved October 28, 2019.
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  37. ^ "ASTM A967 / A967M - 17 Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts". www.astm.org.
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  43. ^ Anchell, Steve. "The Darkroom Cookbook: 3rd Edition (Paperback)". Focal Press. Retrieved January 1, 2013.
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  55. ^ "Japanese Pharmacopoeia, Fifteenth Edition" (PDF). 2006. Archived from the original (PDF) on July 22, 2011. Retrieved 4 February 2010.

External links

Wikimedia Commons has media related to Citric acid.
Wikisource has the text of the 1911 Encyclopædia Britannica article "Citric Acid".
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