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Citric acid is a white, crystalline, weak organic acid present in most plants and many animals as an intermediate in cellular respiration.
Citric acid contains three carboxyl groups making it a carboxylic, more specifically a tricarboxylic, acid.
The name citrus originates from the Greek kedromelon meaning apple of melon for the fruit citron.
CAS: 77-92-9
MF: C6H8O7
MW: 192.12
EINECS: 201-069-1
Synonyms
BETZ 6251;BETZ 0623;CITRIC ACID 99.5+% FCC;CITRIC ACID, 500MG, NEAT;Citric acid, anhydrous, for analysis ACS, 99.6%;Citric acid, anhydrous, for analysis, 99.5%;Citric acid, anhydrous, pure, 99%;anhydrouscitricacid
Greek works mention kitron, kitrion, or kitreos for citron fruit, which is an oblong fruit several inches long from the scrublike tree Citrus medica.
Lemons and limes have high citric acid content, which may account for up to 8% of the fruit's dry weight.
Citric acid is a weak acid and loses hydrogen ions from its three carboxyl groups (COOH) in solution.
the loss of a hydrogen ion from each group in the molecule results in the citrate ion,C3H5O(COO)33.
A citric acid molecule also forms intermediate ions when one or two hydrogen atoms in the carboxyl groups ionize.the citrate ion combines with metals to form salts, the most common of which is calcium citrate.
Citric acid forms esters to produce various citrates, for example trimethyl citrate and triethyl citrate.
Citric acid is a tricarboxylic acid that is propane-1,2,3-tricarboxylic acid bearing a hydroxy substituent at position 2.
Citric acid is an important metabolite in the pathway of all aerobic organisms.
Citric acid has a role as a food acidity regulator, a chelator, an antimicrobial agent and a fundamental metabolite.
Citric acid is a conjugate acid of a citrate(1-) and a citrate anion.
Citric acid is an organic compound with the formula C6H8O7.
Citric acid is a colorless weak organic acid.
Citric acid occurs naturally in citrus fruits.
In biochemistry, Citric acid is an intermediate in the citric acid cycle, which occurs in the metabolism of all aerobic organisms.
More than two million tons of citric acid are manufactured every year.
Citric acid is used widely as acidifier, flavoring, preservative, and chelating agent.
A citrate is a derivative of citric acid; that is, the salts, esters, and the polyatomic anion found in solutions and salts of citric acid.
An example of the former, a salt is trisodium citrate; an ester is triethyl citrate.
When citrate trianion is part of a salt, the formula of the citrate trianion is written as C6H5O73– or C3H5O(COO)33–.
Citric acid is an organic compound that is a weak organic acid.
Citric acid is a colorless, odorless solid with a sour taste.
Citric acid participates in many biochemical reactions, including the citric acid cycle and fatty acid metabolism.
Citric acid can be found in many citrus fruits, such as lemons, limes, and grapefruits.
Citric acid also has antioxidant properties and functions as an anti-inflammatory agent.
Citric acid is used for the production of citrates and citrate salts, which are added to food preservatives or acidic foods such as lemonade or vinegar.
Citric acid can be used to measure water vapor concentration by using fluorescence probes that react with hydrated electrons in water molecules.
Citric acid also has electrochemical impedance spectroscopy (EIS) applications in biomedical research and biotechnology as an enzyme inhibitor or activator.
History
The discovery of citric acid is credited to Jabir ibn Hayyan.
Citric acid was first isolated in 1784 by the Swedish chemist Carl Wilhelm Scheele, who crystallized it from lemon juice.
The crystalline structure of anhydrous citric acid, obtained by cooling hot concentrated solution of the monohydrate form, was first elucidated by Yuill and Bennett in 1934 by X-ray diffraction.
In 1960 Nordman and co-workers further suggested that in the anhydrous form two molecules of the acid are linked through hydrogen bonds between two –COOH groups of each monomer.
Chemical characteristics
Citric acid is known to occur as a monohydrate, though the anhydrous form can be obtained by crystallization from hot water.
Water can be driven off the monohydrate to produce the anhydrate by heating to around 80 °C, though this can also occur at ambient temperatures slowly over time by efflorescence at humidities in range of ~50% or less.
Citric acid dissolves in absolute (anhydrous) ethanol (76 parts of citric acid per 100 parts of ethanol) at 15 °C.
Citric acid decomposes with loss of carbon dioxide above about 175 °C.
Citric acid is a triprotic acid, with pKa values, extrapolated to zero ionic strength, of 3.128, 4.761, and 6.396 at 25 °C.
The pKa of the hydroxyl group has been found, by means of 13C NMR spectroscopy, to be 14.4.
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.
Tables compiled for biochemical studies are available.
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.
The citrate ion forms complexes with metallic cations.
The stability constants for the formation of these complexes are quite large because of the chelate effect.
Consequently, Citric acid 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.
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.
Citric acid Chemical Properties
Melting point: 153-159 °C (lit.)
Boiling point: 310 °C (decomp)
bulk density: 560kg/m3
density: 1.67 g/cm3 at 20 °C
vapor density: 7.26 (vs air)
vapor pressure: <0.1 hPa (20 °C)
refractive index: 1.493~1.509
FEMA: 2306 | CITRIC ACID
Fp: 100 °C
storage temp.: 2-8°C
solubility: Citric acid also dissolves in absolute (anhydrous) ethanol (76 parts of citric acid per 100 parts of ethanol) at 15 °C.
form: grit
pka: 3.14(at 20℃)
color: White
PH: 3.24(1 mM solution);2.62(10 mM solution);2.08(100 mM solution);
Odor: Odorless
Odor Type: odorless
biological source: synthetic
explosive limit: 8%, 65°F
Water Solubility: soluble in Water (1174g/L at 10°C, 1809g/L at 30°C, 3825g/L at 80°C).
Sensitive: Hygroscopic
λmax λ: 260 nm Amax: 0.20
λ: 280 nm Amax: 0.10
Merck: 14,2326
JECFA Number: 218
BRN: 782061
Henry's Law Constant: 3.1×1015 mol/(m3Pa) at 25℃, Burkholder et al. (2019)
Stability: Stable. Incompatible with bases, strong oxidizing agents, reducing agents, metal nitrates.
Cosmetics Ingredients Functions: CHELATING FRAGRANCE BUFFERING
Cosmetic Ingredient Review (CIR): Citric acid (77-92-9)
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)
InChIKey: KRKNYBCHXYNGOX-UHFFFAOYSA-N
LogP: -1.64
CAS DataBase Reference: 77-92-9(CAS DataBase Reference)
NIST Chemistry Reference: 1,2,3-Propanetricarboxylic acid, 2-hydroxy-(77-92-9)
EPA Substance Registry System: Citric acid (77-92-9)
Citric acid is soluble 66 % in water, 33% in alcohol, 3% in ether.
Soluble about 20% in Propylene glycol.
Virtually odorless.
The aqueous solution has a clean acid taste, pleasant in the concentration of 0.02 to 0.08%. Citric acid is a weak organic acid with the formula C6H8O7.
Citric acid is a natural preservative / conservative and is also used to add an acidic, or sour, taste to foods and soft drinks.
In biochemistry, the conjugate base of citric acid, citrate, is important as an intermediate in the citric acid cycle, which occurs in the metabolism of all aerobic organisms.
Citric acid is a commodity chemical, and more than a million tonnes are produced every year by fermentation.
Citric acid is used mainly as an acidifier, as a flavoring, and as a chelating agent.
Physical properties
Citric acid, white crystalline solid, decomposes at higher temperatures, sp gr 1.542. Citric acid is soluble in H2O or alcohol and slightly soluble in ether.
Citric acid is a tribasic acid, forming mono-, di-, and tri- series of salts and esters.
Citric acid occurs in large amounts is citrus fruits, and is used widely in industry as an acidifier, as a flavoring and chelating agent.
pKa values are 5.21, 4.28 and 2.92 at 25 °C (extrapolated to zero ionic strength).
Citric acid is a good buffering agent for solutions between about pH 2 and pH 8.
Citric acid is popular in many buffers in many techniques, electrophoresis (SSC Buffer #), to stop reactions, for biopurifications, crystallography.
In biological systems around pH 7, the two species present are the citrate ion and mono-hydrogen citrate ion.
The pH of a 1 mM solution of citric acid will be about 3.2.
Uses
Citric acid has astringent and anti-oxidant properties.
Citric acid can also be used as a product stabilizer, pH adjuster, and preservative with a low sensitizing potential.
Citric acid is not usually irritating to normal skin, but it can cause burning and redness when applied to chapped, cracked, or otherwise inflamed skin.
Citric acid is derived from citrus fruits.
Citric Acid is an acidulant and antioxidant produced by mold fermentation of sugar solutions and by extraction from lemon juice, lime juice, and pineapple canning residue.
Citric acid is the predominant acid in oranges, lemons, and limes.
Citric acid exists in anhydrous and monohydrate forms.
The anhydrous form is crystallized in hot solutions and the monohydrate form is crystallized from cold (below 36.5°c) solutions.
Anhydrous citric acid has a solubility of 146 g and monohydrate citric acid has a solubility of 175 g/100 ml of distilled water at 20°c.
a 1% solution has a ph of 2.3 at 25°c.
Citric acid is a hygroscopic, strong acid of tart flavor.
Citric acid is used as an acidulant in fruit drinks and carbonated beverages at 0.25-0.40%, in cheese at 3-4%, and in jellies.
Citric acid is used as an antioxidant in instant potatoes, wheat chips, and potato sticks, where it prevents spoilage by trapping the metal ions.
Citric acid is used in combination with antioxidants in the processing of fresh frozen fruits to prevent discoloration.
Citric acid is a weak organic acid that is known as a commodity chemical, as more than a million tonnes are produced every year by mycological fermentation on an industrial scale using crude sugar sol utions, such as molasses and strains of Aspergillus niger.
Citric acid is widely distributed in plants and in animal tissues and fluids and exist in greater than grace amounts in variety of fruits and vegetables, most notably in citrus fruits such as lemon and limes.
Citric acid is mainly used as an acidifier, flavoring agent and chelating agent.
Citric acid was also used as a chemical restrainer particularly in developers for the collodion process and in silver nitrate solutions used for sensitizing salted and albumen papers.
Food and drink
Because Citric acid 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.
Within the European Union 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.
Citric acid has 247 kcal per 100 g.
In the United States the purity requirements for citric acid as a food additive are defined by the Food Chemicals Codex, which is published by the 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.
Citric acid 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.
Biotechnological Applications
Citric acid cycle
Citrate, the conjugate base of 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 foodderived 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" or "Szent-Gyrgyi — Krebs cycle", and the "tricarboxylic acid (TCA) cycle".
Other biological roles
Citrate is a critical component of bone, helping to regulate the size of calcium crystals.
Benefits
Citric acid is not a vitamin or mineral and is not required in the diet.
However, citric acid, not to be confused with ascorbic acid (vitamin C), is beneficial for people with kidney stones.
Citric acid inhibits stone formation and breaks up small stones that are beginning to form.
Citric acid is protective; the more citric acid in your urine, the more protected you are against forming new kidney stones.
Citrate, used in calcium citrate supplements and in some medications (such as potassium citrate), is closely related to citric acid and also has stone prevention benefits.
These medications may be prescribed to alkalinize your urine.
Preparation
By mycological fermentation using molasses and strains of Aspergillus niger; from citrus juices and pineapple wastes.
Biotechnological Production
Fermentation is the technology of choice for citric acid synthesis. Different bacteria (e.g. Arthrobacter paraffinens and Bacillus licheniformis), filamentous fungi (e.g. Aspergilus niger and Penicillium citrinum) and yeasts (e.g. Candida tropicalis and Yarrowia lipolytica) are able to produce citric acid.
Due to high productivity and easy handling, citric acid is usually produced by fermentation with A. niger.
For example, a product concentration of 114 g.L-1 within 168 h has been reached by cultivation of A. niger GCMC 7 on cane molasses.
On the industrial scale, submerged cultivation, surface fermentation and solid-state fermentation are used.
In general, molasses, starch hydrolyzate and starch are used as substrates.
However, there are various studies for alternative raw materials.
Solid-state fermentation of inexpensive agricultural wastes is one possibility.
For example, high yields up to 88 % have been achieved using grape pomace as substrate.
Lowering the cost of product recovery is crucial.
Different methods using precipitation, solvent extraction, adsorption, or in situ product recovery have been described.
One interesting process could be the in situ crystallization of citric acid during fermentation to improve the economics.
Synthesis of other organic compounds
Citric acid is a versatile precursor to many other organic compounds. Dehydration routes give itaconic acid and its anhydride.
Citraconic acid can be produced via thermal isomerization of itaconic acid anhydride.
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:
(HO2CCH2)2C(OH)CO2H → HO2CCH=C(CO2H)CH2CO2H + H2O
Acetonedicarboxylic acid can also be prepared by decarboxylation of citric acid in fuming sulfuric acid.
