Natural occurrence and industrial production of citric acid
- Citric acid is found in high concentrations in citrus fruits such as lemons, oranges, and limes.
- Lemons and limes can contain as much as 8% of citric acid in their juices.
- The concentration of citric acid varies within species and depends on the cultivar and growing conditions.
- Citric acid was first isolated from lemon juice in 1784 by Carl Wilhelm Scheele.
- Industrial-scale production of citric acid began in 1890, using the Italian citrus fruit industry as a base.
- Microbial production of citric acid became important during World War I when Italian citrus exports were disrupted.
- Certain strains of the mold Aspergillus niger were found to be efficient citric acid producers.
- The major industrial route for citric acid production involves feeding A. niger cultures with a sugar-containing medium.
- The source of sugar can be corn steep liquor, molasses, or hydrolyzed corn starch.
- After filtering out the mold, citric acid is isolated by precipitating it with calcium hydroxide and then regenerating it with sulfuric acid.
Chemical characteristics of citric acid
- Citric acid can exist in an anhydrous form or as a monohydrate.
- The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid crystallizes from cold water.
- Citric acid dissolves in absolute ethanol at a ratio of 76 parts per 100 parts of ethanol.
- It decomposes with the loss of carbon dioxide above approximately 175°C.
- Citric acid is a tribasic acid with pK values of 3.128, 4.761, and 6.396 at 25°C.
Global production and usage of citric acid
- Global production of citric acid exceeded 2,000,000 tons in 2018.
- More than 50% of this volume was produced in China.
- Over 50% of citric acid is used as an acidity regulator in beverages.
- Approximately 20% is used in other food applications.
- Around 20% is used for detergent applications, while the remaining 10% is used in cosmetics, pharmaceuticals, and the chemical industry.
Biochemistry - Citric acid cycle
- Citrate is an intermediate in the citric acid cycle, also known as the TCA cycle or Krebs cycle.
- The cycle is a central metabolic pathway for animals, plants, and bacteria.
- Citrate is formed by the condensation of oxaloacetate with acetyl CoA, catalyzed by citrate synthase.
- The cycle ends with the regeneration of oxaloacetate.
- The citric acid cycle is responsible for two-thirds of the food-derived energy in higher organisms.
Applications of citric acid
- Citric acid is commonly used as a flavoring and preservative in food and beverages.
- It is denoted by E number E330 within the European Union.
- Citrate salts of various metals are used in dietary supplements to deliver minerals.
- Citric acid is used in ice cream as an emulsifying agent and in caramel to prevent sucrose crystallization.
- It is used in effervescent formulae and food coloring to balance pH levels.
- Citric acid is an excellent chelating agent, used to remove limescale and treat water.
- It improves the effectiveness of soaps and laundry detergents by chelating metals in hard water.
- Citric acid is used in bathroom and kitchen cleaning solutions and can remove hard water stains from glass.
- It can be used in shampoo to wash out wax and coloring from the hair.
- Citric acid is used to dissolve rust from steel and passivate stainless steels.
- Citric acid is used as an acidulant in creams, gels, and liquids.
- It is an active ingredient in chemical skin peels and facial tissues with antiviral properties.
- Citric acid is used as a buffer to increase the solubility of brown heroin.
- It is used in the production of facial tissues with antiviral properties.
- Citric acid is used in the synthesis of other organic compounds, such as itaconic acid and aconitic acid.
- Citric acid is used to control pH in household cleaners and pharmaceuticals.
- It is used as an odorless alternative to white vinegar for fabric dyeing.
- Sodium citrate, derived from citric acid, is used in Benedicts reagent for identifying reducing sugars.
- Citric acid can be used as an alternative to nitric acid in passivation of stainless steel.
- It can be used as a lower-odor stop bath in photographic film development and as a blood acid regulator.
Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. 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.
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Names | |||
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IUPAC name
Citric acid
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Preferred IUPAC name
2-Hydroxypropane-1,2,3-tricarboxylic acid | |||
Identifiers | |||
3D model (JSmol)
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ChEBI | |||
ChEMBL | |||
ChemSpider | |||
DrugBank | |||
ECHA InfoCard | 100.000.973 | ||
EC Number |
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E number | E330 (antioxidants, ...) | ||
KEGG | |||
PubChem CID
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RTECS number |
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UNII | |||
CompTox Dashboard (EPA)
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Properties | |||
C6H8O7 | |||
Molar mass | 192.123 g/mol (anhydrous), 210.14 g/mol (monohydrate) | ||
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 | ||
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) | |||
Solubility | Soluble in acetone, alcohol, ether, ethyl acetate, DMSO Insoluble in C 6H 6, CHCl3, CS2, toluene | ||
Solubility in ethanol | 62 g/100 g (25 °C) | ||
Solubility in amyl acetate | 4.41 g/100 g (25 °C) | ||
Solubility in diethyl ether | 1.05 g/100 g (25 °C) | ||
Solubility in 1,4-dioxane | 35.9 g/100 g (25 °C) | ||
log P | −1.64 | ||
Acidity (pKa) | pKa1 = 3.13 pKa2 = 4.76 pKa3 = 6.39, 6.40 | ||
Refractive index (nD)
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1.493–1.509 (20 °C) 1.46 (150 °C) | ||
Viscosity | 6.5 cP (50% aq. sol.) | ||
Structure | |||
Monoclinic | |||
Thermochemistry | |||
Heat capacity (C)
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226.51 J/(mol·K) (26.85 °C) | ||
Std molar
entropy (S⦵298) |
252.1 J/(mol·K) | ||
Std enthalpy of
formation (ΔfH⦵298) |
−1543.8 kJ/mol | ||
1985.3 kJ/mol (474.5 kcal/mol, 2.47 kcal/g), 1960.6 kJ/mol 1972.34 kJ/mol (471.4 kcal/mol, 2.24 kcal/g) (monohydrate) | |||
Pharmacology | |||
A09AB04 (WHO) | |||
Hazards | |||
Occupational safety and health (OHS/OSH): | |||
Main hazards
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Skin and eye irritant | ||
GHS labelling: | |||
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Warning | |||
H290, H319, H315 | |||
P305+P351+P338 | |||
NFPA 704 (fire diamond) | |||
Flash point | 155 °C (311 °F; 428 K) | ||
345 °C (653 °F; 618 K) | |||
Explosive limits | 8% | ||
Lethal dose or concentration (LD, LC): | |||
LD50 (median dose)
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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).
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More than two million tons of citric acid are manufactured every year. It is used widely as an acidifier, as a flavoring, and a chelating agent.
A citrate is a derivative of citric acid; that is, the salts, esters, and the polyatomic anion found in solution. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate. When part of a salt, the formula of the citrate anion is written as C
6H
5O3−
7 or C
3H
5O(COO)3−
3.
IPA(key): /ˌsɪtɹɪk ˈæsɪd/
citric acid (uncountable)