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Citric acid is an organic compound with the formula . It is a colorless Weak acid organic acid. It occurs naturally in Citrus. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the metabolism of all .
More than two million tons of citric acid are manufactured every year. It is used widely as acidifier, flavoring, preservative, and chelating agent. (2025). 9783319112329, Springer. ISBN 9783319112329
A citrate is a derivative of citric acid; that is, the salts, , and the polyatomic ion 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 anion is part of a salt, the formula of the citrate trianion is written as or .
Citric acid was first isolated in 1784 by the chemist Carl Wilhelm Scheele, who crystallized it from lemon juice.
Industrial-scale citric acid production first began in 1890 based on the Italian citrus fruit industry, where the juice was treated with hydrated lime (calcium hydroxide) to precipitate calcium citrate, which was isolated and converted back to the acid using diluted sulfuric acid. In 1893, C. Wehmer discovered 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, American food chemist James Currie discovered that certain strains of the mold Aspergillus niger could be efficient citric acid producers, 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. 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 Lever Brothers for the chemical synthesis of citric acid starting either from aconitic or isocitrate (also called alloisocitrate) calcium salts under high pressure conditions; this produced citric acid in near quantitative conversion under what appeared to be a reverse, non-enzymatic Krebs cycle reaction. (US 4056567)
Although industrial-scale production of citric acid by chemical synthesis or extraction from citrus fruits are both feasible, fermentation by molds (and sometimes yeasts) is almost exclusively the only method actually practiced.
Global production was in excess of 2,000,000 tons in 2018. 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.
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. It decarboxylation 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. (Link added 4 August 2024) 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 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.Maniatis, T.; Fritsch, E. F.; Sambrook, J. 1982. Molecular Cloning: A Laboratory Manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY. 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 like oranges and lemons depends on the citric acid concentration, with a higher concentration of citric acid resulting in a lower pH.
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 stability constants for the formation of these complexes are quite large because of 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, .
Citric acid can be at one or more of its three carboxylic acid groups to form any of a variety of mono-, di-, tri-, and mixed esters.
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. (2025). 9780716746843, Freeman. ISBN 9780716746843
Citrate is a vital component of bone, helping to regulate the size of apatite crystals.
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 effervescence formulae, both for ingestion (e.g., powders and tablets) and for personal care ( e.g., bath salts, , and cleaning of Petroleum). 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.
In industry, it is used to dissolve rust from steel, and to passivate .
Citric acid is an alpha hydroxy acid and is an active ingredient in chemical skin peels.
Citric acid is commonly used as a buffer to increase the solubility of brown heroin.
Citric acid is used as one of the active ingredients in the production of facial tissues with antiviral properties.
Citric acid is used as an odorless alternative to white vinegar for fabric dyeing with . It can enhance the Mordant process, crosslinking fabrics and dyes through an esterification reaction.
Sodium citrate is a component of Benedict's reagent, used for both qualitative and quantitative identification of reducing sugars.
Citric acid can be used as an alternative to nitric acid in passivation of stainless steel.
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.
Citric acid is an excellent soldering flux, 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.
Acetonedicarboxylic acid can also be prepared by decarboxylation of citric acid in fuming sulfuric acid.
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