citric acid

Citric acid
IUPAC name	2-hydroxypropane- 1,2,3-tricarboxylic acid
Other names	3-hydroxypentanedioic acid-3-carboxylic acid Hydrogen citrate
Identifiers
CAS number	77-92-9 Y
PubChem	311
ChemSpider ID	305
Properties
Molecular formula	C6H8O7
Molar mass	192.124 g/mol (anhydrous) 210.14 g/mol (monohydrate)
Appearance	crystalline white solid
Density	1.665 g/cm3
Melting point	153 °C
Boiling point	decomposes at 175 °C
Solubility in water	133 g/100 ml (22°C)
Solubility in THF, ethanol, methanol	anhydrous: THF 1.80 M, ethanol 1.6 M, methanol 3.08 M [1] monohydrate: THF 1.52 M, ethanol 1.78 M, methanol 2.27 M [2]
Acidity (pKa)	pKa1=3.15 pKa2=4.77 pKa3=6.40
Hazards
Main hazards	skin and eye irritant
Flash point	?°C
Related compounds
Related compounds	sodium citrate, calcium citrate
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Citric acid is a weak organic acid, and it is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of virtually all 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 fruits and vegetables, most notably citrus fruits. Lemons and limes have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits (about 47 g/L in the juices^[3]). The concentrations of citric acid in citrus fruits range from 0.005 mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes. These values vary depending on the cultivar and the circumstances in which the fruit was grown.

Properties

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 monohydrate. The anhydrous form crystallizes from hot water, whereas the monohydrate forms when citric acid is crystallized from cold water. The monohydrate can be converted to the anhydrous form by heating above 78 °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 acids. When heated above 175°C, it decomposes through the loss of carbon dioxide and water. Citric acid leaves a white crystalline precipitate.

Measurement

Citric acid has been used as an additive to soft drinks, beer, and 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

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

The discovery of citric acid has been credited to the 8th century Muslim alchemist Jabir Ibn Hayyan (Geber).^[4][5][6] 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 Majus (The Great Mirror), compiled by Vincent of Beauvais.^{[citation needed]} Citric acid was first isolated in 1784 by the Swedish chemist Carl Wilhelm Scheele, who crystallized it from lemon juice.^[7][8] Industrial-scale citric acid production began in 1890 based on the 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 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.^[9] After the mould is filtered out of the resulting solution, citric acid is isolated by precipitating it with lime (calcium hydroxide) to yield calcium citrate salt, from which citric acid is regenerated by treatment with sulfuric acid.

Krebs cycle

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 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).

Uses

In 2007, world wide annual production stands at approximately 1,700,000 MT. 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 beverages, especially soft drinks. 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. The buffering properties of citrates are used to control pH in household cleaners and pharmaceuticals. In the United States the purity requirements for citric acid as a food additive are defined by the Food Chemical Codex (FCC), which is published by the United States Pharmacopoeia (USP). Can be used for cherry packing. The market name is color guard.

Water softening

Citric acid's ability to chelate metals makes it useful in soaps and laundry detergents. 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 softeners by stripping off the accumulated metal ions as citrate complexes.The saturation point for citric acid and water is 59%

Others

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]}

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.^[10]

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 exchange their dirty needles for clean needles in an attempt to decrease the spread of AIDS and hepatitis^[11]. 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 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 terrorist activity.^{[citation needed]}

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 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 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.^[12]

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]}

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.^[13]

Citric acid is used as one of the active ingredients in the production of anti-viral tissues.^[14]

Citric acid can be used in food coloring to balance the pH level of the normally basic dye.

Citric acid may be used as the main ripening agent in the first steps of making mozzarella cheese.^[15]

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 good alternative to nitric acid in the process of stainless steel passivation (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 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.^{[citation needed]}

Excessive consumption is capable of eroding the tooth enamel.^[16]

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

The leaflet of Villejuif (also known as the flyer of Villejuif or the list of Villejuif) was a scientifically inaccurate rumour, passed via a leaflet or flyer, that caused 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 carcinogens.

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

References

• Citric Acid MSDS Sheet

External links

Wikimedia Commons has media related to: Citric acid

• CID 311 from PubChem
• The European Citric Acid Manufacturers Association
• NextBio Citric Acid Entry
• Citric acid analysis - free spreadsheet for titration of acids and pH calculation
• Applications of Citric Acid
• MSDS sheet for Citric Acid