Citric acid
From Wikipedia, the free encyclopedia
| 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] |
| PubChem | |
| Properties | |
| Molecular formula | C6H8O7 |
| Molar mass | 192.123 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 (20°C) |
| 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 |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
|
Citric acid is a weak organic acid. 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 almost all living things. It also serves as an environmentally benign cleaning agent and acts as an antioxidant.
Citric acid exists in a variety of fruits and vegetables, most notably citrus fruits. Lemons and limes have particularly high concentrations of the acid; it can comprise as much as 8% of the dry weight of these fruits (1.44 and 1.38 grams per ounce of the juices, respectively[1]).
Contents |
[edit] Properties
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 it above 74 °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.
[edit] 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.
[edit] History
The discovery of citric acid has been credited to the 8th century alchemist Jabir Ibn Hayyan (Geber). 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. Citric acid was first isolated in 1784 by the Swedish chemist Carl Wilhelm Scheele, who crystallized it from lemon juice. Industrial-scale citric acid production began in 1860, 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.
[edit] Production
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,[2] molasses, hydrolyzed corn starch or other inexpensive sugary solutions.[3] After the mold 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.
[edit] 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. It was discovered by Sir Hans Adolf Krebs. 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).
[edit] Uses
In 2007 world wide annual production stands at approximatley 1700.000 MT/y. 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 and chemical industry.
[edit] Food additive
As a food additive, citric acid is used as a flavouring 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 is defined by the Food Chemical Codex (FCC), which is published by the United States Pharmacopoeia (USP).
[edit] 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.
[edit] Others
Citric acid is used in the biotechnology and 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.
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.[4]
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[5]. 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. Purchases of large quantities of citric acid may rouse suspicion of potential terrorist activity.
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. Citric acid aids fermentation and is used in preference to other acids, for example tartaric acid, due to its stability, pleasant flavour and ability to impart brilliance to the finish wine.
When applied to hair, citric acid opens up the outer layer, also known as the cuticle. While the cuticle is open, it allows for a deeper penetration into the hair shaft[citation needed]. It 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.
Citric acid is also used as a stop bath in photography. The developer is normally alkaline, so a mild acid will neutralize it, increasing the effectiveness of the stop bath when compared to plain water.[6]
Citric acid is used as one of the active ingredients in the production of anti-viral tissues.[7]
The concentrations of citric acid in citrus fruits such as oranges and grapefruits is approximately .005 mol/L. The concentration of citric acid is higher for others, approximately .030 mol/L, in citrus fruits such as lemons and limes. These values will vary depending on the circumstances in which the fruit was grown.
[edit] Safety
Citric acid is recognized as safe for use in food by all major national and international food regulatory agencies. It is naturally present in almost all forms of life, and excess citric acid is readily metabolized and eliminated from the body.
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.
Excessive consumption is capable of eroding the tooth enamel.
Close contact to the eyes can cause a slight burning sensation, and may cause loss of sight.
Sometimes a high concentration of citric acid can damage hair, since it opens hair cuticles. It can strip the hair of needed minerals and bleach it.[citation needed]
[edit] See also
- Citric acid intolerance
- Citric acid cycle
- The closely related acids isocitric acid, aconitic acid, and propane-1,2,3-tricarboxylic acid (tricarballylic acid, carballylic acid)
[edit] References
- ^ Penniston KL, Nakada SY, Holmes RP, Assimos DG (2008). "Quantitative Assessment of Citric Acid in Lemon Juice, Lime Juice, and Commercially-Available Fruit Juice Products" (PDF). JOURNAL OF ENDOUROLOGY 22 (3): 567. doi:. PMID 18290732.
- ^ Corn steep liquor is a co-product from the wet milling of maize, see: Corn Steep Liquor
- ^ Citric acid production by a novel Aspergillus niger isolate: II. Optimization of process parameters through statistical experimental designs. Bioresource Technology 98(18) 3470-3477.
- ^ Use of ammoniated citric acid for the chemical cleaning of high pressure boilers.
- ^ Garden, J., Roberts, K., Taylor, A., and Robinson, D. (2003). "Evaluation of the Provision of Single Use Citric Acid Sachets to Injecting Drug Users" (pdf). Scottish Center for Infection and Environmental Health.
- ^ Stopbaths
- ^ "Tissues that fight germs", CNN, 2004-07-14. Retrieved on 2008-05-08.
[edit] External links
