Citric acid
- Citric acid
- 2 -1 ,2,3 -tricarboxylic acid hydroxypropan (IUPAC)
- 3-carboxy- 3- hydroxyglutaric acid
- 2-hydroxy -1 ,2,3- propanetricarboxylic
- 3-carboxy- 3- hydroxypentandisäure
- E 330
- INCI name: Citric Acid
A09AB04
Colorless, odorless solid
Fixed
1.665 g · cm -3 ( 18 ° C)
153 ° C ( anhydrous) 100 ° C ( monohydrate)
Decomposition: 175 ° C
- PKS1 = 3.13
- PKa2 = 4.76
- PKS3 = 6.4
Slightly soluble in water: 750 g · l-1 (20 ° C)
Risk
3000 mg · kg -1 ( LD50, rat, oral)
Template: Infobox chemical / molecular formula search available
Citric acid ( also citric acid) is a colorless, water-soluble carboxylic acid, which is one of the fruit acids. In addition to the anhydrous variant, the citric acid monohydrate ( C6H8O7 · H2O ) containing one molecule of water of crystallization per molecule of citric acid exists. The salts and esters of citric acid are the citrates.
- 4.1 Physical Properties
- 4.2 Chemical Properties
History
Carl Wilhelm Scheele in 1784 for the first time isolated citric acid from lemon juice - hence the name. However, citric acid should already have been aware of the first alchemists, albeit under a different name. Already in the 9th century to the Arab alchemist Jabir ibn Hayyān ( donors ) have discovered citric acid.
Occurrence
Citric acid is one of the most widespread in the plant kingdom acids and occurs as a metabolite found in all organisms. Lemon juice contains citric acid, for example, 5-7%. But occurs in apples pears, cherries, raspberries, blackberries, currants, in conifers, fungi, tobacco, wine, and even in the milk.
Citric acid is widely used because it occurs ( eponymous ) as an intermediate in the citric acid cycle ( tricarboxylic acid cycle also, Krebs cycle ). This process plays a key role in carbohydrate and fatty acid metabolism of all oxygen-consuming creatures, including humans. This cycle also provides the molecular basis for the construction of structures of most amino acids.
Production and representation
Production of citrus fruits
According to the original method of citric acid obtained from citrus fruits. Lemon juice was added to concentrated ammonia solution, concentrated and filtered. The slightly soluble ammonium citrate was converted by precipitation with calcium chloride in the less soluble calcium citrate. The solution was again filtered and the filter cake dissolved in 25 % sulfuric acid, with worse soluble calcium sulfate (gypsum) precipitates. After renewed filtration, a citric acid solution is obtained. The pure citric acid is obtained by crystallization.
Biotechnical Production
Industrial is one citric acid by fermentation of sugar-containing raw materials such as molasses and corn ago. For the fermentation Aspergillus niger strains are used. Especially in the United States and China often come transgenic variants of the mold used in Europe, this is not permitted. In the citric acid production particularly three conditions must be met:
Purity and safety of citric acid on the market show large differences. Various Aspergillus Niger strains produce mycotoxins under certain growth conditions. The production of these substances does not take place under controlled conditions. In highly purified citric acid, there are no traces of mold.
Properties
Physical Properties
Citric acid forms in the anhydrous state rhombic crystals that taste slightly acidic. An aqueous solution of citric acid leads to the electric current, since the carboxyl groups and thus eliminate protons present mobile charge carriers (ions) in the solution. The acid dissociation constants of citric acid are PKS1 = 3.13, pKa2 = 4.76 and PKS3 = 6.4. The partially or completely dissociated acid group of citric acid is referred to as citrate.
Chemical Properties
Citric acid is considered one of the organic tricarboxylic through three carboxyl groups (-COOH). In addition, at the 3-position of the carbon backbone, the hydroxy group (-OH) from a hydroxycarboxylic acid.
Citric acid is one of many typical reactions of carboxylic acids, including
- Deprotonation / salt formation
- Substitution reactions at the carboxyl carbon: esterification
- Halogenation
- Anhydride
- Amidation
Citric acid can be oxidized with various oxidizing agents (eg peroxides or hypochlorites ). Depending on the reaction conditions, thereby β -ketoglutaric acid, oxalic acid, carbon dioxide and water can occur.
Taken in small amounts of citric acid promotes bone growth indirectly, because it promotes the absorption of calcium. However, in larger amounts it is toxic (LD50 in rats: 3 g / kg).
Use
Citric acid acts not only by its acidic action kalklösend, but also by the formation of a calcium complex. It is frequently used in cleaning agents to avoid the unpleasant smell of the vinegar cleaner. However, the complexes are not heat resistant and decompose when heated to relatively sparingly soluble calcium citrate. Citric acid can be ( cold to lukewarm ) for example, used for descaling kettles, immersion heaters, faucets, shower heads, dishwashers and washing machines.
Citric acid and its salts for the preservation and for homogenisation as well as molten salt used as the acidulant of foodstuffs, for example in beverages. In effervescent powder and sticks them together with sodium bicarbonate included. Citric acid is used in particular in sodas and iced tea, comes in fruit juices but also naturally. Citric acid is approved in the EU as a food additive under the E number 330 in most foods unlimited. An exception is, for example, chocolate products and fruit juices for which there is only a limited approval, and some foods, such as honey, milk and butter, for which no approval. Since it also acts as a peptizer, it is used for the preparation of stable suspensions. In fish it can convert biogenic amines in non-volatile salts, thus reducing the fishy odor.
Citric acid and citrates prevent blood clotting. Therefore, one preserved blood donations in bags containing a citrate buffer. For coagulation analyzes to citrate used in which blood with citrate in a ratio of 9:1 (9 parts blood and 1 part 0.11 mol / l sodium citrate) is mixed. One particular use is the use in cell separators. This is removed from the blood of a vein, in the device, the desired blood components are removed (e.g., platelets), and the remaining blood back into the vein. So that the blood does not form dangerous clots in the device, it is treated with citrate buffer.
Citric acid is used as a rinse solution in root canal treatment in dentistry.
Other applications of the salts of citric acid:
- Citric acid is used as a water softener and fabric softener alternative.
- Citric acid is used for the passivation of stainless steel. In this method the free iron fractions are dissolved from the stainless steel surface. Thus, the chromium to iron ratio is positively influenced, which leads to an improvement in the passivation layer, and thus to improve the corrosion resistance of stainless steel.
- Calcium - containing preparations are used in the wellness industry as a dietary supplement.
- Similar preparations are administered dogs for strengthening of teeth and bones.
- Magnesium citrate is a preparation or as a supplement to increase the magnesium levels in the body and thus prevent, for example, leg cramps, and increase the overall performance.
- Citric acid is used as a catalyst in the Biokohleherstellung by hydrothermal carbonization.
- The salts trisodium citrate and Trilithiumcitrat be in construction chemicals - used as retarders or accelerators for the curing of cementitious materials - depending on the amount added.
- Many basic drugs active ingredients are presented as citrates (eg, sildenafil citrate in Viagra).
- Citric acid is used for pH adjustment of cosmetics, such as skin care lotion or cream.
- In high doses, citric acid is used as a rust remover.
Biological Significance
Citrate is considered one of the most important inhibitors of urinary stone formation. Increased Citratwerte in the urine thus reducing the risk of formation of urinary stones. The excreted in the urine citrate is derived on the one hand from the metabolism ( TCA cycle ), on the other hand it is taken with food.