What is the pH of citric acid



Structural formula
General
Surname citric acid
other names
  • Citric acid
  • 3-carboxy-3-hydroxyglutaric acid
  • 3-carboxy-3-hydroxypentanedioic acid
  • 2-hydroxypropane-1,2,3-tricarboxylic acid
  • 2-hydroxy-1,2,3-propane tricarboxylic acid
  • E 330
Molecular formula C.6H8O7
CAS number 77-92-9 (anhydrous)
5949-29-1 (monohydrate)
Brief description colorless, odorless solid[1]
properties
Molar mass 192.43 g mol−1
Physical state firmly
density 1.665 g cm−3 (18 ° C)[1]
Melting point 153 ° C[1]
boiling point Decomposition: from 175 ° C[1]
Vapor pressure

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solubility
safety instructions
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions.

citric acid (mainly in technical terms: Citric acid, systematically according to IUPAC: 2-hydroxypropane-1,2,3-tricarboxylic acid) is a colorless, water-soluble solid acid. With the term Citrate denotes a salt of citric acid and generally deprotonated forms.

In biochemistry, the term citrate is often used when referring to the dissociated ionic form of citric acid that occurs in the aqueous environment of a cell.

In addition to the anhydrous variant, there is also citric acid monohydrate (C.6H8O7 · H2O), which contains one molecule of crystal water per molecule of citric acid.

history

Citric acid was first isolated from lemon juice by Carl Wilhelm Scheele in 1784 - hence the name. However, citric acid was probably already known to the first alchemists, albeit under a different name. The Arab alchemist Jabir ibn Hayyān (Geber) is said to have discovered citric acid as early as the 9th century.

Occurrence

Citric acid is one of the most common acids in the plant kingdom and occurs as a metabolic product in all organisms. For example, lemon juice contains 5-7% citric acid. But it also occurs in apples, pears, raspberries, currants, conifers, mushrooms, tobacco leaves, in wine and even in milk.

This distribution is due to the fact that citric acid occurs as an intermediate product that gives it its name in the so-called citric acid cycle (also known as the tricarboxylic acid cycle, Krebs cycle), which plays a key role in the carbohydrate and fatty acid metabolism of all oxygen-consuming organisms, including humans. This cycle also provides the basic molecular structures for the construction of most of the amino acids.

Extraction and presentation

Manufacture from citrus fruits

Citric acid was obtained from citrus fruits using the original method: concentrated ammonia solution is added to lemon juice, thickened and filtered. The easily soluble ammonium citrate is converted into less soluble calcium citrate by means of a precipitation reaction with calcium chloride. The solution is filtered again and the filter cake is dissolved in 25% sulfuric acid, with calcium sulphate (gypsum) that is even more poorly soluble precipitating out. After another filtration, a citric acid solution is obtained. The pure citric acid is obtained by crystallization.

Biotechnological manufacturing

Citric acid is now used industrially with the help of a transgenic variant of the mold Aspergillus niger won. There are three main requirements for this:

  1. High glucose and oxygen content in the nutrient medium.
  2. Low pH (pH
  3. Low Fe2+-Concentration (

properties

In the anhydrous state, citric acid forms rhombic crystals that taste slightly sour. Ingested in small amounts, citric acid indirectly promotes bone growth because it promotes the absorption of calcium. In larger quantities, however, it has a toxic effect (LD50 in rats: 3 g / kg).

Citric acid is counted among the organic tricarboxylic acids (see carboxylic acid) due to its three carboxyl groups (–COOH). In addition, the hydroxyl group (-OH) at position 3 of the carbon backbone identifies it as a hydroxycarboxylic acid.

An aqueous solution of citric acid conducts the electric current, as the carboxyl groups split off protons and thus mobile charge carriers (ions) are present in the solution. The three acid dissociation constants of citric acid are pK1 = 3.14, pK2 = 4.76 and pK3 = 6.39. The partially or completely dissociated acid residue of citric acid is referred to as citrate (citrate).

Chemical properties

Citric acid can undergo many of the reactions described for carboxylic acids, among others

Citric acid can be oxidized with various oxidizing agents (e.g. peroxides or hypochlorites). Depending on the reaction conditions, this can β-Ketoglutaric acid, oxalic acid, carbon dioxide and water are formed.

use

Citric acid has a lime-dissolving effect not only through its acidic effect, but also through the formation of a calcium complex and is often used in cleaning agents. This avoids the unpleasant smell of vinegar cleaners. However, the complexes are not heat-resistant and decompose when heated to form calcium citrate that is relatively difficult to dissolve.

Citric acid can be used for descaling e.g. B. be used by kettles, immersion heaters, taps, shower heads or by dishwashers or washing machines.

Citric acid and its salts are used to preserve and acidify food, for example in beverages. It is contained in effervescent powder and sticks together with sodium hydrogen carbonate. Citric acid is especially used in lemonades and iced tea, but it also occurs naturally in fruit juices. Citric acid is number one in the EU as a food additive E 330 authorized.

If the pH value is significantly reduced by acidified foods or acids that can form from sugar in the mouth, this can damage the tooth enamel (see dental caries). Citric acid is not specifically responsible for this, but also other acids used in foods or naturally present (e.g. phosphoric acid, ascorbic acid and fruit acids). With a normal diet, however, these acids are buffered by the saliva components in such a way that no damage occurs. On the other hand, filling baby bottles with sugared lemon tea through constant stress led to devastating tooth damage in several years of toddlers until the problem was recognized and remedied.

Citric acid and its salts (citrates) prevent blood clotting. That is why donated blood is preserved in bags containing a citrate buffer solution. The normally viscous blood is also diluted with citrate buffer for analyzes. A special use is in cell separators. Blood is taken from a vein, the desired blood components (e.g. platelets) are filtered out in the device and the remaining blood is returned to the vein. Citrate buffer is added to the blood in the device so that it does not form dangerous clots.

Citric acid is also used as a rinse solution for root canal treatments in dentistry.

Further areas of application for the salts of citric acid:

  • Citric acid is used as a water softener and alternative fabric softener.
  • Citric acid is used to passivate stainless steel. In this process, the free iron components are removed from the stainless steel surface. This has a positive influence on the chromium-iron ratio, which leads to an improvement in the passive layer and thus to an improvement in the corrosion protection of stainless steel.
  • Calcium citrate-containing preparations are touted as dietary supplements by the wellness industry. Similar preparations are also given to dogs to strengthen their teeth and bones.
  • Magnesium citrate is said to increase the magnesium level in the body as a preparation or as a dietary supplement. B. Prevent calf cramps and increase general performance.
  • The salts trisodium citrate and trilithium citrate are used in construction chemistry - depending on the amount added - as a retarder or accelerator for the hardening of cementitious compounds.
  • Many basic active pharmaceutical ingredients are given in a citrate form (e.g. sildenafil citrate in Viagra).

Biological importance

Citrate is one of the most important inhibitors of urinary stone formation. Decreased levels of citrate in the urine therefore increase the risk of the formation of urinary stones. The citrate excreted in the urine comes on the one hand from the metabolism (citric acid cycle), on the other hand it is absorbed with food.

swell

  1. abcdefGH Entry on citric acid in the GESTIS substance database of the BGIA, accessed on December 2, 2007 (JavaScript required)

literature

  • Rolf D. Schmid: Pocket atlas of biotechnology and genetic engineering. 2., act. and exp. Wiley VCH, 2006, ISBN 3527313109

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