Urea

• Kohlensäurediamid
• Carbamide
• UREA (INCI)
• Carbonyldiamid
• Diamide of carbonic acid
• Diaminomethanal
• Piagran (brand name)
• E927b

• B05BC02
• D02AE01

Colorless and odorless, crystalline solid

Fixed

1.32 g · cm -3

From 132.5 to 134.5 ° C ( decomposition)

0.2 Pa ( 75 ° C)

• 26.9 (in DMSO )
• 0.18 ( subj acid in water)

Very well in water:

• 790 g · l -1 ( at 5 ° C)
• 1000 g · l-1 (at 20 ° C)
• 1200 g · l-1 (at 25 ° C)

8471 mg · kg -1 ( LD50, mouse, oral)

-333.1 KJ · mol -1

Template: Infobox chemical / molecular formula search available

Urea (Latin and English urea ), chemically Kohlensäurediamid, is an organic compound that is used by many living things as an end product of the metabolism of nitrogen compounds produced (eg, amino acids) in the so-called urea cycle and excreted in the urine. Purity Urea is a white, crystalline, non-toxic and hygienically acceptable solid, which is not to be confused with uric acid.

History

Urea was first synthesized in 1828 by Friedrich Wöhler prepared by reaction of potassium cyanate and ammonium sulfate.

Urea is considered the first synthesized from inorganic starting materials organic compound. This contradicted the then widespread notion that organic substances could in principle be produced only by living organisms through the so -called " vis vitalis " ( life force ). In fact, Robert 1824 provided by the hydrolysis of cyanogen to oxalic acid to demonstrate that it is the synthesis of organic molecules is not the " life force " needs Wöhler.

In Goethe Humunculus urea synthesis is prominent literary processing.

Physiological significance

Urea is formed in the protein and amino acid metabolism and is one of the urinary metabolites. In mammals, turtles, some fish, and in adult amphibians, he is the main excretion form of the nitrogen contained in particular in proteins dar. birds and most reptiles form uric acid instead; Tadpoles, most species of fish and other aquatic animals excrete the nitrogen than ammonia.

Ammonia is formed when amino acid breakdown and acts in accordance with high concentrations toxic to cells. In order to counteract an increase in the concentration of ammonia, it is converted via the urea cycle in the non-toxic urea and excreted by the kidneys.

Diseases, such as acute or chronic renal failure, as well as a diabetic kidney function, may lead to increased levels of urea in serum / plasma ( normal value: 10-50 mg / dl).

Disorders and special features

A high protein intake also results already with normal renal function to increased levels of urea, which makes him a bad kidney parameters. At ( pre) -stage renal disease, the urea concentration in the serum, however, is more suitable to estimate the severity of uremia, as the serum creatinine concentration.

Applications

Due to its high nitrogen content of 46.62 % urea is the most important nitrogen fertilizer worldwide.

Urea is often used because of its high water-binding capacity beyond as moisturizing factor in cosmetics. In pharmacy is known urea as a keratolytic. This property makes you look at different recipes advantage. For example, he has a highly concentrated ( 40%) in pastes together with an antifungal agent ( antifungal) against nail fungus (onychomycosis ), wherein the urea makes the nail so soft that the infected nail substance can pay off piece by piece. He also serves as a moisturizer in creams to combat atopic eczema and lichen diseases.

Cigarette manufacturers mixing urea with the tobacco so that the nicotine may be absorbed better by increasing the pH value in the lung. Thus, the effect supposedly lighter cigarettes is amplified.

Urea can also be used as road salt substitute, but this is not economical because of its higher price. In Vienna be of relevant use - leading to over-fertilization of the soil - forbidden.

Urea is used in the reduction of nitrogen oxides in the exhaust gas of power plants and combustion engines. In power plants - primarily for smaller systems - SNCR (selective non-catalytic reduction ) applied. The so-called SCR ( Selective Catalytic Reduction ), which is used in power plants and also increasingly in automotive engineering, urea or ammonia is injected into the hot exhaust stream. Urea decomposes to form ammonia, which reduces the nitrogen oxides in the downstream catalytic converter. In automotive engineering, an aqueous solution is used with a 32.5 % share of urea, which is known under the trade name AdBlue. The consumption of the urea solution is about 2 to 8% of the fuel consumption.

Urea is added to foods as a stabilizer. In the EU it is approved as a food additive with the designation E 927b exclusively for chewing gum with no added sugar.

In the dairy cattle feeding urea plays a role as a source of protein.

The addition of urea in higher concentrations to aqueous solutions leads to a denaturation of proteins, urea therefore acts as a denaturant or as a chaotropic compound. However, small concentrations of urea can unfold the opposite effect, namely strengthening the hydrophobic effect and thus stabilize the protein structure.

Properties

Urea condenses on heating above the melting point, with elimination of ammonia ( NH3) to biuret:

Urea reacts with the enzyme urease to carbon dioxide and ammonia.

Industrial production

Urea is produced industrially in large amounts ( 2012: approximately 184 million tonnes worldwide). The first production plant was taken in 1922 by BASF in operation. The ammonium carbamate in a high pressure reactor in the first step at 150 bar formed was subsequently thermally cleaved in decomposers in urea. Excess ammonia was used to produce ammonium sulfate and ammonium nitrate as a fertilizer. In the late 1920s, the process was improved and excess ammonia is returned to the production process. From this developed several total circulation process. All methods have in common that the endothermic in a reactor at high pressure ammonium carbamate formed exothermically in subsequent Zersetzerstufen at low pressure is converted into urea and excess gases are recycled to the reactor, we now employs economic stripping process at the moment.

In countries with large natural gas deposits that were previously often flared simple, natural gas is now converted into urea. These are large plants, which eventually produced from natural gas, air and water in the process steps of hydrogen production ammonia production → → urea synthesis of urea. ( The starting materials mentioned are Strictly implemented in a process gas consisting of H2, N2 and CO2, from which the CO2 is removed. Hydrogen and nitrogen are then converted into ammonia. Pure hydrogen does not come in this chain before ). The separated CO2 to the production of ammonia will be bound only to 2 /3 of the urea. The initially obtained in solution of urea is converted into pellets and sold as sack or loose. The largest plants in the world produce about 4,000 tonnes of urea per day.

Reaction equations:

Possibility of industrial use, urea is the preparation of melamine, which is processed to synthetic resins, for example, with formaldehyde, and urea -formaldehyde resins ( urea resin, so-called UF- resins), which used, for example for the production of chipboard be. Otherwise urea is used mainly as a nitrogenous fertilizer or as a NOx reducing agent in the SNCR process.

Urea as a mineral

Urea was found in 1973 as a natural secondary mineral in Toppin Hill at Lake Rason (Western Australia). It was recognized by the International Mineralogical Association (IMA ) as a separate mineral. This leads it according to the classification of minerals according to Strunz ( 9th edition ) as " Miscellaneous organic compounds " under the system no. " 10.CA.35 ". The also common in English-speaking classification of minerals according to Dana leads the mineral under the system no. " 50.4.6.1 ". Urea crystallizes in the tetragonal crystal system. It develops colorless to pale - yellow or pale - brown, needle-shaped crystals.

Urea is a component of the urine or the feces of birds and bats and therefore part in guano and bat guano ( Chiropterit ). The mineral urea is not stable and can only be formed under arid conditions.