Urea
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Names | |||
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Pronunciation | urea /jʊəˈriːə/, carbamide /ˈkɑːrbəmaɪd/ | ||
Preferred IUPAC name
Urea[1] | |||
Systematic IUPAC name
Carbonic diamide[1] | |||
Other names
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Identifiers | |||
3D model (
JSmol ) |
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635724 | |||
ChEBI | |||
ChEMBL | |||
ChemSpider | |||
DrugBank | |||
ECHA InfoCard
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100.000.286 | ||
E number | E927b (glazing agents, ...) | ||
1378 | |||
IUPHAR/BPS |
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KEGG | |||
PubChem CID
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RTECS number
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UNII | |||
CompTox Dashboard (EPA)
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Properties | |||
CO(NH2)2 | |||
Molar mass | 60.06 g/mol | ||
Appearance | White solid | ||
Density | 1.32 g/cm3 | ||
Melting point | 133 to 135 °C (271 to 275 °F; 406 to 408 K) | ||
Boiling point | decomposes | ||
545 g/L (at 25 °C)[2] | |||
Solubility | 500 g/L glycerol[3]
50 g/L ethanol | ||
Basicity (pKb) | 13.9[5] | ||
−33.4·10−6 cm3/mol | |||
Structure | |||
4.56 D | |||
ThermochemistryCRC Handbook | |||
Std enthalpy of (ΔfH⦵298)formation |
−333.19 kJ/mol | ||
Gibbs free energy (ΔfG⦵)
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−197.15 kJ/mol | ||
Pharmacology | |||
B05BC02 (WHO) D02AE01 (WHO) | |||
Hazards | |||
GHS labelling: | |||
NFPA 704 (fire diamond) | |||
Flash point | Non-flammable | ||
Lethal dose or concentration (LD, LC): | |||
LD50 (median dose)
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8500 mg/kg (oral, rat) | ||
Safety data sheet (SDS) | ICSC 0595 | ||
Related compounds | |||
Related ureas
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Thiourea Hydroxycarbamide | ||
Related compounds
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Urea, also called carbamide (because it is a
Urea serves an important role in the cellular
It is a colorless, odorless solid, highly soluble in water, and practically non-toxic (LD50 is 15 g/kg for rats).[6] Dissolved in water, it is neither acidic nor alkaline. The body uses it in many processes, most notably nitrogen excretion. The liver forms it by combining two ammonia molecules (NH3) with a carbon dioxide (CO2) molecule in the urea cycle. Urea is widely used in fertilizers as a source of nitrogen (N) and is an important raw material for the chemical industry.
In 1828, Friedrich Wöhler discovered that urea can be produced from inorganic starting materials, which was an important conceptual milestone in chemistry. This showed for the first time that a substance previously known only as a byproduct of life could be synthesized in the laboratory without biological starting materials, thereby contradicting the widely held doctrine of vitalism, which stated that only living organisms could produce the chemicals of life.
Properties
Molecular and crystal structure
The urea molecule is planar when in a solid crystal because of sp2 hybridization of the N orbitals.[7][8] It is non-planar with C2 symmetry when in the gas phase[9] or in aqueous solution,[8] with C–N–H and H–N–H bond angles that are intermediate between the trigonal planar angle of 120° and the tetrahedral angle of 109.5°. In solid urea, the oxygen center is engaged in two N–H–O hydrogen bonds. The resulting dense and energetically favourable hydrogen-bond network is probably established at the cost of efficient molecular packing: The structure is quite open, the ribbons forming tunnels with square cross-section. The carbon in urea is described as sp2 hybridized, the C-N bonds have significant double bond character, and the carbonyl oxygen is relatively basic. Urea's high aqueous solubility reflects its ability to engage in extensive hydrogen bonding with water.
By virtue of its tendency to form porous frameworks, urea has the ability to trap many organic compounds. In these so-called
As the helices are interconnected, all helices in a crystal must have the same molecular handedness. This is determined when the crystal is nucleated and can thus be forced by seeding. The resulting crystals have been used to separate racemic mixtures.[10]
Reactions
Urea is basic and is protonated readily. It is also a
Urea reacts with malonic esters to make barbituric acids.
Decomposition
Molten urea decomposes into ammonium cyanate at about 152 °C, and into ammonia and isocyanic acid above 160 °C:[11]
- CO(NH2)2 → [NH4]+[OCN]− → NH3 + HNCO
Heating above 160 °C yields biuret NH2CONHCONH2 and triuret NH2CONHCONHCONH2 via reaction with isocyanic acid:[12][11]
- CO(NH2)2 + HNCO → NH2CONHCONH2
- NH2CONHCONH2 + HNCO → NH2CONHCONHCONH2
At higher temperatures it converts to a range of condensation products, including cyanuric acid (CNOH)3, guanidine HNC(NH2)2, and melamine.[12][11]
In aqueous solution, urea slowly equilibrates with ammonium cyanate. This
Analysis
Urea is readily quantified by a number of different methods, such as the diacetyl monoxime colorimetric method, and the Berthelot reaction (after initial conversion of urea to ammonia via urease). These methods are amenable to high throughput instrumentation, such as automated flow injection analyzers[17] and 96-well micro-plate spectrophotometers.[18]
Related compounds
Uses
Agriculture
More than 90% of world industrial production of urea is destined for use as a nitrogen-release
Resins
Urea is a raw material for the manufacture of
Explosives
Urea can be used in a reaction with
Automobile systems
Urea is used in
- 4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O
When urea is used, a pre-reaction (hydrolysis) occurs to first convert it to ammonia:
- CO(NH2)2 + H2O → 2 NH3 + CO2
Being a solid highly
Laboratory uses
Urea in concentrations up to 10 M is a powerful protein denaturant as it disrupts the noncovalent bonds in the proteins. This property can be exploited to increase the solubility of some proteins. A mixture of urea and choline chloride is used as a deep eutectic solvent (DES), a substance similar to ionic liquid. When used in a deep eutectic solvent, urea gradually denatures the proteins that are solubilized.[20]
Urea can in principle serve as a hydrogen source for subsequent power generation in fuel cells. Urea present in urine/wastewater can be used directly (though bacteria normally quickly degrade urea). Producing hydrogen by electrolysis of urea solution occurs at a lower voltage (0.37 V) and thus consumes less energy than the electrolysis of water (1.2 V).[21]
Urea in concentrations up to 8 M can be used to make fixed brain tissue transparent to visible light while still preserving fluorescent signals from labeled cells. This allows for much deeper imaging of neuronal processes than previously obtainable using conventional one photon or two photon confocal microscopes.[22]
Medical use
Urea has also been studied as a diuretic. It was first used by Dr. W. Friedrich in 1892.[27] In a 2010 study of ICU patients, urea was used to treat euvolemic hyponatremia and was found safe, inexpensive, and simple.[28]
Like saline, urea has been injected into the uterus to induce abortion, although this method is no longer in widespread use.[29]
The
Urea has also been studied as an excipient in Drug-coated Balloon (DCB) coating formulation to enhance local drug delivery to stenotic blood vessels.
Urea labeled with
Miscellaneous uses
- An ingredient in .
- A component of nitrogento promote growth
- A non-corroding alternative to
- A main ingredient in hair removers such as Nair and Veet
- A browning agent in factory-produced pretzels
- An ingredient in some
- A cloud seeding agent, along with other salts[37]
- A flame-proofing agent, commonly used in dry chemical fire extinguisher charges such as the urea-potassium bicarbonatemixture
- An ingredient in many tooth whitening products
- An ingredient in dish soap
- Along with
- A nutrient used by ocean nourishment experiments for geoengineeringpurposes
- As an additive to extend the working temperature and open time of hide glue
- As a solubility-enhancing and moisture-retaining additive to dye baths for textile dyeing or printing[38]
- As an optical parametric oscillator in nonlinear optics[39][40]
Physiology
Amino acids from ingested food (or produced from catabolism of muscle protein) that are used for the synthesis of proteins and other biological substances can be oxidized by the body as an alternative source of energy, yielding urea and carbon dioxide.[41] The oxidation pathway starts with the removal of the amino group by a transaminase; the amino group is then fed into the urea cycle. The first step in the conversion of amino acids into metabolic waste in the liver is removal of the alpha-amino nitrogen, which produces ammonia. Because ammonia is toxic, it is excreted immediately by fish, converted into uric acid by birds, and converted into urea by mammals.[42]
Ammonia (NH3) is a common byproduct of the metabolism of nitrogenous compounds. Ammonia is smaller, more volatile, and more mobile than urea. If allowed to accumulate, ammonia would raise the pH in cells to toxic levels. Therefore, many organisms convert ammonia to urea, even though this synthesis has a net energy cost. Being practically neutral and highly soluble in water, urea is a safe vehicle for the body to transport and excrete excess nitrogen.
Urea is synthesized in the body of many organisms as part of the
In water, the amine groups undergo slow displacement by water molecules, producing ammonia,
Humans
The
By action of the
The equivalent nitrogen content (in
Other species
In
Adverse effects
Urea can be irritating to skin, eyes, and the respiratory tract. Repeated or prolonged contact with urea in fertilizer form on the skin may cause dermatitis.[47]
High concentrations in the blood can be damaging. Ingestion of low concentrations of urea, such as are found in typical human urine, are not dangerous with additional water ingestion within a reasonable time-frame. Many animals (e.g. camels, rodents or dogs) have a much more concentrated urine which may contain a higher urea amount than normal human urine.
Urea can cause algal blooms to produce toxins, and its presence in the runoff from fertilized land may play a role in the increase of toxic blooms.[48]
The substance decomposes on heating above melting point, producing toxic gases, and reacts violently with strong oxidants, nitrites, inorganic chlorides, chlorites and perchlorates, causing fire and explosion.[49]
History
Urea was first discovered in urine in 1727 by the Dutch scientist Herman Boerhaave,[50] although this discovery is often attributed to the French chemist Hilaire Rouelle as well as William Cruickshank.[51]
Boerhaave used the following steps to isolate urea:[52][53]
- Boiled off water, resulting in a substance similar to fresh cream
- Used filter paper to squeeze out remaining liquid
- Waited a year for solid to form under an oily liquid
- Removed the oily liquid
- Dissolved the solid in water
- Used recrystallization to tease out the urea
In 1828, the German chemist Friedrich Wöhler obtained urea artificially by treating silver cyanate with ammonium chloride.[54][55][56]
- AgNCO + [NH4]Cl → CO(NH2)2 + AgCl
This was the first time an organic compound was artificially synthesized from inorganic starting materials, without the involvement of living organisms. The results of this experiment implicitly discredited
- "I must tell you that I can make urea without the use of kidneys, either man or dog. Ammonium cyanate is urea."
In fact, his second sentence was incorrect.
Uremic frost was first described in 1865 by Harald Hirschsprung, the first Danish pediatrician in 1870 who also described the disease that carries his name in 1886. Uremic frost has become rare since the advent of dialysis. It is the classical pre-dialysis era description of crystallized urea deposits over the skin of patients with prolonged kidney failure and severe uremia.[58]
Historical preparation
Urea was first noticed by
Laboratory preparation
Ureas in the more general sense can be accessed in the laboratory by reaction of phosgene with primary or secondary amines:
- COCl2 + 4 RNH2 → (RNH)2CO + 2 [RNH3]+Cl−
These reactions proceed through an isocyanate intermediate. Non-symmetric ureas can be accessed by the reaction of primary or secondary amines with an isocyanate.
Urea can also be produced by heating ammonium cyanate to 60 °C.
- [NH4]+[OCN]− → (NH2)2CO
Industrial production
In 2020, worldwide production capacity was approximately 180 million tonnes.[64]
For use in industry, urea is produced from synthetic
Synthesis
The basic process, patented in 1922, is called the
The second is urea conversion: the slower
The overall conversion of NH3 and CO2 to urea is exothermic, with the reaction heat from the first reaction driving the second. The conditions that favor urea formation (high temperature) have an unfavorable effect on the carbamate formation equilibrium. The process conditions are a compromise: the ill-effect on the first reaction of the high temperature (around 190 °C) needed for the second is compensated for by conducting the process under high pressure (140–175 bar), which favors the first reaction. Although it is necessary to compress gaseous carbon dioxide to this pressure, the ammonia is available from the ammonia production plant in liquid form, which can be pumped into the system much more economically. To allow the slow urea formation reaction time to reach equilibrium, a large reaction space is needed, so the synthesis reactor in a large urea plant tends to be a massive pressure vessel.
Reactant recycling
Because the urea conversion is incomplete, the urea must be separated from the unconverted reactants, including the ammonium carbamate. Various commercial urea processes are characterized by the conditions under which urea forms and the way that unconverted reactants are further processed.
Conventional recycle processes
In early "straight-through" urea plants, reactant recovery (the first step in "recycling") was done by letting down the system pressure to atmospheric to let the carbamate decompose back to ammonia and carbon dioxide. Originally, because it was not economic to recompress the ammonia and carbon dioxide for recycle, the ammonia at least would be used for the manufacture of other products such as ammonium nitrate or ammonium sulfate, and the carbon dioxide was usually wasted. Later process schemes made recycling unused ammonia and carbon dioxide practical. This was accomplished by the "total recycle process", developed in the 1940s to 1960s and now called the "conventional recycle process". It proceeds by depressurizing the reaction solution in stages (first to 18–25 bar and then to 2–5 bar) and passing it at each stage through a steam-heated carbamate decomposer, then recombining the resulting carbon dioxide and ammonia in a falling-film carbamate condenser and pumping the carbamate solution back into the urea reaction vessel.[12]
Stripping recycle process
The "conventional recycle process" for recovering and reusing the reactants has largely been supplanted by a stripping process, developed in the early 1960s by Stamicarbon in The Netherlands, that operates at or near the full pressure of the reaction vessel. It reduces the complexity of the multi-stage recycle scheme, and it reduces the amount of water recycled in the carbamate solution, which has an adverse effect on the equilibrium in the urea conversion reaction and thus on overall plant efficiency. Effectively all new urea plants use the stripper, and many total recycle urea plants have converted to a stripping process.[12][67]
In the conventional recycle processes, carbamate decomposition is promoted by reducing the overall pressure, which reduces the partial pressure of both ammonia and carbon dioxide, allowing these gasses to be separated from the urea product solution. The stripping process achieves a similar effect without lowering the overall pressure, by suppressing the partial pressure of just one of the reactants in order to promote carbamate decomposition. Instead of feeding carbon dioxide gas directly to the urea synthesis reactor with the ammonia, as in the conventional process, the stripping process first routes the carbon dioxide through the stripper. The stripper is a carbamate decomposer that provides a large amount of gas-liquid contact. This flushes out free ammonia, reducing its partial pressure over the liquid surface and carrying it directly to a carbamate condenser (also under full system pressure). From there, reconstituted ammonium carbamate liquor is passed to the urea production reactor. That eliminates the medium-pressure stage of the conventional recycle process.[12][67]
Side reactions
The three main side reactions that produce impurities have in common that they decompose urea.
Urea hydrolyzes back to ammonium carbamate in the hottest stages of the synthesis plant, especially in the stripper, so residence times in these stages are designed to be short.[12]
Biuret is formed when two molecules of urea combine with the loss of a molecule of ammonia.
- 2 NH2CONH2 → NH2CONHCONH2 + NH3
Normally this reaction is suppressed in the synthesis reactor by maintaining an excess of ammonia, but after the stripper, it occurs until the temperature is reduced.[12] Biuret is undesirable in urea fertilizer because it is toxic to crop plants to varying degrees,[68] but it is sometimes desirable as a nitrogen source when used in animal feed.[69]
Isocyanic acid HNCO and ammonia NH3 results from the thermal decomposition of ammonium cyanate [NH4]+[OCN]−, which is in chemical equilibrium with urea:
- CO(NH2)2 → [NH4]+[OCN]− → HNCO + NH3
This decomposition is at its worst when the urea solution is heated at low pressure, which happens when the solution is concentrated for prilling or granulation (see below). The reaction products mostly volatilize into the overhead vapours, and recombine when these condense to form urea again, which contaminates the process condensate.[12]
Corrosion
Ammonium carbamate solutions are highly corrosive to metallic construction materials – even to resistant forms of stainless steel – especially in the hottest parts of the synthesis plant such as the stripper. Historically corrosion has been minimized (although not eliminated) by continuous injection of a small amount of oxygen (as air) into the plant to establish and maintain a passive oxide layer on exposed stainless steel surfaces. Highly corrosion resistant materials have been introduced to reduce the need for passivation oxygen, such as specialized duplex stainless steels in the 1990s, and zirconium or zirconium-clad titanium tubing in the 2000s.[12]
Finishing
Urea can be produced in solid forms (prills, granules, pellets or crystals) or as solutions.
Solid forms
For its main use as a fertilizer urea is mostly marketed in solid form, either as prills or granules. Prills are solidified droplets, whose production predates satisfactory urea granulation processes. Prills can be produced more cheaply than granules, but the limited size of prills (up to about 2.1 mm in diameter), their low crushing strength, and the caking or crushing of prills during bulk storage and handling make them inferior to granules. Granules are produced by acretion onto urea seed particles by spraying liquid urea in a succession of layers. Formaldehyde is added during the production of both prills and granules in order to increase crushing strength and suppress caking. Other shaping techniques such as pastillization (depositing uniform-sized liquid droplets onto a cooling conveyor belt) are also used.[12]
Liquid forms
Solutions of urea and ammonium nitrate in water (UAN) are commonly used as a liquid fertilizer. In admixture, the combined solubility of ammonium nitrate and urea is so much higher than that of either component alone that it gives a stable solution with a total nitrogen content (32%) approaching that of solid ammonium nitrate (33.5%), though not, of course, that of urea itself (46%). UAN allows use of ammonium nitrate without the explosion hazard.[12] In UAN accounts for 80% of the liquid fertilizers in the US.[70]
See also
References
- ^ ISBN 978-0-85404-182-4.
The compound H2N-CO-NH2 has the retained name 'urea', which is the preferred IUPAC name, with locants N and N′, as shown above the structure below. The systematic name is 'carbonic diamide', (…).
- ^ ISBN 9781439802465.
- ^ "Solubility of Various Compounds in Glycerine" (PDF). msdssearch.dow.com. Archived from the original (PDF) on 13 April 2014. Retrieved 12 April 2014.
- .
- ^ Calculated from 14−pKa. The value of pKa is given as 0.10 by the CRC Handbook of Chemistry and Physics, 49th edition (1968–1969). A value of 0.18 is given by Williams, R. (24 October 2001). "pKa Data" (PDF). Archived from the original (PDF) on 24 August 2003.
- ^ "Urea - Registration Dossier - ECHA". echa.europa.eu.
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- ^ ISSN 1520-6106.
- PMID 26371867.
- ^ ISBN 3-540-17307-2.
- ^ ISSN 0040-6031.
- ^ ISBN 978-3527306732.
- ^ Aldrich, Sigma. "Urea Solution Product Information" (PDF). Retrieved 7 February 2023.
- ^ OCLC 463300660.
- ISBN 978-0-12-182083-1. Retrieved 24 February 2023.
- PMID 24161613.
- S2CID 42485569.
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- ^ Duo et al., (1992). Can. J. Chem. Eng, 70, 1014–1020.
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- ^ Carow, Colleen (14 November 2008). "Researchers develop urea fuel cell". Ohio University (Press release). Archived from the original on 29 June 2017. Retrieved 6 January 2022.
- S2CID 28281721.
- ^ "UriSec 40 How it Works". Odan Laboratories. January 2009. Archived from the original on 2 February 2011. Retrieved 15 February 2011.
- ^ "UriSec 40% Cream". Odan Laboratories. Retrieved 20 August 2021.
- ISBN 978-0-323-08037-8.
- ^ "Carbamide Peroxide Drops GENERIC NAME(S): CARBAMIDE PEROXIDE". WebMD. Retrieved 19 August 2021.
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- ^ "Lacura Multi Intensive Serum – Review – Excellent value for money – Lacura Multi Intensive Serum "Aqua complete"". Dooyoo.co.uk. 19 June 2009. Retrieved 28 December 2010.
- .
- ^ Burch, Paula E. (13 November 1999). "Dyeing FAQ: What is urea for, in dyeing? Is it necessary?". All About Hand Dyeing. Retrieved 24 August 2020.
- ^ "Optical parametric oscillator using urea crystal". Google Patents.
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- PMID 14144484.
- ^ "Urea". Imperial College London. Retrieved 23 March 2015.
- ISBN 1-4160-2328-3. Page 837
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- ISBN 978-1-4051-3525-2.
- ^ PubChem. "urea cycle". pubchem.ncbi.nlm.nih.gov. Retrieved 28 June 2021.
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- ^ International Chemical Safety Cards: UREA. cdc.gov
- ^
Boerhaave called urea "sal nativus urinæ" (the native, i.e., natural, salt of urine). See:
- The first mention of urea is as "the essential salt of the human body" in: Peter Shaw and Ephraim Chambers, A New Method of Chemistry …, vol 2, (London, England: J. Osborn and T. Longman, 1727), page 193: Process LXXXVII.
- Boerhaave, Herman Elementa Chemicae …, volume 2, (Leipzig ("Lipsiae"), (Germany): Caspar Fritsch, 1732), page 276.
- For an English translation of the relevant passage, see: Peter Shaw, A New Method of Chemistry …, 2nd ed., (London, England: T. Longman, 1741), page 198: Process CXVIII: The native salt of urine
- Lindeboom, Gerrit A. Boerhaave and Great Britain …, (Leiden, Netherlands: E.J. Brill, 1974), page 51.
- Backer, H. J. (1943) "Boerhaave's Ontdekking van het Ureum" (Boerhaave's discovery of urea), Nederlands Tijdschrift voor Geneeskunde (Dutch Journal of Medicine), 87 : 1274–1278 (in Dutch).
- .
- ^ "Why Pee is Cool – entry #5 – "How Pee Unites You With Rocks"". Science minus details. 11 October 2011. Retrieved 9 August 2016.
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- ^ Wöhler, Friedrich (1828) "Ueber künstliche Bildung des Harnstoffs" (On the artificial formation of urea), Annalen der Physik und Chemie, 88 (2) : 253–256. Available in English at Chem Team.
- ^
ISBN 978-3-527-30983-2.
- ^
Gibb BC (April 2009). "Teetering towards chaos and complexity". Nature Chemistry. 1 (1): 17–8. PMID 21378787.
- .
- ^ "The discovery of urea and the end of vitalism - Hektoen International". hekint.org. 15 April 2024. Retrieved 17 April 2024.
- ^ Rouelle (1773) "Observations sur l'urine humaine, & sur celle de vache & de cheval, comparées ensemble" (Observations on human urine and on that of the cow and horse, compared to each other), Journal de Médecine, de Chirurgie et de Pharmacie, 40 : 451–468. Rouelle describes the procedure he used to separate urea from urine on pages 454–455.
- ^ Fourcroy and Vauquelin (1799) "Extrait d’un premier mémoire des cit. Fourcroy et Vaulquelin, pour servir à l’histoire naturelle, chimique et médicale de l’urine humaine, contenant quelques faits nouveaux sur son analyse et son altération spontanée" (Extract of a first memoir by citizens Fourcroy and Vauquelin, for use in the natural, chemical, and medical history of human urine, containing some new facts of its analysis and its spontaneous alteration), Annales de Chimie, 31 : 48–71. On page 69, urea is named "urée".
- ^ Fourcroy and Vauqeulin (1800) "Deuxième mémoire: Pour servir à l’histoire naturelle, chimique et médicale de l’urine humaine, dans lequel on s’occupe spécialement des propriétés de la matière particulière qui le caractérise," (Second memoir: For use in the natural, chemical and medical history of human urine, in which one deals specifically with the properties of the particular material that characterizes it), Annales de Chimie, 32 : 80–112; 113–162. On page 91, urea is again named "urée".
- ISBN 978-90-5699-645-1.
- PMID 20895332.
- ^ "Urea production statistics". www.ifastat.org. International Fertilizer Association. Retrieved 19 April 2023.
- ^ US 1429483, Carl Bosch & Wilhelm Meiser, "Process of Manufacturing Urea", issued 1922-09-19, assigned to BASF
- ^ a b Brouwer, Mark. "Thermodynamics of the Urea Process" (PDF). ureaknowhow.com. Retrieved 26 February 2023.
- ^ ISSN 0009-286X.
- S2CID 5970745.
- ISSN 0021-8812.
- PMID 34413867.
External links
- Urea in the Pesticide Properties DataBase (PPDB)