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Urea or carbamide is an organic compound with the chemical formula (NH2)2CO. The molecule has two amine (-NH2) residues joined by a carbonyl (-CO-) functional group.
Urea plays an important role in the metabolism of nitrogen-containing compounds by animals, and is the main nitrogen-containing substance in the urine of mammals. Being solid, colourless, odorless, neither acidic nor basic, highly soluble in water, and relatively non-toxic, urea is widely used in fertilizers as a convenient source of nitrogen. Urea is also an important feedstock for the chemical industry. The synthesis of this organic compound by Friedrich Wöhler 1828 from an inorganic precursor was an important milestone in the development of chemistry.
Uses
Agriculture
More than 90% of world production is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use (46.7%). Therefore, it has the lowest transportation costs per unit of nitrogen nutrient.
In the soil, it hydrolyses back to ammonia and carbon dioxide. The ammonia is oxidized by bacteria in the soil to nitrate which can be absorbed by the plants. Urea is also used in many multi-component solid fertilizer formulations. Urea is highly soluble in water and is, therefore, also very suitable for use in fertilizer solutions (in combination with ammonium nitrate: UAN), e.g., in 'foliar feed' fertilizers. For fertilizer use, granules are preferred over prills because of their narrower particle size distribution which is an advantage for mechanical application.
The most common impurity of synthetic urea, biuret, must be present at less than 2%, as it impairs plant growth.
Urea is usually spread at rates of between 40 and 300 kg/ha, but actual spreading rates will vary according to farm type and region. It is better to make several small to medium applications at intervals to minimise leaching losses and increase efficient use of the N applied, compared with single heavy applications. During summer, urea should be spread just before, or during rain to reduce possible losses from volatilization (process wherein nitrogen is lost to the atmosphere as ammonia gas). Urea should not be mixed for any length of time with other fertilizers, as problems of physical quality may result.
Because of the high nitrogen concentration in urea, it is very important to achieve an even spread. The application equipment must be correctly calibrated and properly used. Drilling must not occur on contact with or close to seed, due to the risk of germination damage. Urea dissolves in water for application as a spray or through irrigation systems.
In grain and cotton crops, urea is often applied at the time of the last cultivation before planting. It should be applied into or be incorporated into the soil. In high rainfall areas and on sandy soils (where nitrogen can be lost through leaching) and where good in-season rainfall is expected, urea can be side- or top-dressed during the growing season. Top-dressing is also popular on pasture and forage crops. In cultivating sugarcane, urea is side-dressed after planting, and applied to each ratoon crop.
In irrigated crops, urea can be applied dry to the soil, or dissolved and applied through the irrigation water. Urea will dissolve in its own weight in water, but it becomes increasingly difficult to dissolve as the concentration increases. Dissolving urea in water is endothermic, causing the temperature of the solution to fall when urea dissolves.
As a practical guide, when preparing urea solutions for fertigation (injection into irrigation lines), dissolve no more than 30 kg urea per 100 L water.
In foliar sprays, urea concentrations of 0.5% – 2.0% are often used in horticultural crops. As urea sprays may damage crop foliage, specific advice should be sought before use. Low-biuret grades of urea should be used if urea sprays are to be applied regularly or to sensitive horticultural crops.
Like most nitrogen products, urea absorbs moisture from the atmosphere. Therefore it should be stored either in closed/sealed bags on pallets, or, if stored in bulk, under cover with a tarpaulin. As with most solid fertilizers, it should also be stored in a cool, dry, well-ventilated area.
Chemical industry
Urea is a raw material for the manufactuer of many important chemicals, such as
* Various plastics, especially the urea-formaldehyde resins.
* Various adhesives, such as urea-formaldehyde or the urea-melamine-formaldehyde used in marine plywood.
* Potassium cyanate, another industrial feedstock.
* Urea nitrate, an explosive.
Urea has the ability to trap many organic compounds in the form of clathrates. The organic compounds are held in channels formed by interpenetrating helices comprising of hydrogen-bonded urea molecules. This behaviour can be used to separate mixtures, and has been used in the production of aviation fuel and lubricating oils, and in the separation of paraffins.
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.
Automobile systems
Urea is used in SNCR and SCR reactions to reduce the NOx pollutants in exhaust gases from combustion, for example, from power plants and diesel engines. The BlueTec system, for example, injects water-based urea solution into the exhaust system. The ammonia produced by decomposition of the urea reacts with the nitrogen oxide emissions and is converted into nitrogen and water within the catalytic converter.
Other commercial uses
* A stabilizer in nitrocellulose explosives.
* A component of animal feed, providing a relatively cheap source of nitrogen to promote growth.
* A non-corroding alternative to rock salt for road de-icing, and the resurfacing of snowboarding halfpipes and terrain parks.
* A flavor-enhancing additive for cigarettes.
* A browning agent in factory-produced pretzels.
* An ingredient in some hair conditioners, facial cleansers, bath oils, skin softeners, and lotions.
* A reactant in some ready-to-use cold compresses for first-aid use, due to the endothermic reaction it creates when mixed with water.
* A cloud seeding agent, along with other salts.
* A flame-proofing agent, commonly used in dry chemical fire extinguisher charges such as the urea-potassium bicarbonate mixture.
* An ingredient in many tooth whitening products.
* An ingredient in dish soap.
* Along with ammonium phosphate, as a yeast nutrient, for fermentation of sugars into ethanol.
* An nutrient used by plankton in ocean nourishment experiments for geoengineering purposes.
* 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.
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, a type of ionic liquid.
Urea can serve as a hydrogen source, for subsequent power generation in a fuel cell. 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 and uses less energy than by electrolysis of water.[4]
Medical use
Urea is used in topical dermatological products to promote rehydration of the skin. If covered by an occlusive dressing, 40% urea preparations may also be used for nonsurgical debridement of nails. This drug is also used as an earwax removal aid.
Like saline, urea injection is used to perform abortions. It is also the main component of an alternative medicinal treatment referred to as urine therapy.
The blood urea nitrogen (BUN) test is a measure of the amount of nitrogen in the blood that comes from urea. It is used as a marker of renal function.
Urea labeled with carbon-14 or carbon-13 is used in the urea breath test, which is used to detect the presence of the bacteria Helicobacter pylori (H. pylori) in the stomach and duodenum of humans, associated with ulcers. The test detects the characteristic enzyme urease, produced by H. pylori, by a reaction that produces ammonia from urea. This increases the pH (reduces acidity) of the stomach environment around the bacteria. Similar bacteria species to H. pylori can be identified by the same test in animals such as apes, dogs, and cats (including big cats).
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