Urea or carbamide is an organic compound with the chemical
formula CO(NH2). The molecule has two —NH2 groups joined by a carbonyl (C=O)
functional group.
Urea serves an important role in the metabolism of
nitrogen-containing compounds by animals and is the main nitrogen-containing
substance in the urine of mammals. It is a colorless, odorless solid, highly
soluble in water and practically non-toxic (LD50 is 15 g/kg for rat). Dissolved
in water, it is neither acidic nor alkaline. The body uses it in many
processes, the most notable one being nitrogen excretion. Urea is widely used
in fertilizers as a convenient source of nitrogen. Urea is also an important
raw material for the chemical industry.
The discovery by Friedrich Wöhler in 1828 that urea can be
produced from inorganic starting materials was an important conceptual
milestone in chemistry, as it showed for the first time that a substance
previously known only as a byproduct of life could be synthesized in the
laboratory without any biological starting materials, contradicting the widely
held doctrine of vitalism.
Related compounds
The terms urea is also used for a class of chemical
compounds sharing the same functional group RR'N—CO—NRR', namely a carbonyl
group attached to two organic amine residues. Examples include carbamide
peroxide, allantoin, and hydantoin. Ureas are closely related to biurets and
related in structure to amides, carbamates, carbodiimides, and thiocarbamides.
History
Urea was first discovered in urine in 1727 by the Dutch scientist
Herman Boerhaave, though this discovery is often attributed to the French
chemist Hilaire Rouelle.
In 1828, the German chemist Friedrich Wöhler obtained urea
artificially by treating silver cyanate with ammonium chloride.
AgNCO + NH4Cl → (NH2)2CO
+ 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
vitalism — the theory that the chemicals of living organisms are fundamentally
different from those of inanimate matter. This insight was important for the
development of organic chemistry. His discovery prompted Wöhler to write
triumphantly to Berzelius: "I must tell you that I can make urea without
the use of kidneys, either man or dog. Ammonium cyanate is urea." For this
discovery, Wöhler is considered by many[who?] the father of organic chemistry.
Physiology
Urea is synthesized in the body of many organisms as part of
the urea cycle, either from the oxidation of amino acids or from ammonia. In
this cycle, amino groups donated by ammonia and L-aspartate are converted to
urea, while L-ornithine, citrulline, L-argininosuccinate, and L-arginine act as
intermediates. Urea production occurs in the liver and is regulated by
N-acetylglutamate. Urea is then dissolved into the blood (in the reference
range of 2.5 to 6.7 mmol/liter) and further transported and excreted by the
kidney as a component of urine. In addition, a small amount of urea is excreted
(along with sodium chloride and water) in sweat.
Amino acids from ingested food that are not used for the
synthesis of proteins and other biological substances are oxidized by the body,
yielding urea and carbon dioxide, as an alternative source of energy.[8] The
oxidation pathway starts with the removal of the amino group by a transaminase;
the amino group is then fed into the urea cycle.
Ammonia (NH3) is another 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.
In water, the amine groups undergo slow displacement by
water molecules, producing ammonia, ammonium ion, and bicarbonate ion. For this
reason, old, stale urine has a stronger odor than fresh urine.
In humans
The handling of urea by the kidneys is a vital part of
mammalian metabolism. Besides its role as carrier of waste nitrogen, urea also
plays a role in the countercurrent exchange system of the nephrons, that allows
for re-absorption of water and critical ions from the excreted urine. Urea is
reabsorbed in the inner medullary collecting ducts of the nephrons,[9] thus
raising the osmolarity in the medullary interstitium surrounding the thin
ascending limb of the loop of Henle, which in turn causes water to be
reabsorbed. By action of the urea transporter 2, some of this reabsorbed urea
will eventually flow back into the thin ascending limb of the tubule, through
the collecting ducts, and into the excreted urine. This mechanism, which is
controlled by the antidiuretic hormone, allows the body to create hyperosmotic
urine, that has a higher concentration of dissolved substances than the blood
plasma. This mechanism is important to prevent the loss of water, to maintain
blood pressure, and to maintain a suitable concentration of sodium ions in the
blood plasmas.
The equivalent nitrogen content (in gram) of urea (in mmol)
can be estimated by the conversion factor 0.028 g/mmol. Furthermore, 1 gram of
nitrogen is roughly equivalent to 6.25 grams of protein, and 1 gram of protein
is roughly equivalent to 5 grams of muscle tissue. In situations such as muscle
wasting, 1 mmol of excessive urea in the urine (as measured by urine volume in
litres multiplied by urea concentration in mmol/l) roughly corresponds to a
muscle loss of 0.67 gram.
In other species
In aquatic organisms the most common form of nitrogen waste
is ammonia, whereas land-dwelling organisms convert the toxic ammonia to either
urea or uric acid. Urea is found in the urine of mammals and amphibians, as
well as some fish. Birds and saurian reptiles have a different form of nitrogen
metabolism that requires less water, and leads to nitrogen excretion in the
form of uric acid. It is noteworthy that tadpoles excrete ammonia but shift to
urea production during metamorphosis. Despite the generalization above, the
urea pathway has been documented not only in mammals and amphibians but in many
other organisms as well, including birds, invertebrates, insects, plants, yeast,
fungi, and even microorganisms.
Production
Urea is produced on an industrial scale: In 2012, worldwide
production capacity was approximately 184 million tonnes.
Industrial methods
For use in industry, urea is produced from synthetic ammonia
and carbon dioxide. As large quantities of carbon dioxide are produced during
the ammonia manufacturing process as a byproduct from hydrocarbons
(predominantly natural gas, less often petroleum derivatives), or occasionally
from coal, urea production plants are almost always located adjacent to the
site where the ammonia is manufactured. Although natural gas is both the most
economical and the most widely available ammonia plant feedstock, plants using
it do not produce quite as much carbon dioxide from the process as is needed to
convert their entire ammonia output into urea. In recent years new technologies
such as the KM-CDR process have been developed to recover supplementary carbon
dioxide from the combustion exhaust gases produced in the fired reforming
furnace of the ammonia synthesis gas plant, allowing operators of stand-alone
nitrogen fertilizer complexes to avoid the need to handle and market ammonia as
a separate product and also to reduce their 'greenhouse gas' emissions to the
atmosphere.
Synthesis
The basic process, developed in 1922, is also called the
Bosch–Meiser urea process after its discoverers. The various commercial urea
processes are characterized by the conditions under which urea formation takes
place and the way in which unconverted reactants are further processed. The
process consists of two main equilibrium reactions, with incomplete conversion
of the reactants. The first is carbamate formation: the fast exothermic
reaction of liquid ammonia with gaseous carbon dioxide (CO2) at high
temperature and pressure to form ammonium carbamate (H2N-COONH4):
2NH3 + CO2 is in
equilibrium with H2N-COONH4
The second is urea conversion: the slower endothermic
decomposition of ammonium carbamate into urea and water:
H2N-COONH4 is in
equilibrium with (NH2)2CO + H2O
The overall conversion of NH3 and CO2 to urea is exothermic,
the reaction heat from the first reaction driving the second. Like all chemical
equilibria, these reactions behave according to Le Chatelier's principle, and
the conditions that most favour carbamate formation have an unfavourable effect
on the urea conversion equilibrium. The process conditions are, therefore, a
compromise: the ill-effect on the first reaction of the high temperature
(around 190̂̂°C) needed for the second is compensated for by conducting the
process under high pressure (140–175 bar), which favours the first reaction.
Although it is necessary to compress gaseous carbon dioxide to this pressure,
the ammonia is available from the ammonia 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.
Because the urea conversion is incomplete, the product has
to be separated from unchanged ammonium carbamate. In early
"straight-through" urea plants this was done by letting down the
system pressure to atmospheric so as to allow the carbamate to 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, for example ammonium
nitrate or sulfate. (The carbon dioxide would be wasted, as likely as not.)
Later process schemes were developed to allow recycling of the unused ammonia
and carbon dioxide. This was accomplished 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 resultant carbon dioxide and ammonia in a falling-film
"carbamate condenser" and pumping the carbamate solution into the previous
stage.
The stripping concept
There are two main disadvantages to the "total
recycle" concept just outlined. The first is the complexity of the flow
scheme and, consequently, the amount of process equipment needed; the second is
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 the
overall efficiency of the plant. The stripping concept, developed in the early
1960s by Stamicarbon in The Netherlands, addressed both problems. It also
improved heat recovery and reuse in the process.
The position of the equilibrium in the carbamate
formation/decomposition depends on the product of the partial pressures of the
reactants. In the total recycle processes carbamate decomposition is promoted
by reducing the overall pressure, which reduces the partial pressure of both
ammonia and carbon dioxide. But it is possible to achieve a similar effect
without lowering the overall pressure by suppressing the partial pressure of
just one of the reactants. Instead of being fed directly to the reactor along
with the ammonia, as in the total recycle process, in the stripping process the
carbon dioxide gas is first routed through a stripper (a carbamate decomposer
operating under the full system pressure which is configured to provide maximum
gas-liquid contact), flushing out free ammonia and thus reducing its partial
pressure over the liquid surface, and carrying it directly to a carbamate
condenser (also operating at full system pressure), from which reconstituted
ammonium carbamate liquor passes directly to the reactor. That allows the
medium-pressure stage of the total recycle process to be omitted altogether.
The stripping concept proved to be such a major advance that
competitors such as Snamprogetti – now Saipem – (Italy), the former Montedison
(Italy), Toyo Engineering Corporation (Japan) and Urea Casale (Switzerland) all
developed their own versions of it. Today effectively all new urea plants use
the principle, and many total recycle urea plants have been converted to
stripping processes. No radical alternative to it has been proposed; the main
thrust of technological development today, in response to industry demands for
ever larger individual plants, is directed at reconfiguring and reorientating major
items in the plant to reduce their size and the overall height of the plant, as
well as at meeting ever more challenging environmental performance targets.
Side reactions
It is fortunate that the urea conversion reaction is a slow
one, because if it were not it would go into reverse in the stripper. As it is,
the succeeding stages of the process must be designed to minimize residence
times, at least until the temperature has been reduced to the point where the
reversion reaction is very slow.
Two reactions produce impurities. Biuret is formed when two
molecules of urea combine with the loss of a molecule of ammonia.
2NH2CONH2 →
H2NCONHCONH2 + NH3
Normally this reaction is suppressed in the synthesis
reactor by maintaining an excess of ammonia, but after the stripper it will
occur until the temperature is reduced. Biuret is undesirable in fertilizer
urea because it is toxic to crop plants, although to what extent depends on the
nature of the crop and the method of application of the urea.[18] (Biuret is
actually welcome in urea when is used as a cattle feed supplement.)
Isocyanic acid results from the thermal decomposition of
ammonium cyanate, with which urea is interchangeable:
NH2CONH2 → NH4NCO
→ HNCO + NH3
This reaction 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 are condensed to form urea
again, which contaminates the process condensate.
Corrosion
Ammonium carbamate solutions are notoriously corrosive
towards metallic materials of construction, even the more resistant forms of
stainless steel, especially in the hottest parts of the plant such as the
stripper. Traditionally corrosion has been minimized (although not eliminated)
by continuously injecting a minor amount of oxygen (as air) into the plant to
establish and maintain a passive oxide layer on exposed stainless steel
surfaces. But because the carbon dioxide feed is recovered from ammonia
synthesis gas it contains traces of hydrogen which can mingle with the
passivation air to form an explosive mixture if allowed to accumulate in the
plant. In the mid 1990s two duplex (ferritic-austenitic) stainless steels were
introduced (DP28W, jointly developed by Toyo Engineering and Sumitomo Metals
Industries and Safurex, jointly
developed by Stamicarbon and Sandvik Materials Technology (Sweden) which have allowed the amount of passivation
oxygen to be drastically reduced and can, in theory, operate without oxygen at
all.
Saipem now uses either zirconium stripper tubes or
bimetallic tubes comprising a titanium body (which is cheaper but less
erosion-resistant) to which is metallurgically bonded an internal lining of
zirconium. These tubes are fabricated by ATI Wah Chang (USA), a leading
specialist in refractory and 'reactive' metals, using its Omegabond technique.
Finishing
Urea can be produced as prills, granules, pellets, crystals,
and solutions.
Solid forms
For its main use as a fertilizer urea is mostly marketed in
solid form, either as prills or granules. The advantage of prills is that, in
general, they can be produced more cheaply than granules and that the technique
was firmly established in industrial practice long before a satisfactory urea
granulation process was commercialized. However, on account of the limited size
of particles that can be produced with the desired degree of sphericity and
their low crushing and impact strength, the performance of prills during bulk
storage, handling and use is generally (although not always!) considered
inferior to that of granules.
High-quality compound fertilizers containing nitrogen
co-granulated with other components such as phosphates have been produced
routinely since the beginnings of the modern fertilizer industry, but on
account of the low melting point and hygroscopic nature of urea it took courage
to apply the same kind of technology to granulate urea on its own. But at the
end of the 1970s three companies began to develop fluidized-bed granulation.
The first in the field was Nederlandse Stikstof Maatschappij, which later
became part of Hydro Agri (now Yara International). Yara eventually sold this
technology to Uhde GmbH, whose Uhde Fertilizer Technology (UFT) subsidiary now
markets it. Around the same time Toyo Engineering Corporation developed its
spouted-bed process, comprising a fluidized bed deliberately agitated to produce
turbulent ebullation. Stamicarbon also undertook development work on its own
fluidized-bed granulation system, using film sprays rather than atomizing
sprays to introduce the urea melt, but shelved it until the 1990s, when there
was for a time considerable doubt about the commercial future of the Hydro
(UFT) process. As a result, the Stamicarbon technology is now commercialized
and highly successful. More recently, Urea Casale has introduced a
fluidized-bed granulation system with a difference: the urea is sprayed in
laterally from the side walls of the granulator instead of from the bottom so
that the bed organizes into two cylindrical masses contrarotating on parallel
longitudinal axes. The raw product is stated to be so uniform that screens are
unnecessary.
Surprisingly, perhaps, considering the product particles are
anything but spherical, pastillation using a Rotoform steel-belt pastillator is
beginning to gain ground as a urea particle-forming process as a result of
development work by Stamicarbon in collaboration with Sandvik Process Systems
(Germany). Single-machine capacity is limited to 175 t/d, but the machines are
simple and need little maintenance, specific power consumption is much lower
than for granulation, and the product is very uniform. The robustness of the
product appears to make up for its very non-spherical shape.
UAN solutions
In admixture, the combined solubility of ammonium nitrate
and urea is so much higher than that of either component alone that it is
possible to obtain a stable solution (known as UAN) with a total nitrogen
content (32%) approaching that of solid ammonium nitrate (33.5%), though not,
of course, that of urea itself (46%). Given the ongoing safety and security
concerns surrounding fertilizer-grade solid ammonium nitrate, UAN provides a
considerably safer alternative without entirely sacrificing the agronomic
properties that make ammonium nitrate more attractive than urea as a fertilizer
in areas with short growing seasons. It is also more convenient to store and
handle than a solid product and easier to apply accurately to the land by
mechanical means.
Laboratory preparation
Ureas in the more general sense can be accessed in the
laboratory by reaction of phosgene with primary or secondary amines, proceeding
through an isocyanate intermediate. Non-symmetric ureas can be accessed by
reaction of primary or secondary amines with an isocyanate.
Also, urea is produced when phosgene reacts with ammonia:
COCl2 + 4 NH3 →
(NH2)2CO + 2 NH4Cl
Historical process
Urea was first noticed by Hermann Boerhaave in the early
18th century from evaporates of urine. In 1773, Hilaire Rouelle obtained
crystals containing urea from human urine by evaporating it and treating it
with alcohol in successive filtrations.This method was aided by Carl Wilhelm
Scheele's discovery that urine treated by concentrated nitric acid precipitated
crystals. Antoine François, comte de Fourcroy and Louis Nicolas Vauquelin
discovered in 1799 that the nitrated crystals were identical to Rouelle's
substance and invented the term "urea." Berzelius made further
improvements to its purification and finally William Prout, in 1817, succeeded
in obtaining and determining the chemical composition of the pure substance. In
the evolved procedure, urea was precipitated as urea nitrate by adding strong
nitric acid to urine. To purify the resulting crystals, they were dissolved in
boiling water with charcoal and filtered. After cooling, pure crystals of urea
nitrate form. To reconstitute the urea from the nitrate, the crystals are
dissolved in warm water, and barium carbonate added. The water is then
evaporated and anhydrous alcohol added to extract the urea. This solution is
drained off and allowed to evaporate resulting in pure urea.
Chemical properties
Molecular and crystal structure
The urea molecule is planar in the crystal structure, but
the geometry around the nitrogens is pyramidal in the gas-phase minimum-energy
structure. 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 basic compared
to, say, formaldehyde. Urea's high aqueous solubility reflects its ability to engage
in extensive hydrogen bonding with water.
By virtue of its tendency to form a porous frameworks, urea
has the ability to trap many organic compounds. In these so-called clathrates,
the organic "guest" molecules are held in channels formed by interpenetrating
helices composed of hydrogen-bonded urea molecules. This behaviour can be used
to separate mixtures, e.g. in the production of aviation fuel and lubricating
oils, and in the separation of hydrocarbons.
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.
Reactions
Urea reacts with alcohols to form urethanes. Urea reacts
with malonic esters to make barbituric acids.
Uses
Agriculture
More than 90% of world industrial production of urea is
destined for use as a nitrogen-release fertilizer. Urea has the highest
nitrogen content of all solid nitrogenous fertilizers in common use. Therefore,
it has the lowest transportation costs per unit of nitrogen nutrient. The
standard crop-nutrient rating (NPK rating) of urea is 46-0-0.
Many soil bacteria possess the enzyme urease, which
catalyzes the conversion of the urea to ammonia or ammonium ion and bicarbonate
ion, thus urea fertilizers are very rapidly transformed to the ammonium form in
soils. Among soil bacteria known to carry urease, some ammonia-oxidizing
bacteria (AOB), such as species of Nitrosomonas, are also able to assimilate
the carbon dioxide released by the reaction to make biomass via the Calvin
Cycle, and harvest energy by oxidizing ammonia (the other product of urease) to
nitrite, a process termed nitrification. Nitrite-oxidizing bacteria, especially
Nitrobacter, oxidize nitrite to nitrate, which is extremely mobile in soils
because of its negative charge and is a major cause of water pollution from
agriculture. Ammonium and nitrate are readily absorbed by plants, and are the
dominant sources of nitrogen for plant growth. 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 is biuret, which
impairs plant growth.
Urea is usually spread at rates of between 40 and 300 kg/ha
but rates vary. Smaller applications incur lower losses due to leaching. During
summer, urea is often spread just before or during rain to minimize losses from
volatilization (process wherein nitrogen is lost to the atmosphere as ammonia
gas). Urea is not compatible with other fertilizers.
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. 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 3 g urea
per 1 L water.
In foliar sprays, urea concentrations of 0.5% – 2.0% are
often used in horticultural crops. Low-biuret grades of urea are often
indicated.
Urea absorbs moisture from the atmosphere and therefore is
typically stored either in closed/sealed bags on pallets or, if stored in bulk,
under cover with a tarpaulin. As with most solid fertilizers, storage in a
cool, dry, well-ventilated area is recommended.
Chemical industry
Urea is a raw material for the manufacture of two main
classes of materials: urea-formaldehyde resins and urea-melamine-formaldehyde
used in marine plywood.
Explosive
Urea can be used to make urea nitrate, a high explosive that
is used industrially and as part of some improvised explosive devices. It is a
stabilizer in nitrocellulose explosives.
Automobile systems
Urea is used in SNCR and SCR reactions to reduce the NOx
pollutants in exhaust gases from combustion from Diesel, dual fuel, and
lean-burn natural gas engines. The BlueTec system, for example, injects
water-based urea solution into the exhaust system. The ammonia produced by the
hydrolysis of the urea reacts with the nitrogen oxide emissions and is
converted into nitrogen and water within the catalytic converter.
Niche
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 hardening of ski-resort
terrain park take-offs and landings
A flavor-enhancing
additive for cigarettes
A main ingredient
in hair removers such as Nair and Veet
A browning agent
in factory-produced pretzels
An ingredient in
some skin cream,[40] moisturizers, hair conditioners
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
A 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 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 (0.37V) and
thus consumes less energy than the electrolysis of water (1.2V).
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
then previously obtainable using conventional one photon or two photon confocal
microscopes.
Medical use
Urea-containing creams are used as topical dermatological
products to promote rehydration of the skin. Urea 40% is indicated for
psoriasis, xerosis, onychomycosis, ichthyosis, eczema, keratosis, keratoderma,
corns, and calluses. If covered by an occlusive dressing, 40% urea preparations
may also be used for nonsurgical debridement of nails. Urea 40% "dissolves
the intercellular matrix" of the nail plate. Only diseased or dystrophic
nails are removed, as there is no effect on healthy portions of the
nail.[citation needed] This drug is also used as an earwax removal
aid.[citation needed]
Urea can also be used as a diuretic. It was first used as a
diuretic by a Dr. W. Friedrich in 1892. In a 2010 study of ICU patients in Belgium,
urea was used as a diuretic to treat euvolemic hyponatremia and was found to be
a safe, inexpensive and simple treatment.
Certain types of instant cold packs (or ice packs) contain
water and separated urea crystals. Rupturing the internal water bag starts an
endothermic reaction and allows the pack to be used to reduce
swelling.[citation needed]
Like saline, urea injection is used to perform
abortion.[citation needed]
Urea is the main component of an alternative medicinal
treatment referred to as urine therapy.[citation needed]
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, though it is inferior to other markers such as creatinine
because blood urea levels are influenced by other factors such as diet and
dehydration.
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
peptic 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).
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[47] and 96-well micro-plate spectrophotometers.
Safety
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.[citation needed]
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., dogs) have a much more concentrated urine and it contains a
higher urea amount than normal human urine; this can prove dangerous as a
source of liquids for consumption in a life-threatening situation (such as in a
desert).
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.
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.
SUBSCRIBERS - ( LINKS) :FOLLOW / REF / 2 /
findleverage.blogspot.com
Krkz77@yahoo.com
+234-81-83195664
No comments:
Post a Comment