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= Sodium =
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Introduction
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Sodium is a chemical element; it has symbol Na (from Neo-Latin ) and
atomic number 11. It is a soft, silvery-white, highly reactive metal.
Sodium is an alkali metal, being in group 1 of the periodic table. Its
only stable isotope is 23Na. The free metal does not occur in nature
and must be prepared from compounds. Sodium is the sixth most abundant
element in the Earth's crust and exists in numerous minerals such as
feldspars, sodalite, and halite (NaCl). Many salts of sodium are
highly water-soluble: sodium ions have been leached by the action of
water from the Earth's minerals over eons, and thus sodium and
chlorine are the most common dissolved elements by weight in the
oceans.
Sodium was first isolated by Humphry Davy in 1807 by the electrolysis
of sodium hydroxide. Among many other useful sodium compounds, sodium
hydroxide (lye) is used in soap manufacture, and sodium chloride
(edible salt) is a de-icing agent and a nutrient for animals including
humans.
Sodium is an essential element for all animals and some plants. Sodium
ions are the major cation in the extracellular fluid (ECF) and as such
are the major contributor to the ECF osmotic pressure. Animal cells
actively pump sodium ions out of the cells by means of the
sodium-potassium pump, an enzyme complex embedded in the cell
membrane, in order to maintain a roughly ten-times higher
concentration of sodium ions outside the cell than inside. In nerve
cells, the sudden flow of sodium ions into the cell through
voltage-gated sodium channels enables transmission of a nerve impulse
in a process called the action potential.
Physical
==========
Sodium at standard temperature and pressure is a soft silvery metal
that combines with oxygen in the air, forming sodium oxides. Bulk
sodium is usually stored in oil or an inert gas. Sodium metal can be
easily cut with a knife. It is a good conductor of electricity and
heat.
The melting (98 °C) and boiling (883 °C) points of sodium are lower
than those of lithium but higher than those of the heavier alkali
metals potassium, rubidium, and caesium, following periodic trends
down the group. These properties change dramatically at elevated
pressures: at 1.5 Mbar, the color changes from silvery metallic to
black; at 1.9 Mbar the material becomes transparent with a red color;
and at 3 Mbar, sodium is a clear and transparent solid. All of these
high-pressure allotropes are insulators and electrides.
In a flame test, sodium and its compounds glow yellow because the
excited 3s electrons of sodium emit a photon when they fall from 3p to
3s; the wavelength of this photon corresponds to the D line at about
589.3 nm. Spin-orbit interactions involving the electron in the 3p
orbital split the D line into two, at 589.0 and 589.6 nm; hyperfine
structures involving both orbitals cause many more lines.
Isotopes
==========
Twenty isotopes of sodium are known, but only 23Na is stable. 23Na is
created in the carbon-burning process in stars by fusing two carbon
atoms together; this requires temperatures above 600 megakelvins and a
star of at least three solar masses. Two radioactive, cosmogenic
isotopes are the byproduct of cosmic ray spallation: 22Na has a
half-life of 2.6 years and 24Na, a half-life of 15 hours; all other
isotopes have a half-life of less than one minute.
Two nuclear isomers have been discovered, the longer-lived one being
24mNa with a half-life of around 20.2 milliseconds. Acute neutron
radiation, as from a nuclear criticality accident, converts some of
the stable 23Na in human blood to 24Na; the neutron radiation dosage
of a victim can be calculated by measuring the concentration of 24Na
relative to 23Na.
Chemistry
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Sodium atoms have 11 electrons, one more than the stable configuration
of the noble gas neon. The first and second ionization energies are
495.8 kJ/mol and 4562 kJ/mol, respectively. As a result, sodium
usually forms ionic compounds involving the Na+ cation.
Metallic sodium
=================
Metallic sodium is generally less reactive than potassium and more
reactive than lithium. Sodium metal is highly reducing, with the
standard reduction potential for the Na+/Na couple being −2.71 volts,
though potassium and lithium have even more negative potentials.
Salts and oxides
==================
Sodium compounds are of immense commercial importance, being
particularly central to industries producing glass, paper, soap, and
textiles. The most important sodium compounds are table salt (NaCl),
soda ash (Na2CO3), baking soda (NaHCO3), caustic soda (NaOH), sodium
nitrate (NaNO3), di- and tri-sodium phosphates, sodium thiosulfate
(Na2S2O3·5H2O), and borax (Na2B4O7·10H2O). In compounds, sodium is
usually ionically bonded to water and anions and is viewed as a hard
Lewis acid.
Most soaps are sodium salts of fatty acids. Sodium soaps have a higher
melting temperature (and seem "harder") than potassium soaps.
Like all the alkali metals, sodium reacts exothermically with water.
The reaction produces caustic soda (sodium hydroxide) and flammable
hydrogen gas. When burned in air, it forms primarily sodium peroxide
with some sodium oxide.
Aqueous solutions
===================
Sodium tends to form water-soluble compounds, such as halides,
sulfates, nitrates, carboxylates and carbonates. The main aqueous
species are the aquo complexes [Na(H2O)'n']+, where 'n' = 4-8; with
'n' = 6 indicated from X-ray diffraction data and computer
simulations.
Direct precipitation of sodium salts from aqueous solutions is rare
because sodium salts typically have a high affinity for water. An
exception is sodium bismuthate (NaBiO3), which is insoluble in cold
water and decomposes in hot water. Because of the high solubility of
its compounds, sodium salts are usually isolated as solids by
evaporation or by precipitation with an organic antisolvent, such as
ethanol; for example, only 0.35 g/L of sodium chloride will dissolve
in ethanol. A crown ether such as 15-crown-5 may be used as a
phase-transfer catalyst.
Sodium content of samples is determined by atomic absorption
spectrophotometry or by potentiometry using ion-selective electrodes.
Electrides and sodides
========================
Like the other alkali metals, sodium dissolves in ammonia and some
amines to give deeply colored solutions; evaporation of these
solutions leaves a shiny film of metallic sodium. The solutions
contain the coordination complex [Na(NH3)6]+, with the positive charge
counterbalanced by electrons as anions; cryptands permit the isolation
of these complexes as crystalline solids. Sodium forms complexes with
crown ethers, cryptands and other ligands.
For example, 15-crown-5 has a high affinity for sodium because the
cavity size of 15-crown-5 is 1.7-2.2 Å, which is enough to fit the
sodium ion (1.9 Å). Cryptands, like crown ethers and other ionophores,
also have a high affinity for the sodium ion; derivatives of the
alkalide Na− are obtainable by the addition of cryptands to solutions
of sodium in ammonia via disproportionation.
Organosodium compounds
========================
Many organosodium compounds have been prepared. Because of the high
polarity of the C-Na bonds, they behave like sources of carbanions
(salts with organic anions). Some well-known derivatives include
sodium cyclopentadienide (NaC5H5) and trityl sodium ((C6H5)3CNa).
Sodium naphthalene, Na+[C10H8•]−, a strong reducing agent, forms upon
mixing Na and naphthalene in ethereal solutions.
Intermetallic compounds
=========================
Sodium forms alloys with many metals, such as potassium, calcium,
lead, and the group 11 and 12 elements. Sodium and potassium form KNa2
and NaK. NaK is 40-90% potassium and it is liquid at ambient
temperature. It is an excellent thermal and electrical conductor.
Sodium-calcium alloys are by-products of the electrolytic production
of sodium from a binary salt mixture of NaCl-CaCl2 and ternary mixture
NaCl-CaCl2-BaCl2. Calcium is only partially miscible with sodium, and
the 1-2% of it dissolved in the sodium obtained from said mixtures can
be precipitated by cooling to 120 °C and filtering.
In a liquid state, sodium is completely miscible with lead. There are
several methods to make sodium-lead alloys. One is to melt them
together and another is to deposit sodium electrolytically on molten
lead cathodes. NaPb3, NaPb, Na9Pb4, Na5Pb2, and Na15Pb4 are some of
the known sodium-lead alloys. Sodium also forms alloys with gold
(NaAu2) and silver (NaAg2). Group 12 metals (zinc, cadmium and
mercury) are known to make alloys with sodium. NaZn13 and NaCd2 are
alloys of zinc and cadmium. Sodium and mercury form NaHg, NaHg4,
NaHg2, Na3Hg2, and Na3Hg.
History
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Because of its importance in human health, salt has long been an
important commodity. In medieval Europe, a compound of sodium with the
Latin name of 'sodanum' was used as a headache remedy. The name sodium
is thought to originate from the Arabic 'suda', meaning headache, as
the headache-alleviating properties of sodium carbonate or soda were
well known in early times.
Although sodium, sometimes called 'soda', had long been recognized in
compounds, the metal itself was not isolated until 1807 by Sir Humphry
Davy through the electrolysis of sodium hydroxide. In 1809, the German
physicist and chemist Ludwig Wilhelm Gilbert proposed the names
'Natronium' for Humphry Davy's "sodium" and 'Kalium' for Davy's
"potassium".
The chemical abbreviation for sodium was first published in 1814 by
Jöns Jakob Berzelius in his system of atomic symbols, and is an
abbreviation of the element's Neo-Latin name 'natrium', which refers
to the Egyptian 'natron', a natural mineral salt mainly consisting of
hydrated sodium carbonate. Natron historically had several important
industrial and household uses, later eclipsed by other sodium
compounds.
Sodium imparts an intense yellow color to flames. As early as 1860,
Kirchhoff and Bunsen noted the high sensitivity of a sodium flame
test, and stated in Annalen der Physik und Chemie:
In a corner of our 60 m3 room farthest away from the apparatus, we
exploded 3 mg of sodium chlorate with milk sugar while observing the
nonluminous flame before the slit. After a while, it glowed a bright
yellow and showed a strong sodium line that disappeared only after 10
minutes. From the weight of the sodium salt and the volume of air in
the room, we easily calculate that one part by weight of air could not
contain more than 1/20 millionth weight of sodium.
Occurrence
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The Earth's crust contains 2.27% sodium, making it the sixth most
abundant element on Earth and the fourth most abundant metal, behind
aluminium, iron, calcium, and magnesium and ahead of
potassium.Sodium's estimated oceanic abundance is 10.8 grams per
liter. Because of its high reactivity, it is never found as a pure
element. It is found in many minerals, some very soluble, such as
halite and natron, others much less soluble, such as amphibole and
zeolite. The insolubility of certain sodium minerals such as cryolite
and feldspar arises from their polymeric anions, which in the case of
feldspar is a polysilicate. In the universe, sodium is the 15th most
abundant element with a 20,000 parts-per-billion abundance, making
sodium 0.002% of the total atoms in the universe.
Astronomical observations
===========================
Atomic sodium has a very strong spectral line in the yellow-orange
part of the spectrum (the same line as is used in sodium-vapour street
lights). This appears as an absorption line in many types of stars,
including the Sun. The line was first studied in 1814 by Joseph von
Fraunhofer during his investigation of the lines in the solar
spectrum, now known as the Fraunhofer lines. Fraunhofer named it the
"D" line, although it is now known to actually be a group of closely
spaced lines split by a fine and hyperfine structure.
The strength of the D line allows its detection in many other
astronomical environments. In stars, it is seen in any whose surfaces
are cool enough for sodium to exist in atomic form (rather than
ionised). This corresponds to stars of roughly F-type and cooler. Many
other stars appear to have a sodium absorption line, but this is
actually caused by gas in the foreground interstellar medium. The two
can be distinguished via high-resolution spectroscopy, because
interstellar lines are much narrower than those broadened by stellar
rotation.
Sodium has also been detected in numerous Solar System environments,
including the exospheres of Mercury and the Moon, and numerous other
bodies. Some comets have a sodium tail, which was first detected in
observations of Comet Hale-Bopp in 1997. Sodium has even been detected
in the atmospheres of some extrasolar planets via transit
spectroscopy.
Commercial production
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Employed in rather specialized applications, about 100,000 tonnes of
metallic sodium are produced annually. Metallic sodium was first
produced commercially in the late nineteenth century by carbothermal
reduction of sodium carbonate at 1100 °C, as the first step of the
Deville process for the production of aluminium:
:Na2CO3 + 2 C → 2 Na + 3 CO
The high demand for aluminium created the need for the production of
sodium. The introduction of the Hall-Héroult process for the
production of aluminium by electrolysing a molten salt bath ended the
need for large quantities of sodium. A related process based on the
reduction of sodium hydroxide was developed in 1886.
Sodium is now produced commercially through the electrolysis of molten
sodium chloride (common salt), based on a process patented in 1924.
This is done in a Downs cell in which the NaCl is mixed with calcium
chloride to lower the melting point below 700 °C. As calcium is less
electropositive than sodium, no calcium will be deposited at the
cathode. This method is less expensive than the previous Castner
process (the electrolysis of sodium hydroxide).
If sodium of high purity is required, it can be distilled once or
several times.
The market for sodium is volatile due to the difficulty in its storage
and shipping; it must be stored under a dry inert gas atmosphere or
anhydrous mineral oil to prevent the formation of a surface layer of
sodium oxide or sodium superoxide.
Uses
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Though metallic sodium has some important uses, the major applications
for sodium use compounds; millions of tons of sodium chloride,
hydroxide, and carbonate are produced annually. Sodium chloride is
extensively used for anti-icing and de-icing and as a preservative;
examples of the uses of sodium bicarbonate include baking, as a
raising agent, and sodablasting. Along with potassium, many important
medicines have sodium added to improve their bioavailability; though
potassium is the better ion in most cases, sodium is chosen for its
lower price and atomic weight. Sodium hydride is used as a base for
various reactions (such as the aldol reaction) in organic chemistry.
Metallic sodium is used mainly for the production of sodium
borohydride, sodium azide, indigo, and triphenylphosphine. A
once-common use was the making of tetraethyllead and titanium metal;
because of the move away from TEL and new titanium production methods,
the production of sodium declined after 1970. Sodium is also used as
an alloying metal, an anti-scaling agent, and as a reducing agent for
metals when other materials are ineffective.
Note the free element is not used as a scaling agent, ions in the
water are exchanged for sodium ions. Sodium plasma ("vapor") lamps are
often used for street lighting in cities, shedding light that ranges
from yellow-orange to peach as the pressure increases. By itself or
with potassium, sodium is a desiccant; it gives an intense blue
coloration with benzophenone when the desiccate is dry.
In organic synthesis, sodium is used in various reactions such as the
Birch reduction, and the sodium fusion test is conducted to
qualitatively analyse compounds. Sodium reacts with alcohols and gives
alkoxides, and when sodium is dissolved in ammonia solution, it can be
used to reduce alkynes to trans-alkenes. Lasers emitting light at the
sodium D line are used to create artificial laser guide stars that
assist in the adaptive optics for land-based visible-light telescopes.
Heat transfer
===============
Liquid sodium is used as a heat transfer fluid in sodium-cooled fast
reactors because it has the high thermal conductivity and low neutron
absorption cross section required to achieve a high neutron flux in
the reactor. The high boiling point of sodium allows the reactor to
operate at ambient (normal) pressure, but drawbacks include its
opacity, which hinders visual maintenance, and its strongly reducing
properties. Sodium will explode in contact with water, although it
will only burn gently in air.
Radioactive sodium-24 may be produced by neutron bombardment during
operation, posing a slight radiation hazard; the radioactivity stops
within a few days after removal from the reactor. If a reactor needs
to be shut down frequently, sodium-potassium alloy (NaK) is used.
Because NaK is a liquid at room temperature, the coolant does not
solidify in the pipes. The pyrophoricity of the NaK means extra
precautions must be taken to prevent and detect leaks.
Another heat transfer application of sodium is in poppet valves in
high-performance internal combustion engines; the valve stems are
partially filled with sodium and work as a heat pipe to cool the
valves.
Biological role in humans
===========================
In humans, sodium is an essential mineral that regulates blood volume,
blood pressure, osmotic equilibrium and pH. The minimum physiological
requirement for sodium is estimated to range from about 120 milligrams
per day in newborns to 500 milligrams per day over the age of 10.
Diet
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Sodium chloride, also known as 'edible salt' or 'table salt' (chemical
formula ), is the principal source of sodium () in the diet and is
used as seasoning and preservative in such commodities as pickled
preserves and jerky. For Americans, most sodium chloride comes from
processed foods. Other sources of sodium are its natural occurrence in
food and such food additives as monosodium glutamate (MSG), sodium
nitrite, sodium saccharin, baking soda (sodium bicarbonate), and
sodium benzoate.
The U.S. Institute of Medicine set its tolerable upper intake level
for sodium at 2.3 grams per day, but the average person in the United
States consumes 3.4 grams per day. The American Heart Association
recommends no more than 1.5 g of sodium per day.
The Committee to Review the Dietary Reference Intakes for Sodium and
Potassium, which is part of the National Academies of Sciences,
Engineering, and Medicine, has determined that there isn't enough
evidence from research studies to establish Estimated Average
Requirement (EAR) and Recommended Dietary Allowance (RDA) values for
sodium. As a result, the committee has established Adequate Intake
(AI) levels instead, as follows. The sodium AI for infants of 0-6
months is established at 110 mg/day, 7-12 months: 370 mg/day; for
children 1-3 years: 800 mg/day, 4-8 years: 1,000 mg/day; for
adolescents: 9-13 years - 1,200 mg/day, 14-18 years 1,500 mg/day; for
adults regardless of their age or sex: 1,500 mg/day.
Sodium chloride () contains approximately 39.34% of its total mass as
elemental sodium (). This means that of sodium chloride contains
approximately of elemental sodium. For example, to find out how much
sodium chloride contains 1500 mg of elemental sodium (the value of
1500 mg sodium is the adequate intake (AI) for an adult), we can use
the proportion:
:393.4 mg Na : 1000 mg NaCl = 1500 mg Na : x mg NaCl
Solving for 'x' gives us the amount of sodium chloride that contains
1500 mg of elemental sodium
:x = (1500 mg Na × 1000 mg NaCl) / 393.4 mg Na = 3812.91 mg
This mean that 3812.91 mg of sodium chloride contain 1500 mg of
elemental sodium.
High sodium consumption
=========================
High sodium consumption is unhealthy, and can lead to alteration in
the mechanical performance of the heart. High sodium consumption is
also associated with chronic kidney disease, high blood pressure,
cardiovascular diseases, and stroke.
High blood pressure
=====================
There is a strong correlation between higher sodium intake and higher
blood pressure. Studies have found that lowering sodium intake by 2 g
per day tends to lower systolic blood pressure by about two to four mm
Hg. It has been estimated that such a decrease in sodium intake would
lead to 9-17% fewer cases of hypertension.
Hypertension causes 7.6 million premature deaths worldwide each year.
Since edible salt contains about 39.3% sodium--the rest being chlorine
and trace chemicals; thus, 2.3 g sodium is about 5.9 g, or 5.3 ml, of
salt--about one US teaspoon.
One scientific review found that people with or without hypertension
who excreted less than 3 grams of sodium per day in their urine (and
therefore were taking in less than 3 g/d) had a 'higher' risk of
death, stroke, or heart attack than those excreting 4 to 5 grams per
day. Levels of 7 g per day or more in people with hypertension were
associated with higher mortality and cardiovascular events, but this
was not found to be true for people without hypertension. The US FDA
states that adults with hypertension and prehypertension should reduce
daily sodium intake to 1.5 g.
Physiology
============
The renin-angiotensin system regulates the amount of fluid and sodium
concentration in the body. Reduction of blood pressure and sodium
concentration in the kidney result in the production of renin, which
in turn produces aldosterone and angiotensin, which stimulates the
reabsorption of sodium back into the bloodstream. When the
concentration of sodium increases, the production of renin decreases,
and the sodium concentration returns to normal. The sodium ion (Na+)
is an important electrolyte in neuron function, and in osmoregulation
between cells and the extracellular fluid. This is accomplished in all
animals by Na+/K+-ATPase, an active transporter pumping ions against
the gradient, and sodium/potassium channels. The difference in
extracellular and intracellular ion concentration, maintained by the
sodium-potassium pump, produce electrical signals in the form of
action potentials that supports cardiac muscle contraction and promote
long distance communication between neurons. Sodium is the most
prevalent metallic ion in extracellular fluid.
In humans, unusually low or high sodium levels in the blood is
recognized in medicine as hyponatremia and hypernatremia. These
conditions may be caused by genetic factors, ageing, or prolonged
vomiting or diarrhea.
Biological role in plants
===========================
In C4 plants, sodium is a micronutrient that aids metabolism,
specifically in regeneration of phosphoenolpyruvate and synthesis of
chlorophyll. In others, it substitutes for potassium in several roles,
such as maintaining turgor pressure and aiding in the opening and
closing of stomata. Excess sodium in the soil can limit the uptake of
water by decreasing the water potential, which may result in plant
wilting; excess concentrations in the cytoplasm can lead to enzyme
inhibition, which in turn causes necrosis and chlorosis.
In response, some plants have developed mechanisms to limit sodium
uptake in the roots, to store it in cell vacuoles, and restrict salt
transport from roots to leaves. Excess sodium may also be stored in
old plant tissue, limiting the damage to new growth. Halophytes have
adapted to be able to flourish in sodium rich environments.
Safety and precautions
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Sodium forms flammable hydrogen and caustic sodium hydroxide on
contact with water; ingestion and contact with moisture on skin, eyes
or mucous membranes can cause severe burns. Sodium spontaneously
explodes in the presence of water due to the formation of hydrogen
(highly explosive) and sodium hydroxide (which dissolves in the water,
liberating more surface). However, sodium exposed to air and ignited
or reaching autoignition (reported to occur when a molten pool of
sodium reaches about ) displays a relatively mild fire.
In the case of massive (non-molten) pieces of sodium, the reaction
with oxygen eventually becomes slow due to formation of a protective
layer. Fire extinguishers based on water accelerate sodium fires.
Those based on carbon dioxide and bromochlorodifluoromethane should
not be used on sodium fire. Metal fires are Class D, but not all Class
D extinguishers are effective when used to extinguish sodium fires. An
effective extinguishing agent for sodium fires is Met-L-X. Other
effective agents include Lith-X, which has graphite powder and an
organophosphate flame retardant, and dry sand.
Sodium fires are prevented in nuclear reactors by isolating sodium
from oxygen with surrounding pipes containing inert gas. Pool-type
sodium fires are prevented using diverse design measures called catch
pan systems. They collect leaking sodium into a leak-recovery tank
where it is isolated from oxygen.
Liquid sodium fires are more dangerous to handle than solid sodium
fires, particularly if there is insufficient experience with the safe
handling of molten sodium. In a technical report for the United States
Fire Administration, R. J. Gordon writes (emphasis in original)
External links
======================================================================
* [
http://www.periodicvideos.com/videos/011.htm Sodium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://www.balashon.com/2008/07/neter-and-nitrogen.html Etymology
of "natrium" - source of symbol Na]
* [
http://www.theodoregray.com/PeriodicTable/Elements/011/index.html
The Wooden Periodic Table Table's Entry on Sodium]
*
[
https://web.archive.org/web/20070712011829/http://ie.lbl.gov/education/parent/Na_iso.htm
Sodium isotopes data from 'The Berkeley Laboratory Isotopes
Project's']
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Sodium
