Sodium is a
chemical element with the
symbol Na (from Latin ''natrium'') and
atomic number 11. It is a soft, silvery-white, highly reactive
metal
A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
. 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
In the context of nutrition, a mineral is a chemical element required as an essential nutrient by organisms to perform functions necessary for life. However, the four major structural elements in the human body by weight ( oxygen, hydrogen, ca ...
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 and ECF compartment volume. Loss of water from the ECF compartment increases the sodium concentration, a condition called
hypernatremia.
Isotonic loss of water and sodium from the ECF compartment decreases the size of that compartment in a condition called ECF
hypovolemia.
By means of the
sodium-potassium pump, living human cells pump three sodium ions out of the cell in exchange for two potassium ions pumped in; comparing ion concentrations across the cell membrane, inside to outside,
potassium measures about 40:1, and sodium, about 1:10. In
nerve cells
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...
, the electrical charge across the cell membrane enables transmission of the nerve impulse—an
action potential—when the charge is dissipated; sodium plays a key role in that activity.
Characteristics
Physical
Sodium at
standard temperature and pressure is a soft silvery metal that combines with oxygen in the air and forms grayish white
sodium oxide
Sodium oxide is a chemical compound with the formula Na2 O. It is used in ceramics and glasses. It is a white solid but the compound is rarely encountered. Instead "sodium oxide" is used to describe components of various materials such as gla ...
unless immersed in oil or inert gas, which are the conditions it is usually stored in. Sodium metal can be easily cut with a knife and is a good conductor of electricity and heat because it has only one electron in its valence shell, resulting in weak
metallic bonding and free electrons, which carry energy. Due to having low atomic mass and large atomic radius, sodium is third-least dense of all elemental metals and is one of only three metals that can float on water, the other two being lithium and potassium.
[Greenwood and Earnshaw, p. 75]
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 bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea l ...
, 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
The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in the cores of massive stars (at least 8 \beginM_\odot\end at birth) that combines carbon into other elements. It requires high temperatures (> 5& ...
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
Sodium atoms have 11 electrons, one more than the stable configuration of the
noble gas neon. The first and second
ionization energies
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
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.
The thermal, fluidic, chemical, and nuclear properties of molten sodium metal have caused it to be one of the main coolants of choice for the
fast breeder reactor. Such nuclear reactors are seen as a crucial step for the production of clean energy.
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 (Na
Cl),
soda ash (Na
2 CO3),
baking soda (Na
HCO3),
caustic soda (NaOH),
sodium nitrate (Na
NO3), di- and tri-
sodium phosphates
Sodium phosphate is a generic term for a variety of salts of sodium (Na+) and phosphate (PO43−). Phosphate also forms families or condensed anions including di-, tri-, tetra-, and polyphosphates. Most of these salts are known in both anhydrou ...
,
sodium thiosulfate (Na
2 S2O3·5H
2O), and
borax
Borax is a salt (ionic compound), a hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a basic solution. It is commonly available in powder or granular form ...
(Na
2 B4O
7·10H
2O).
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.
Sodium containing mixed oxides are promising catalysts and photocatalysts. Photochemically intercalated sodium ion enhances the photoelectrocatalytic activity of
WO3.
Like all the
alkali metals, sodium reacts
exothermically with water. The reaction produces caustic soda (
sodium hydroxide) and flammable
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
gas. When burned in air, it forms primarily
sodium peroxide
Sodium peroxide is an inorganic compound with the formula Na2O2. This yellowish solid is the product of sodium ignited in excess oxygen. It is a strong base. This metal peroxide exists in several hydrates and peroxyhydrates including Na2O2·2H2O ...
with some
sodium oxide
Sodium oxide is a chemical compound with the formula Na2 O. It is used in ceramics and glasses. It is a white solid but the compound is rarely encountered. Instead "sodium oxide" is used to describe components of various materials such as gla ...
.
[Greenwood and Earnshaw, p. 84]
Aqueous solutions
Sodium tends to form water-soluble compounds, such as
halides,
sulfates,
nitrates,
carboxylates
In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge.
Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxyla ...
and
carbonates. The main aqueous species are the aquo complexes
2O)''n''">a(H2O)''n''sup>+, 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 (NaBiO
3). 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.
Crown ethers, like
15-crown-5, may be used as a
phase-transfer catalyst
In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of heterogeneous catalysis. Ionic reac ...
.
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(NH
3)
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
Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp, where Cp− is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are p ...
(NaC
5H
5) and trityl sodium ((C
6H
5)
3CNa).
Sodium naphthalene, Na
+ 10H8•">10H8•sup>−, 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
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
,
lead, and the
group 11 and
12 elements. Sodium and potassium form KNa
2 and
NaK
In data networking, telecommunications, and computer buses, an acknowledgment (ACK) is a signal that is passed between communicating processes, computers, or devices to signify acknowledgment, or receipt of message, as part of a communications ...
. NaK is 40–90% potassium and it is liquid at
ambient temperature
Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
. 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-CaCl
2 and ternary mixture NaCl-CaCl
2-BaCl
2. 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. NaPb
3, NaPb, Na
9Pb
4, Na
5Pb
2, and Na
15Pb
4 are some of the known sodium-lead alloys. Sodium also forms alloys with
gold (NaAu
2) and
silver (NaAg
2). Group 12 metals (
zinc,
cadmium and
mercury) are known to make alloys with sodium. NaZn
13 and NaCd
2 are alloys of zinc and cadmium. Sodium and mercury form NaHg, NaHg
4, NaHg
2, Na
3Hg
2, and Na
3Hg.
History
Because of its importance in human health, salt has long been an important commodity, as shown by the English word ''salary'', which derives from ''salarium'', the wafers of salt sometimes given to Roman soldiers along with their other wages. 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
Ludwig Wilhelm Gilbert (12 August 1769 – 7 March 1824) was a German physicist and chemist, and professor of physics at the University of Leipzig. From 1799-1824 he published the "''Annalen der Physik''", of which Poggendorffs "''Annalen d ...
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
New 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
''Annalen der Physik'' (English: ''Annals of Physics'') is one of the oldest scientific journals on physics; it has been published since 1799. The journal publishes original, peer-reviewed papers on experimental, theoretical, applied, and math ...
:
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
The Earth's crust contains 2.27% sodium, making it the
seventh most abundant element on Earth and the fifth most abundant metal, behind
aluminium,
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
,
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
, and
magnesium and ahead of potassium.
[Greenwood and Earnshaw, p. 69.] 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.
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
Mercury's atmosphere, the
exosphere of 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
Employed only in rather specialized applications, only about 100,000 tonnes of metallic sodium are produced annually.
Metallic sodium was first produced commercially in the late 19th century
by
carbothermal reduction of
sodium carbonate at 1100 °C, as the first step of the
Deville process for the production of aluminium:
:Na
2CO
3 + 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, based on a process patented in 1924.
[Pauling, Linus, ''General Chemistry'', 1970 ed., Dover Publications] 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
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
is less
electropositive than sodium, no calcium will be deposited at the cathode. This method is less expensive than the previous
Castner process
The Castner process is a process for manufacturing sodium metal by electrolysis of molten sodium hydroxide at approximately 330 °C. Below that temperature, the melt would solidify; above that temperature, the molten sodium would start to dissol ...
(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
Sodium oxide is a chemical compound with the formula Na2 O. It is used in ceramics and glasses. It is a white solid but the compound is rarely encountered. Instead "sodium oxide" is used to describe components of various materials such as gla ...
or
sodium superoxide
Sodium superoxide is the inorganic compound with the formula Na O2. This yellow-orange solid is a salt of the superoxide anion. It is an intermediate in the oxidation of sodium by oxygen.
Preparation
NaO2 is prepared by treating sodium peroxide ...
.
Uses
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
Deicing is the process of removing snow, ice or frost from a surface. Anti-icing is the application of chemicals that not only deice but also remain on a surface and continue to delay the reformation of ice for a certain period of time, or prev ...
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, and as a reducing agent in inorganic chemistry.
Metallic sodium is used mainly for the production of
sodium borohydride,
sodium azide,
indigo, and
triphenylphosphine
Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists ...
. 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.
[Alfred Klemm, Gabriele Hartmann, Ludwig Lange, "Sodium and Sodium Alloys" in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. ] Sodium is also used as an alloying metal, an
anti-scaling agent
Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water requires less soap for the same cleaning effort, as soap is not wasted bonding with calcium ions. Soft water also ext ...
, 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
The Birch reduction is an organic reaction that is used to convert arenes to cyclohexadienes. The reaction is named after the Australian chemist Arthur Birch and involves the organic reduction of aromatic rings in an amine solvent (traditionally ...
, and the
sodium fusion test is conducted to qualitatively analyse compounds. Sodium reacts with alcohol 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
[Sodium as a Fast Reactor Coolant](_blank)
presented by Thomas H. Fanning. Nuclear Engineering Division. U.S. Department of Energy. U.S. Nuclear Regulatory Commission. Topical Seminar Series on Sodium Fast Reactors. 3 May 2007 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,
NaK
In data networking, telecommunications, and computer buses, an acknowledgment (ACK) is a signal that is passed between communicating processes, computers, or devices to signify acknowledgment, or receipt of message, as part of a communications ...
is used. Because NaK is a liquid at room temperature, the coolant does not solidify in the pipes.
In this case, the
pyrophoricity of potassium requires extra precautions to prevent and detect leaks. Another heat transfer application is
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
Biological role in humans
In humans, sodium is an essential mineral that regulates
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
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
Sodium chloride (
salt) 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.
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 between 9 and 17% fewer cases of
hypertension.
Hypertension causes 7.6 million premature deaths worldwide each year. (Note that salt contains about 39.3% sodiumthe rest being chlorine and trace chemicals; thus, 2.3 g sodium is about 5.9 g, or 5.3 ml, of saltabout one
US teaspoon.
)
One study 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
The renin–angiotensin system (RAS), or renin–angiotensin–aldosterone system (RAAS), is a hormone system that regulates blood pressure, fluid and electrolyte balance, and systemic vascular resistance.
When renal blood flow is reduced, ...
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
Renin (etymology and pronunciation), also known as an angiotensinogenase, is an aspartic protease protein and enzyme secreted by the kidneys that participates in the body's renin–angiotensin–aldosterone system (RAAS)—also known as the ...
, 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. 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
Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and ...
, 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
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)
See also
References
Bibliography
*
External links
Sodiumat ''
The Periodic Table of Videos'' (University of Nottingham)
Etymology of "natrium" – source of symbol Na
{{Authority control
Chemical elements
Alkali metals
Sodium minerals
Desiccants
Dietary minerals
Reducing agents
Nuclear reactor coolants
Chemical elements with body-centered cubic structure