Encyclopedia
Sodium is a
chemical element which has the symbol Na , atomic number 11, atomic mass 22.9898 g/mol, oxidation number +1. Sodium is a soft, silvery, highly reactive element and is a member of the alkali metals within ‘group 1’ . It has only one stable isotope,
23Na. Sodium was first isolated by Sir Humphry Davy in 1807 by passing an electric current through molten
sodium hydroxide. Sodium quickly oxidizes in air so must be stored in an inert environment such as kerosene. Sodium is in great quantity in the earths oceans as chloride. It is also a component of very many earthly minerals.
Notable Characteristics
Compared with the other alkali metals, sodium is generally more reactive than lithium and less so than potassium in accordance with ‘periodic law’; for example, their reaction in water, chlorine gas, etc., the reactivity of their nitrates, chlorates, perchlorates, etc. One violation of the periodic law is regarding density – lithium and potassium are less dense than sodium , all of them being less dense than water. Owing to it’s high reactivity sodium is found in nature only as a compound and never as the free element. Sodium reacts exothermically with water: Small pea sized pieces will swim around the surface of the water until they are consumed by it whereas large pieces will explode. The reaction with water produces very caustic sodium hydroxide and highly flammable hydrogen gas. In any case these are considered an extreme hazard and will cause severe skin and eye injury. When burned in air sodium forms sodium peroxide Na
2O2, with limited oxygen the oxide Na
2O . If burned in oxygen under pressure sodium superoxide NaO
2 will be produced. Sodium peroxide is reactive with water in which hydrogen peroxide and sodium hydroxide are formed. The superoxide also generates these as well as oxygen gas.
When sodium or its compounds are introduced into a flame it will contribute a bright yellow. In chemistry most sodium compounds are considered soluble but nature provides us with very many insoluble sodium compounds such as the feldspar’s. There are other insoluble sodium salts such as sodium bismuthate, sodium octamolybdate, sodium thioplatinate. Sodium meta-antimonate's solubility is 0.03g/L as is the pyro form of this salt .
Sodium ions are necessary for regulation of blood and body fluids, transmission of nerve impulses, heart activity, and certain metabolic functions. Interestingly, sodium is needed by animals, which maintain high concentrations in their blood and extracellular fluids, but the ion is not needed by plants. A completely plant-based diet, therefore, will be very low in sodium. This requires some herbivores to obtain their sodium from salt licks and other mineral sources. The animal need for sodium is probably the reason for the highly conserved ability to taste sodium ion, as "salty." Receptors for the pure salty taste respond best to sodium, and otherwise only to a few other small monovalent cations .
Calcium chloride also tastes somewhat salty, but also quite bitter. Sodium salts without exception are soluble in water, and all of them are salty to the taste.
The most common sodium salt, sodium chloride , used for seasoning and food preservation, has been an important commodity in human activities .
The human requirement for sodium in the diet is less than 500 mg per day, which is typically less than a tenth as much as many diets "seasoned to taste." Most people consume far more sodium than is physiologically needed. For certain people with salt-sensitive blood pressure, this extra intake may cause a negative effect on health. See
edible salt.
Applications
Sodium in its metallic form can be used to refine some reactive metals, such as
zirconium and
potassium, from their compounds. This alkali metal as the Na
+ ion is vital to animal life. Other uses:
- In certain alloys to improve their structure.
- In soap, in combination with fatty acids. Sodium soaps are harder soaps than potassium soaps.
- To descale metal .
- To purify molten metals.
- In sodium vapor lamps, an efficient means of producing light from electricity , often used for street lighting in cities. Low-pressure sodium lamps give a distinctive yellow-orange light which consists primarily of the twin sodium D spectral lines. High-pressure sodium lamps give a more natural peach-colored light, composed of wavelengths spread much more widely across the spectrum.
- As a heat transfer fluid in some types of nuclear reactors and inside the hollow valves of high-performance internal combustion engines.
- NaCl, a compound of sodium ions and chloride ions, is an important heat transfer material.
- In organic synthesis, sodium is used as a reducing agent, for example in the Birch reduction.
- In chemistry, sodium is often used either alone or with potassium in an alloy, NaK as a desiccant for drying solvents. Used with benzophenone, it forms an intense blue coloration when the solvent is dry and oxygen-free.
History
Sodium has long been recognized in compounds, but was not isolated until 1807 by Sir
Humphry Davy through the
electrolysis of
caustic soda. In
medieval Europe a compound of sodium with the
Latin name of
sodanum was used as a headache remedy. Sodium's symbol, Na, comes from the
neo-Latin name for a common sodium compound named
natrium, which comes from the Greek
nítron, a kind of natural
salt. The difference between the English name, Soda, and the abbreviation,
Na stems from
Berzelius' publication of his system of atomic symbols in Thomas Thomson's Annals of Philosophy.
Sodium imparts an intense yellow color to flames. As early as 1860
Kirchhoff and
Bunsen noted the high sensitivity that a flame test for sodium could give. They state in Annalen der Physik und der Chemie in the paper "Chemical Analysis by Observation of Spectra":
In a corner of our 60 cu.m. 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 few minutes, the flame gradually turned 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
- See also .
Sodium is relatively abundant in stars and the D
spectral lines of this element are among the most prominent in star light. Sodium makes up about 2.6% by weight of the
Earth's crust making it the fourth most abundant element overall and the most abundant alkali metal.
At the end of the 19th century, sodium was chemically prepared by heating
sodium carbonate with
carbon to 1100 °C.
- Na2CO3 + 2 C ? 2 Na + 3 CO .
It is now produced commercially through the
electrolysis of liquid
sodium chloride. This is done in a Down's cell in which the NaCl is mixed with
calcium chloride to lower the melting point below 700 °C. As calcium is more electropositive than sodium, no calcium will be formed at the cathode. This method is less expensive than the previous method of electrolyzing
sodium hydroxide.
Metallic sodium cost about 15 to 20 US cents per pound in 1997 but reagent grade sodium cost about US$35 per pound in 1990.
Phase behavior under pressure
Under extreme pressure, sodium departs from common melting behavior. Most materials require higher temperatures to melt under pressure than they do at normal atmospheric pressure. This is because they expand on melting due to loser molecular packing in the liquid, and thus pressure forces equilibrium in the direction of the denser solid phase.
At a pressure of 30 gigapascals , the melting temperature of sodium begins to drop. At around 100 gigapascals, sodium will melt at near room temperature. A possible explanation for the aberrant behavior of sodium is that this element has one free
electron that is pushed closer to the other 10 electrons when placed under pressure, forcing interactions that are not normally present. While under pressure, solid sodium assumes several odd
crystal structures suggesting that the liquid might have unusual properties such as
superconduction or
superfluidity.
Compounds
- See also .
Sodium chloride or
halite, better known as common salt, is the most common compound of sodium, but sodium occurs in many other
minerals, such as
amphibole,
cryolite, soda niter and
zeolite. Sodium compounds are important to the chemical,
glass, metal,
paper,
petroleum, soap, and
textile industries. Hard soaps are generally sodium salt of certain fatty acids .
The sodium compounds that are the most important to industry are common salt ,
soda ash ,
baking soda ,
caustic soda , Chile saltpeter , di- and tri-sodium phosphates,
sodium thiosulfate , and
borax .
Isotopes
There are thirteen isotopes of sodium that have been recognized. The only stable isotope is
23Na. Sodium has two
radioactive cosmogenic isotopes .
Acute neutron radiation exposure converts some of the stable
23Na in human blood plasma to
24Na. By measuring the concentration of this isotope, the neutron radiation dosage to the victim can be computed.
Precautions
Extreme care is required in handling elemental/metallic sodium. Sodium is potentially explosive in water and is a caustic poison, since it is rapidly converted to sodium hydroxide on contact with moisture. The powdered form may combust spontaneously in air or oxygen. Sodium must be stored either in an inert atmosphere , or under a liquid hydrocarbon such as mineral oil or kerosene.
The reaction of sodium and water is a familiar one in chemistry labs, and is reasonably safe if amounts of sodium smaller than a pencil eraser are used and the reaction is done behind a plastic shield by people wearing eye protection. However, the sodium-water reaction does not scale up well, and is treacherous when larger amounts of sodium are used. Larger pieces of sodium melt under the heat of the reaction, and the molten ball of metal is buoyed up by hydrogen and may appear to be stably reacting with water, until splashing covers more of the reaction mass, causing thermal runaway and an explosion which scatters molten sodium metal, lye solution, and sometimes flame. This behavior is unpredictable, and among the alkali metals it is usually sodium which invites this surprise phenomenon, because lithium is not reactive enough to do it, and potassium is so reactive that chemistry students are not tempted to try the reaction with larger potassium pieces.
Sodium is much more reactive than magnesium. When the metal itself catches fire it burns at high temperatures and also melts, which spreads the flame and exposes even more surface area to the air.
Few common fire extinguishers work on sodium fires. Water, of course, exacerbates sodium fires, as do water-based foams. CO
2 and Halon are often ineffective on sodium fires, which reignite when the extinguisher dissipates. Among the very few materials effective on a sodium metal fire are Pyromet and Met-L-X. Pyromet is a NaCl/
2HPO
4 mix, with flow/anti-clump agents. It smothers the fire, drains away heat, and melts to form an impermeable crust. This is the standard dry-powder canister fire extinguisher for all classes of fires. Met-L-X is mostly sodium chloride, NaCl, with approximately 5% Saran plastic as a crust-former, and flow/anti-clumping agents. It is most commonly hand-applied, with a scoop. Other extreme fire extinguishing materials include Lith-X, a graphite based dry powder with an organophosphate flame retardant; and Na-X, a Na
2CO
3-based material.
Because of the reaction scale problems discussed above, disposing of large quantities of sodium must be done through a licensed hazardous materials disposer. Smaller quantities may be broken up and neutralized carefully with
ethanol , or even
methanol , but care should nevertheless be taken, as the caustic products from the ethanol or methanol reaction are just as hazardous to eyes and skin as those from water. After the alcohol reaction appears complete and all pieces of reaction debris have been broken up or dissolved, a mixture of alcohol and water, then pure water, may then be carefully used for a final cleaning. This should be allowed to stand a few minutes until the reaction products are diluted more thoroughly and flushed down the drain. The purpose of the final water soak and wash of any reaction mass which may contain sodium is to ensure that alcohol does not carry unreacted sodium into the sink trap, where a water reaction may generate hydrogen in the trap space which can then be potentially ignited, causing a confined sink trap explosion.
Physiology and sodium ions
Sodium ions play a diverse and important role in many physiological processes. Excitable animal cells, for example, rely on the entry of Na
+ to cause a
depolarization. An example of this is signal transduction in the human
central nervous system, which depends on sodium ion motion in all nerves.
Some potent neurotoxins, such as
batrachotoxin, increase the sodium ion permeability of the
cell membranes in nerves and muscles, causing a massive and irreversible
depolarization of the membranes, with potentially fatal consequences. However, drugs with smaller effects on sodium ion motion in nerves may have diverse pharmacological effects which range from anti-depressant to anti-seizure actions.
Sodium is the primary cation in extracellular fluids in animals and humans. These fluids, such as blood plasma and extracellular fluids in other tissues, bathe cells and carry out transport functions for nutrients and wastes. Sodium is also the principal cation in seawater, although the concentration there is about 3.8 times what it is normally in extracellular body fluids. This suggests that animal life moved from the sea to dry land at a time when the seas were far less salty than they are now.
Although the system for maintaining optimal salt and water balance in the body is a complex one, one of the primary ways in which the human body keeps track of loss of body water is that osmoreceptors in the
hypothalamus sense a balance of sodium and water concentration in extracellular fluids. Relative loss of body water will cause sodium concentration to rise higher than normal, a condition known as hypernatremia. This ordinarily results in thirst. Conversely, an excess of body water caused by drinking will result in too little sodium in the blood , a condition which is again sensed by the
hypothalamus, causing a decrease in vasopressin hormone secretion from the
posterior pituitary, and a consequent loss of water in the urine, which acts to restore blood sodium concentrations to normal.
Severely dehydrated persons, such as people rescued from ocean or desert survival situations, usually have very high blood sodium concentrations. These must be very carefully and slowly returned to normal, since too-rapid correction of hyponatremia may result in brain damage from cellular swelling as water moves suddenly into cells with high osmolar content.
Because the hypothalamus/osmoreceptor system ordinarily works well to cause drinking or urination to restore the body's sodium concentrations to normal, this system can be used in medical treatment to regulate the body's total fluid content by controlling the body's sodium content. Thus, when a powerful diuretic drug is given which causes the kidneys to excrete sodium, the effect is usually followed by an excretion of body water, as the osmoreceptor system senses the sodium loss, and then directs compensatory urinary loss of water in order to correct the hyponatremia, or state.
See also
References
External links
