Sodium chloride, also known as salt, common salt, table salt or halite, is an ionic compound
with the formula NaCl, representing equal proportions of sodium and chlorine. Sodium
chloride is the salt most responsible for the salinity of the ocean and of the extracellular
fluid of many multicellular organisms. In the form of edible or table salt it is commonly
used as a condiment and food preservative. Large quantities of sodium chloride are used
in many industrial processes, and it is a major source of sodium and chlorine compounds
used as feedstocks for further chemical syntheses. A second major consumer of sodium chloride
is deicing of roadways in sub-freezing weather.
Chemistry Solid sodium chloride
In solid sodium chloride, each ion is surrounded by six ions of the opposite charge as expected
on electrostatic grounds. The surrounding ions are located at the vertices of a regular
octahedron. In the language of close-packing, the larger chloride ions are arranged in a
cubic array whereas the smaller sodium ions fill all the cubic gaps between them. This
same basic structure is found in many other compounds and is commonly known as the halite
or rock-salt crystal structure. It can be represented as a face-centered cubic lattice
with a two-atom basis or as two interpenetrating face centered cubic lattices. The first atom
is located at each lattice point, and the second atom is located half way between lattice
points along the fcc unit cell edge. Thermal conductivity of NaCl as a function
of temperature has a maximum of 2.03 W/(cm K) at 8 K and decreases to 0.069 at 314 K.
It also decreases with doping. Aqueous solutions
The attraction between the Na+ and Cl− ions in the solid is so strong that only highly
polar solvents like water dissolve NaCl well. When dissolved in water, the sodium chloride
framework disintegrates as the Na+ and Cl− ions become surrounded by the polar water
molecules. These solutions consist of metal aquo complex with the formula [Na(H2O)8]+,
with the Na-O distance of 250 pm. The chloride ions are also strongly solvated, each being
surrounded by an average of 6 molecules of water. Solutions of sodium chloride have very
different properties from pure water. The freezing point is −21.12 °C for 23.31
wt% of salt, and the boiling point of saturated salt solution is near 108.7 °C. From cold
solutions, salt crystallises as the dihydrate NaCl·2H2O.
Unexpected stable stoichiometric variants Common salt has a well-established 1:1 molar
ratio of sodium and chlorine. In 2013, compounds of sodium and chloride of different stoichiometries
have been discovered by a team at Stony Brook University; five new compounds were predicted.
The existence of some of them has been tested and experimentally confirmed: cubic and orthorhombic
NaCl3 and two-dimensional metallic tetragonal Na3Cl. This indicates that compounds violating
chemical intuition are possible, in simple systems under nonambient conditions.
Occurrence Small particles of sea salt are the dominant
cloud condensation nuclei well out at sea, which allow the formation of clouds in otherwise
non-polluted air. Production
Salt is currently mass-produced by evaporation of seawater or brine from brine wells and
salt lakes. Mining of rock salt is also a major source. China is the world's main supplier
of salt. In 2010, world production was estimated at 270 million tonnes, the top five producers
being China, United States, Germany, India and Canada. Salt is also a byproduct of potassium
mining. Uses
In addition to the familiar domestic uses of salt, more dominant applications of the
approximately 250 megatons/year production include chemicals and de-icing.
Chemicals production Salt is the source, directly or indirectly,
for the production of many chemicals, which consume most of the world's production.
Chlor-alkali industry
It is the starting point for the Chloralkali process, which provides the world with chlorine
and sodium hydroxide according to the chemical equation
2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH This electrolysis is conducted in either a
mercury cell, a diaphragm cell, or a membrane cell. Each of those use a different method
to separate the chlorine from the sodium hydroxide. Other technologies are under development due
to the high energy consumption of the electrolysis, whereby small improvements in the efficiency
can have large economic paybacks. Some applications of chlorine include PVC, disinfectants, and
solvents. Sodium hydroxide enables industries that produce paper, soap, and aluminium.
Soda ash industry Via the Solvay process, sodium chloride is
used to produce sodium carbonate and calcium chloride. Sodium carbonate is used to produce
glass, sodium bicarbonate, and dyes as well as a myriad of other chemicals. In the Mannheim
process and in the Hargreaves process, it is used for the production of sodium sulfate
and hydrochloric acid. Standard
Sodium Chloride has an international standard that is created by ASTM International. The
standard is named ASTM E534-13 and it is the standard test methods for chemical analysis
of sodium chloride. These methods listed provide procedures for analyzing sodium chloride to
determine if it is suitable for its intended use and application.
Miscellaneous industrial uses Sodium chloride is heavily used, so even miscellaneous
applications can consume massive quantities. In oil and gas exploration, salt is an important
component of drilling fluids in well drilling. It is used to flocculate and increase the
density of the drilling fluid to overcome high downwell gas pressures. Whenever a drill
hits a salt formation, salt is added to the drilling fluid to saturate the solution and
to minimize the dissolution within the salt stratum. Salt is also used to increase the
curing of concrete in cemented casings. In textiles and dyeing, salt is used as a
brine rinse to separate organic contaminants, to promote “salting out” of dyestuff precipitates,
and to blend with concentrated dyes to standardize them. One of its main roles is to provide
the positive ion charge to promote the absorption of negatively charged ions of dyes.
It also is used in processing aluminium, beryllium, copper, steel and vanadium. In the pulp and
paper industry, salt is used to bleach wood pulp. It also is used to make sodium chlorate,
which is added along with sulfuric acid and water to manufacture chlorine dioxide, an
excellent oxygen-based bleaching chemical. The chlorine dioxide process, which originated
in Germany after World War I, is becoming more popular because of environmental pressures
to reduce or eliminate chlorinated bleaching compounds. In tanning and leather treatment,
salt is added to animal hides to inhibit microbial activity on the underside of the hides and
to attract moisture back into the hides. In rubber manufacture, salt is used to make
buna, neoprene and white rubber types. Salt brine and sulfuric acid are used to coagulate
an emulsified latex made from chlorinated butadiene.
Salt also is added to secure the soil and to provide firmness to the foundation on which
highways are built. The salt acts to minimize the effects of shifting caused in the subsurface
by changes in humidity and traffic load. Water softening
Hard water contains calcium and magnesium ions that interfere with action of soap and
contribute to the buildup of a scale or film of alkaline mineral deposits in household
and industrial equipment and pipes. Commercial and residential water-softening units use
ion exchange resins to remove the offending ions that cause the hardness. These resins
are generated and regenerated using sodium chloride.
Road salt
The second major application of salt is for de-icing and anti-icing of roads, both in
grit bins and spread by winter service vehicles. In anticipation of snowfall, roads are optimally
"anti-iced" with brine, which prevents bonding between the snow-ice and the road surface.
This procedure obviates the heavy use of salt after the snowfall. For de-icing, mixtures
of brine and salt are used, sometimes with additional agents such as calcium and magnesium
chloride. The use of salt or brine becomes ineffective below −10 °C.
Salt for de-icing in the United Kingdom predominantly comes from a single mine in Winsford in Cheshire.
Prior to distribution it is mixed with <100 ppm of sodium hexacyanoferrate(II) as an anti-caking
agent, which enables rock salt to flow freely out of the gritting vehicles despite being
stockpiled prior to use. In recent years this additive has also been used in table salt.
Other additives had been used in road salt to reduce the total costs. For example, in
the US, a byproduct carbohydrate solution from sugar beet processing was mixed with
rock salt and adhered to road surfaces about 40% better than loose rock salt alone. Because
it stayed on the road longer, the treatment did not have to be repeated several times,
saving time and money. In the technical terms of physical chemistry,
the minimum freezing point of a water-salt mixture is −21.12 °C for 23.31 wt% of
salt. Freezing near this concentration is however so slow that the eutectic point of
−22.4 °C can be reached with about 25 wt% of salt.
Environmental effects Road salt ends up in fresh water bodies and
could harm aquatic plants and animals by disrupting their osmoregulation ability. The omnipresence
of salt poses a problem in any coastal coating application, as trapped salts cause great
problems in adhesion. Naval authorities and ship builders monitor the salt concentrations
on surfaces during construction. Maximum salt concentrations on surfaces are dependent on
the authority and application. The IMO regulation is mostly used and sets salt levels to a maximum
of 50 mg/m2 soluble salts measured as sodium chloride. These measurements are done by means
of a Bresle test. In highway de-icing, salt has been associated
with corrosion of bridge decks, motor vehicles, reinforcement bar and wire, and unprotected
steel structures used in road construction. Surface runoff, vehicle spraying, and windblown
actions also affect soil, roadside vegetation, and local surface water and groundwater supplies.
Although evidence of environmental loading of salt has been found during peak usage,
the spring rains and thaws usually dilute the concentrations of sodium in the area where
salt was applied. Food industry, medicine and agriculture
Many micro organisms cannot live in an overly salty environment: water is drawn out of their
cells by osmosis. For this reason salt is used to preserve some foods, such as smoked
bacon, fish, or cabbage. It can also be used to detach leeches that have attached themselves
to feed. It is also used to disinfect wounds. Salt is added to food, either by the food
producer or by the consumer, as a flavor enhancer, preservative, binder, fermentation-control
additive, texture-control agent and color developer. The salt consumption in the food
industry is subdivided, in descending order of consumption, into other food processing,
meat packers, canning, baking, dairy and grain mill products. Salt is added to promote color
development in bacon, ham and other processed meat products. As a preservative, salt inhibits
the growth of bacteria. Salt acts as a binder in sausages to form a binding gel made up
of meat, fat, and moisture. Salt also acts as a flavor enhancer and as a tenderizer.
In many dairy industries, salt is added to cheese as a color-, fermentation-, and texture-control
agent. The dairy subsector includes companies that manufacture creamery butter, condensed
and evaporated milk, frozen desserts, ice cream, natural and processed cheese, and specialty
dairy products. In canning, salt is primarily added as a flavor enhancer and preservative.
It also is used as a carrier for other ingredients, dehydrating agent, enzyme inhibitor and tenderizer.
In baking, salt is added to control the rate of fermentation in bread dough. It also is
used to strengthen the gluten and as a flavor enhancer, such as a topping on baked goods.
The food-processing category also contains grain mill products. These products consist
of milling flour and rice and manufacturing cereal breakfast food and blended or prepared
flour. Salt is also used a seasoning agent, e.g. in potato chips, pretzels, cat and dog
food. Sodium chloride is used in veterinary medicine
as emesis causing agent. It is given as warm saturated solution. Emesis can also be caused
by pharyngeal placement of small amount of plain salt or salt crystals.
Firefighting
Sodium chloride is the principal extinguishing agent in fire extinguishers used on combustible
metal fires such as magnesium, potassium, sodium, and NaK alloys. Thermoplastic powder
is added to the mixture, along with waterproofing and anti-caking materials to form the extinguishing
agent. When it is applied to the fire, the salt acts like a heat sink, dissipating heat
from the fire, and also forms an oxygen-excluding crust to smother the fire. The plastic additive
melts and helps the crust maintain its integrity until the burning metal cools below its ignition
temperature. This type of extinguisher was invented in the late 1940s in the cartridge-operated
type shown here, although stored pressure versions are now popular. Common sizes are
30 pounds portable and 350 pounds wheeled. Cleanser
Since at least medieval times, people have used salt as a cleansing agent rubbed on household
surfaces. It is also used in many brands of shampoo, toothpaste and popularly to de-ice
driveways and patches of ice. Optical usage
Defect-free NaCl crystals have an optical transmittance of about 90% between 200 nm
and 20 µm. They were therefore used in optical components operating in the infrared spectral
range, where few non-absorbing alternatives exist and where requirements for absence of
microscopic inhomogeneities are less strict than in the visible range. While inexpensive,
NaCl crystals are soft and hygroscopic – when exposed to the ambient air they gradually
cover with "frost". This limits application of NaCl to dry environments or for short-term
uses such as prototyping. Nowadays materials like zinc selenide, which are stronger mechanically
and are less sensitive to moisture, are used instead of NaCl for the IR spectral range.
Biological functions
The long held belief that a high-salt diet raises the risk of cardio-vascular disease
is coming under scrutiny. More recently, dietary salt was demonstrated to attenuate nitric
oxide production. Nitric oxide contributes to vessel homeostasis by inhibiting vascular
smooth muscle contraction and growth, platelet aggregation, and leukocyte adhesion to the
endothelium.
See also
References This article incorporates public domain
material from the United States Geological Survey document: "Salt".
External links The Salt Manufacturers Association website
Salt United States Geological Survey Statistics and Information
"Using Salt and Sand for Winter Road Maintenance". Road Management Journal. December 1997.
Calculators: surface tensions, and densities, molarities and molalities of aqueous NaCl
vapour pressure of NaCl solution JtBaker MSDS