This article is about the chemical. For its familiar form, common table salt, seeSalt. For the medical solutions, seeSaline (medicine). For the mineral, seeHalite.
Sodium chloride/ˌsoʊdiəmˈklɔːraɪd/,[8] commonly known astable salt, is anionic compound with thechemical formulaNaCl, representing a 1:1 ratio ofsodium andchlorideions. It is transparent or translucent, brittle,hygroscopic, and occurs as the mineralhalite. In its edible form, it is commonly used as acondiment andfood preservative. Large quantities of sodium chloride are used in many industrial processes, and it is a major source of sodium and chlorine compounds used asfeedstocks for furtherchemical syntheses. Another major application of sodium chloride isde-icing of roadways in sub-freezing weather.
In addition to the many familiar domestic uses of salt, more dominant applications of the approximately 250 million tonnes per year production (2008 data) include chemicals and de-icing.[9]
This electrolysis is conducted in either a mercury cell, a diaphragm cell, or a membrane cell. Each of those uses 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 includePVCthermoplastics production, disinfectants, and solvents.
Sodium hydroxide is extensively used in many different industries enabling production of paper, soap, aluminum, and more.
Sodium chloride is heavily used, so even relatively minor applications can consume massive quantities. Inoil andgas exploration, salt is an important component of drilling fluids in well drilling.[11] It is used toflocculate and increase thedensity 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 in order to minimize the dissolution within the salt stratum.[9] Salt is also used to increase the curing of concrete in cemented casings.[10]
In textiles and dyeing, salt is used as a brine rinse to separate organic contaminants,[12] to promote "salting out" of dyestuff precipitates, and to blend with concentrated dyes to increase yield in dyebaths and make the colors look sharper. One of its main roles is to provide the positive ion charge to promote the absorption of negatively charged ions of dyes.[10]
In tanning and leather treatment, salt is added to animalhides to inhibit microbial activity on the underside of the hides and to attract moisture back into the hides.[10]
In rubber manufacture, salt is used to makebuna,neoprene, and white rubber types. Salt brine and sulfuric acid are used to coagulate an emulsifiedlatex made from chlorinatedbutadiene.[10][9]
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.[10]
Hard water containscalcium andmagnesium ions that interfere with action ofsoap 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 useion-exchange resins to remove ions that cause the hardness. These resins are generated and regenerated using sodium chloride.[10][9]
The second major application of salt is fordeicing and anti-icing of roads, both ingrit bins and spread bywinter service vehicles. In anticipation of snowfall, roads are optimally "anti-iced" with brine (concentratedsolution of salt in water), which prevents bonding between the snow-ice and the road surface. This procedure obviates the heavy use of salt after the snowfall. For deicing, mixtures of brine and salt are used, sometimes with additional agents such ascalcium chloride and/ormagnesium chloride. The use of salt or brine becomes ineffective below −10 °C (14 °F).
Mounds of road salt for use in winter
Salt for deicing in the United Kingdom predominantly comes from a single mine inWinsford inCheshire. Prior to distribution it is mixed with <100 ppm ofsodium ferrocyanide as ananticaking 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 have 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.[10]
In the technical terms of physical chemistry, the minimum freezing point of a water-salt mixture is −21.12 °C (−6.02 °F) for 23.31 wt% of salt. Freezing near this concentration is however so slow that theeutectic point of −22.4 °C (−8.3 °F) can be reached with about 25 wt% of salt.[13]
Road salt ends up in fresh-water bodies and could harm aquatic plants and animals by disrupting theirosmoregulation ability.[14] The omnipresence of salt in coastal areas poses a problem in anycoating application, because trapped salts cause great problems in adhesion. Naval authorities and ship builders monitor the salt concentrations on surfaces during construction. Maximal salt concentrations on surfaces are dependent on the authority and application. TheIMO 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 aBresle test. Salinization (increasing salinity, akafreshwater salinization syndrome) and subsequent increased metal leaching is an ongoing problem throughout North America and European fresh waterways.[15]
In highway de-icing, salt has been associated withcorrosion of bridge decks, motor vehicles, reinforcement bar and wire, and unprotected steel structures used in road construction.Surface runoff, vehicle spraying, and windblown salt 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.[10] A 2009 study found that approximately 70% of the road salt being applied in theMinneapolis-St Paul metro area is retained in the local watershed.[16]
Some agencies are substitutingbeer,molasses, andbeet juice instead of road salt.[17] Airlines utilize moreglycol andsugar rather than salt-based solutions for deicing.[18]
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 insausages to form a binding gel made up of meat, fat, and moisture. Salt also acts as a flavor enhancer and as atenderizer.[10]
It is used as a cheap and safedesiccant because of itshygroscopic properties, makingsalting an effective method offood preservation historically; the salt draws water out of bacteria throughosmotic pressure, keeping it from reproducing, a major source of food spoilage. Even though more effective desiccants are available, few are safe for humans to ingest. Manymicroorganisms cannot live in a salty environment: water is drawn out of theircells byosmosis. For this reason salt is used topreserve some foods, such as bacon, fish, or cabbage.
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 andpreservative. 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 thegluten (the elastic protein-water complex in certain doughs) 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 in products such as potato chips,pretzels, and cat and dog food.[10]
Sodium chloride is used in veterinary medicine asemesis-causing agent. It is given as warm saturated solution. Emesis can also be caused bypharyngeal placement of small amount of plain salt or salt crystals.
For watering plants to use sodium chloride (NaCl) as a fertilizer, moderate concentration is used to avoid potential toxicity: 1–3 grams (0.035–0.106 oz) per liter is generally safe and effective for most plants.[19][20][21]
Sodium chloride is the principal extinguishing agent in dry-powderfire extinguishers that are used on combustible metal fires, for metals such as magnesium, zirconium, titanium, and lithium (Class D extinguishers). The salt forms an oxygen-excluding crust that smothers the fire.[23]
Since at leastmedieval times, people have used salt as a cleansing agent rubbed on household surfaces. It is also used in many brands ofshampoo and toothpaste and is commonly used to deice driveways, parking lots, and walkways.
Sodium chloride crystals have atransmittance of at least 90% (through 1 mm) forinfrared light having wavelengths in the range 0.2– 18 μm.[24] They were used in optical components such as windows and lenses, where few non-absorbing alternatives existed in that spectral range. While inexpensive, NaCl crystals are soft andhygroscopic – when exposed to the water in ambient air, they gradually cover with "frost". This limits application of NaCl to dry environments, vacuum-sealed areas, or short-term uses such as prototyping. Materials that are mechanically stronger and less sensitive to moisture, such aszinc selenide andchalcogenide glasses, are more widely used than NaCl.
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 regularoctahedron. In the language ofclose-packing, the largerchlorideions (167 pm in size[25]) are arranged in a cubic array whereas the smallersodium ions (116 pm[25]) fill all the cubic gaps (octahedral voids) between them. This same basic structure is found in many othercompounds and is commonly known as theNaCl structure or rock salt crystal structure. It can be represented as aface-centered cubic (fcc) 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 halfway between lattice points along the fcc unit cell edge.
Solid sodium chloride has a melting point of 801 °C and liquid sodium chloride boils at 1465 °C. Atomic-resolution real-time video imaging allows visualization of the initial stage of crystal nucleation of sodium chloride.[26]
TheThermal conductivity of sodium chloride as a function of temperature has a maximum of 2.03 W/(cm K) at 8 K (−265.15 °C; −445.27 °F) and decreases to 0.069 at 314 K (41 °C; 106 °F). It also decreases withdoping.[27]
View of one slab ofhydrohalite, NaCl·2H2O. (red = O, white = H, green = Cl, purple = Na).[28]
The attraction between the Na+ and Cl− ions in the solid is so strong that only highlypolar solvents like water dissolve NaCl well.
When dissolved in water, the sodium chloride framework disintegrates as the Na+ and Cl− ions become surrounded by polar water molecules. These solutions consist ofmetal 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 six molecules of water.[31] Solutions of sodium chloride have very different properties from pure water. Theeutectic point is −21.12 °C (−6.02 °F) for 23.31%mass fraction of salt, and the boiling point of saturated salt solution is near 108.7 °C (227.7 °F).[13]
ThepH of a sodium chloride solution remains ≈7 due to the extremely weak basicity of the Cl− ion, which is the conjugate base of thestrong acid HCl. In other words, NaCl has no effect on system pH[32] in diluted solutions where the effects of ionic strength and activity coefficients are negligible.
Solubility of NaCl (g NaCl / 1 kg of solvent at 25 °C (77 °F))[33]
Common salt has a 1:1 molar ratio of sodium and chlorine. In 2013, compounds of sodium and chloride of differentstoichiometries have been discovered; five new compounds were predicted (e.g., Na3Cl, Na2Cl, Na3Cl2, NaCl3, and NaCl7). The existence of some of them has been experimentally confirmed at high pressures and other conditions: cubic and orthorhombic NaCl3, two-dimensional metallic tetragonal Na3Cl and exotic hexagonal NaCl.[34] This indicates that compounds violating chemical intuition are possible, in simple systems under non-ambient conditions.[35]
Salt is currentlymass-produced byevaporation ofseawater orbrine frombrine wells andsalt lakes.Mining of rock salt is also a major source. China is the world's main supplier of salt.[10] In 2017, world production was estimated at 280 milliontonnes, the top five producers (in million tonnes) being China (68.0), United States (43.0), India (26.0), Germany (13.0), and Canada (13.0).[38] Salt is also a byproduct ofpotassium mining.
^Wells, John C. (2008),Longman Pronunciation Dictionary (3rd ed.), Longman, pp. 143 and 755,ISBN9781405881180
^abcdWestphal, Gisbertet al. (2002) "Sodium Chloride" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheimdoi:10.1002/14356007.a24_317.pub4.
^abcdefghijklKostick, Dennis S. (October 2010)."Salt"(PDF).U.S. Geological Survey, 2008 Minerals Yearbook.Archived(PDF) from the original on 1 July 2017. Retrieved4 May 2011.
^Caenn, Ryen; Darley, H. C. H.; Gray, George Robert; Gray, George Robert (2011).Composition and properties of drilling and completion fluids (6th ed.). Amsterdam ; Boston, MA: Gulf Professional Pub.ISBN978-0-12-383858-2.
^Maathuis, Frans J. M. (22 October 2013). "Sodium in plants: Perception, signalling, and regulation of sodium fluxes".Journal of Experimental Botany.65 (3):849–858.doi:10.1093/jxb/ert326.PMID24151301.
^Waynant, Ronald W.; Ediger, Marwood N. (2000). "Chapter 11: Optical Materials: Visible and Infrared".Electro-optics Handbook. New York: McGraw-Hill Professional Publishing. p. 11.20.ISBN0-07-068716-1.Archived from the original on 1 May 2024. Retrieved1 May 2024.
^abR. D. Shannon (1976). "Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides".Acta Crystallogr A.32 (5):751–767.Bibcode:1976AcCrA..32..751S.doi:10.1107/S0567739476001551.
^Lincoln, S. F.; Richens, D. T. and Sykes, A. G. (2003) "Metal Aqua Ions" Comprehensive Coordination Chemistry II Volume 1, pp. 515–555.doi:10.1016/B0-08-043748-6/01055-0.
^"Acidic, Basic, and Neutral Salts".Flinn Scientific Chem Fax. 2016.Archived from the original on 19 September 2018. Retrieved18 September 2018.Neutralization of a strong acid and a strong base gives a neutral salt.
^Burgess, J (1978).Metal Ions in Solution. New York: Ellis Horwood.ISBN978-0-85312-027-8.
^Tikhomirova, K. A.; Tantardini, C.; Sukhanova, E. V.; Popov, Z. I.; Evlashin, S. A.; Tarkhov, M. A.; Zhdanov, V. L. (2020). "Exotic Two-Dimensional Structure: The first case of Hexagonal NaCl".The Journal of Physical Chemistry Letters.11 (10):3821–3827.doi:10.1021/acs.jpclett.0c00874.PMID32330050.S2CID216130640.
^Bolen, Wallace P. (January 2018)."Salt"(PDF).U.S. Geological Survey, 2018 Mineral Commodity Summaries.Archived(PDF) from the original on 22 September 2018. Retrieved22 September 2018.
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