Sodium chloride

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Sodium chloride
Halit-Kristalle.jpg
NaCl polyhedra.png
Names
IUPAC name
Sodium chloride
Other names
  • Common salt
  • Halite
  • Rock salt
  • Saline
  • Sodium chloride
  • Table salt
  • Regular salt
  • Sea salt
Identifiers
3D model (JSmol)
3534976
ChEBI
ChemSpider
ECHA InfoCard 100.028.726
EC Number 231-598-3
13673
KEGG
MeSH Sodium+chloride
RTECS number VZ4725000
UNII
Properties
NaCl
Molar mass 58.44 g mol−1
Appearance Colorless crystals
Odor Odorless
Density 2.165 g/cm3
Melting point 801 °C (1,474 °F; 1,074 K)
Boiling point 1,413 °C (2,575 °F; 1,686 K)
359 g/L
Solubility in ammonia 21.5 g/L
Solubility in methanol 14.9 g/L
−30.3·10−6 cm3/mol
1.5442 (at 589 nm)
Structure
Face-centered cubic
(see text), cF8
Fm3m, No. 225
a = 564.02 pm
Octahedral (Na+)
Octahedral (Cl)
Thermochemistry
36.79 J K−1 mol−1
72.11 J K−1 mol−1
−411.12 kJ mol−1
Pharmacology
A12CA01 (WHO) B05CB01 (WHO), B05XA03 (WHO), S01XA03 (WHO)
Hazards
Safety data sheet See: data page
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g., sodium chloride Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Lethal dose or concentration (LD, LC):
3 g/kg (oral, rats)[1]
Related compounds
Other anions
Sodium fluoride
Sodium bromide
Sodium iodide
Sodium astatide
Other cations
Lithium chloride
Potassium chloride
Rubidium chloride
Caesium chloride
Francium chloride
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Thermodynamic
data
Phase behaviour
solid–liquid–gas
UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

Sodium chloride /ˌsdiəm ˈklɔːrd/,[2] also known as salt or halite, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g·mol−1, respectively, 100 g of NaCl contain 39.34 g Na and 60.66 g Cl. Sodium chloride is the salt most responsible for the salinity of seawater 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.

Salt in its natural form is known as rock salt or halite. Salt is present in vast quantities in seawater, where it is the main mineral constituent. The open ocean has about 35 grams (1.2 oz) of solids per liter, a salinity of 3.5%.

Salt is essential for life in general, and saltiness is one of the basic human tastes. The tissues of animals contain larger quantities of salt than do plant tissues. Salt is one of the oldest and most ubiquitous food seasonings, and salting is an important method of food preservation.

Some of the earliest evidence of salt processing dates to around 8,000 years ago, when people living in an area in what is now known as the country of Romania were boiling spring water to extract the salts; a salt-works in China dates to approximately the same period. Salt was also prized by the ancient Hebrews, the Greeks, the Romans, the Byzantines, the Hittites, Egyptians, and the Indians. Salt became an important article of trade and was transported by boat across the Mediterranean Sea, along specially built salt roads, and across the Sahara in camel caravans. The scarcity and universal need for salt has led nations to go to war over it and use it to raise tax revenues. Salt is used in religious ceremonies and has other cultural significance.

Salt is processed from salt mines, or by the evaporation of seawater (sea salt) or mineral-rich spring water in shallow pools. Its major industrial products are caustic soda and chlorine, and is used in many industrial processes including the manufacture of polyvinyl chloride, plastics, paper pulp and many other products.

Edible salt is sold in forms such as sea salt and table salt which usually contains an anti-caking agent and may be iodised to prevent iodine deficiency. As well as its use in cooking and at the table, salt is present in many processed foods.

Sodium is an essential nutrient for human health via its role as an electrolyte and osmotic solute.[3] Excessive salt consumption can increase the risk of cardiovascular diseases, such as hypertension, in children and adults. Such health effects of salt have long been studied. Accordingly, numerous world health associations and experts in developed countries recommend reducing consumption of popular salty foods.[4] The World Health Organization recommends that adults should consume less than 2,000 mg of sodium, equivalent to 5 grams of salt per day.[5]

History[edit]

Salt production in Halle, Saxony-Anhalt (1670)

Humans have always tended to build communities either around sources of salt, or where they can trade for it.

All through history the availability of salt has been pivotal to civilization. The word "salary" comes from the Latin word for salt because the Roman Legions were sometimes paid in salt.[6] The Natron Valley was a key region that supported the Egyptian Empire to its north, because it supplied it with a kind of salt that came to be called by its name, natron.

Even before this, what is now thought to have been the first city in Europe is Solnitsata, in Bulgaria, which was a salt mine, providing the area now known as the Balkans with salt since 5400 BC.[7] Even the name Solnisata means "salt works".

While people have used canning and artificial refrigeration to preserve food for the last hundred years or so, salt has been the best-known food preservative, especially for meat, for many thousands of years.[8] A very ancient salt-works operation has been discovered at the Poiana Slatinei archaeological site next to a salt spring in Lunca, Neamț County, Romania. Evidence indicates that Neolithic people of the Precucuteni Culture were boiling the salt-laden spring water through the process of briquetage to extract the salt as far back as 6050 BC.[9] The salt extracted from this operation may have had a direct correlation to the rapid growth of this society's population soon after its initial production began.[10] The harvest of salt from the surface of Xiechi Lake near Yuncheng in Shanxi, China, dates back to at least 6000 BC, making it one of the oldest verifiable saltworks.[11]

There is more salt in animal tissues such as meat, blood and milk, than there is in plant tissues.[12] Nomads who subsist on their flocks and herds do not eat salt with their food, but agriculturalists, feeding mainly on cereals and vegetable matter, need to supplement their diet with salt.[13] With the spread of civilization, salt became one of the world's main trading commodities. It was of high value to the ancient Hebrews, the Greeks, the Romans, the Byzantines, the Hittites and other peoples of antiquity. In the Middle East, salt was used to ceremonially seal an agreement, and the ancient Hebrews made a "covenant of salt" with God and sprinkled salt on their offerings to show their trust in him.[14] An ancient practice in time of war was salting the earth: scattering salt around in a defeated city in order to prevent plant growth. The Bible tells the story of King Abimelech who was ordered by God to do this at Shechem,[15] and various texts claim that the Roman general Scipio Aemilianus Africanus ploughed over and sowed the city of Carthage with salt after it was defeated in the Third Punic War (146 BC).[16]

Ponds near Maras, Peru, fed from a mineral spring and used for salt production since the time of the Incas.

Salt may have been used for barter in connection with the obsidian trade in Anatolia in the Neolithic Era.[17] Herodotus described salt trading routes across Libya back in the 5th century BC. In the early years of the Roman Empire, roads such as the Via Salaria were built for the transportation of salt from the salt pans of Ostia to the capital.[18] Salt was included among funeral offerings found in ancient Egyptian tombs from the third millennium BC, as were salted birds, and salt fish.[19] From about 2800 BC, the Egyptians began exporting salt fish to the Phoenicians in return for Lebanon cedar, glass and the dye Tyrian purple; the Phoenicians traded Egyptian salt fish and salt from North Africa throughout their Mediterranean trade empire.[20]

In Africa, salt was used as currency south of the Sahara, and slabs of rock salt were used as coins in Abyssinia.[13] Moorish merchants in the 6th century traded salt for gold, weight for weight. The Tuareg have traditionally maintained routes across the Sahara especially for the transportation of salt by Azalai (salt caravans). The caravans still cross the desert from southern Niger to Bilma, although much of the trade now takes place by truck. Each camel takes two bales of fodder and two of trade goods northwards and returns laden with salt pillars and dates.[21] In Gabon, before the coming of the white man, the coast people carried on a remunerative trade with those of the interior by the medium of sea salt. This was gradually displaced by the salt the white men brought in sacks, so that the coast natives lost their previous profits; as of the author's writing in 1958, sea salt was still the currency best appreciated in the interior.[22]

Salzburg, Hallstatt, and Hallein lie within 17 km (11 mi) of each other on the river Salzach in central Austria in an area with extensive salt deposits. Salzach literally means "salt river" and Salzburg "salt castle", both taking their names from the German word Salz meaning salt and Hallstatt was the site of the world's first salt mine.[23] The town gave its name to the Hallstatt culture that began mining for salt in the area in about 800 BC. Around 400 BC, the townsfolk, who had previously used pickaxes and shovels, began open pan salt making. During the first millennium BC, Celtic communities grew rich trading salt and salted meat to Ancient Greece and Ancient Rome in exchange for wine and other luxuries.[8] The word salary comes from the Latin word for salt. The reason for this is unknown; a persistent modern claim that the Roman Legions were sometimes paid in salt is baseless.[24][25][26][27] The word salad literally means "salted", and comes from the ancient Roman practice of salting leaf vegetables.[28]

Wars have been fought over salt. Venice fought and won a war with Genoa over the product, and it played an important part in the American Revolution. Cities on overland trade routes grew rich by levying duties,[29] and towns like Liverpool flourished on the export of salt extracted from the salt mines of Cheshire.[30] Various governments have at different times imposed salt taxes on their peoples. The voyages of Christopher Columbus are said to have been financed from salt production in southern Spain, and the oppressive salt tax in France was one of the causes of the French Revolution. After being repealed, this tax was reimposed by Napoleon when he became emperor to pay for his foreign wars, and was not finally abolished until 1945.[29] In 1930, Mahatma Gandhi led at least 100,000 people on the "Dandi March" or "Salt Satyagraha", in which protesters made their own salt from the sea thus defying British rule and avoiding paying the salt tax. This civil disobedience inspired millions of common people, and elevated the Indian independence movement from an elitist movement to a national struggle.[31]

Religion[edit]

Bread and salt at a Russian wedding ceremony
Salt deposits beside the Dead Sea
Red rock salt from the Khewra Salt Mine in Pakistan

Salt has long held an important place in religion and culture. At the time of Brahmanic sacrifices, in Hittite rituals and during festivals held by Semites and Greeks at the time of the new moon, salt was thrown into a fire where it produced crackling noises.[32] The ancient Egyptians, Greeks and Romans invoked their gods with offerings of salt and water and some people think this to be the origin of Holy Water in the Christian faith.[33] In Aztec mythology, Huixtocihuatl was a fertility goddess who presided over salt and salt water.[34]

Salt is considered to be a very auspicious substance in Hinduism and is used in particular religious ceremonies like house-warmings and weddings.[35] In Jainism, devotees lay an offering of raw rice with a pinch of salt before a deity to signify their devotion and salt is sprinkled on a person's cremated remains before the ashes are buried.[36] Salt is believed to ward off evil spirits in Mahayana Buddhist tradition, and when returning home from a funeral, a pinch of salt is thrown over the left shoulder as this prevents evil spirits from entering the house.[37] In Shinto, salt is used for ritual purification of locations and people (harae, specifically shubatsu), and small piles of salt are placed in dishes by the entrance of establishments for the two-fold purposes of warding off evil and attracting patrons.[38]

In the Hebrew Bible, there are thirty-five verses which mention salt.[39] One of these mentions Lot's wife, who was turned into a pillar of salt when she looked back at the cities of Sodom and Gomorrah (Genesis 19:26) as they were destroyed. When the judge Abimelech destroyed the city of Shechem, he is said to have "sown salt on it," probably as a curse on anyone who would re-inhabit it (Judges 9:45). The Book of Job contains the first mention of salt as a condiment. "Can that which is unsavoury be eaten without salt? or is there any taste in the white of an egg?" (Job 6:6).[39] In the New Testament, six verses mention salt. In the Sermon on the Mount, Jesus referred to his followers as the "salt of the earth". The apostle Paul also encouraged Christians to "let your conversation be always full of grace, seasoned with salt" (Colossians 4:6).[39] Salt is mandatory in the rite of the Tridentine Mass.[40] Salt is used in the third item (which includes an Exorcism) of the Celtic Consecration (cf. Gallican Rite) that is employed in the consecration of a church. Salt may be added to the water "where it is customary" in the Roman Catholic rite of Holy water.[40]

In Judaism, it is recommended to have either a salty bread or to add salt to the bread if this bread is unsalted when doing Kiddush for Shabbat. It is customary to spread some salt over the bread or to dip the bread in a little salt when passing the bread around the table after the Kiddush.[41] To preserve the covenant between their people and God, Jews dip the Sabbath bread in salt.[33]

In Wicca, salt is symbolic of the element Earth. It is also believed to cleanse an area of harmful or negative energies. A dish of salt and a dish of water are almost always present on an altar, and salt is used in a wide variety of rituals and ceremonies.[42]

Uses[edit]

Salt shaker

Salt is essential to the health of humans and other animals, and is one of the five basic taste sensations.[43]

Salt is used in many cuisines around the world, and is often found in salt shakers on diners' eating tables for their personal use on food. Salt is also an ingredient in many manufactured foodstuffs. Table salt is a refined salt containing about 97 to 99 percent sodium chloride.[44][45][46] Usually, anticaking agents such as sodium aluminosilicate or magnesium carbonate are added to make it free-flowing. Iodized salt, containing potassium iodide, is widely available. Some people put a desiccant, such as a few grains of uncooked rice[47] or a saltine cracker, in their salt shakers to absorb extra moisture and help break up salt clumps that may otherwise form.[48]

Fortified table salt[edit]

Some table salt sold for consumption contain additives which address a variety of health concerns, especially in the developing world. The identities and amounts of additives vary widely from country to country. Iodine is an important micronutrient for humans, and a deficiency of the element can cause lowered production of thyroxine (hypothyroidism) and enlargement of the thyroid gland (endemic goitre) in adults or cretinism in children.[49] Iodized salt has been used to correct these conditions since 1924[50] and consists of table salt mixed with a minute amount of potassium iodide, sodium iodide or sodium iodate. A small amount of dextrose may also be added to stabilize the iodine.[51] Iodine deficiency affects about two billion people around the world and is the leading preventable cause of mental retardation.[52] Iodized table salt has significantly reduced disorders of iodine deficiency in countries where it is used.[53]

The amount of iodine and the specific iodine compound added to salt varies from country to country. In the United States, the Food and Drug Administration (FDA) recommends [21 CFR 101.9 (c)(8)(iv)] 150 micrograms of iodine per day for both men and women. US iodized salt contains 46–77 ppm (parts per million), whereas in the UK the iodine content of iodized salt is recommended to be 10–22 ppm.[54]

Sodium ferrocyanide, also known as yellow prussiate of soda, is sometimes added to salt as an anticaking agent. The additive is considered safe for human consumption.[55][56] Such anti-caking agents have been added since at least 1911 when magnesium carbonate was first added to salt to make it flow more freely.[57] The safety of sodium ferrocyanide as a food additive was found to be provisionally acceptable by the Committee on Toxicity in 1988.[55] Other anticaking agents sometimes used include tricalcium phosphate, calcium or magnesium carbonates, fatty acid salts (acid salts), magnesium oxide, silicon dioxide, calcium silicate, sodium aluminosilicate and calcium aluminosilicate. Both the European Union and the United States Food and Drug Administration permitted the use of aluminium in the latter two compounds.[58]

In "doubly fortified salt", both iodide and iron salts are added. The latter alleviates iron deficiency anaemia, which interferes with the mental development of an estimated 40% of infants in the developing world. A typical iron source is ferrous fumarate.[59] Another additive, especially important for pregnant women, is folic acid (vitamin B9), which gives the table salt a yellow color. Folic acid helps prevent neural tube defects and anaemia, which affect young mothers, especially in developing countries.[59]

A lack of fluorine in the diet is the cause of a greatly increased incidence of dental caries.[60] Fluoride salts can be added to table salt with the goal of reducing tooth decay, especially in countries that have not benefited from fluoridated toothpastes and fluoridated water. The practice is more common in some European countries where water fluoridation is not carried out. In France, 35% of the table salt sold contains added sodium fluoride.[59]

Other kinds[edit]

Irregular crystals of sea salt

Unrefined sea salt contains small amounts of magnesium and calcium halides and sulfates, traces of algal products, salt-resistant bacteria and sediment particles. The calcium and magnesium salts confer a faintly bitter overtone, and they make unrefined sea salt hygroscopic (i.e., it gradually absorbs moisture from air if stored uncovered). Algal products contribute a mildly "fishy" or "sea-air" odour, the latter from organobromine compounds. Sediments, the proportion of which varies with the source, give the salt a dull grey appearance. Since taste and aroma compounds are often detectable by humans in minute concentrations, sea salt may have a more complex flavor than pure sodium chloride when sprinkled on top of food. When salt is added during cooking however, these flavors would likely be overwhelmed by those of the food ingredients.[61] The refined salt industry cites scientific studies saying that raw sea and rock salts do not contain enough iodine salts to prevent iodine deficiency diseases.[62]

Different natural salts have different mineralities depending on their source, giving each one a unique flavour. Fleur de sel, a natural sea salt from the surface of evaporating brine in salt pans, has a unique flavour varying with the region from which it is produced. In traditional Korean cuisine, so-called "bamboo salt" is prepared by roasting salt[63] in a bamboo container plugged with mud at both ends. This product absorbs minerals from the bamboo and the mud, and has been claimed to increase the anticlastogenic and antimutagenic properties of doenjang (a fermented bean paste).[64]

Kosher salt, though refined, contains no iodine and has a much larger grain size than most refined salts. This can give it different properties when used in cooking, and can be useful for preparing kosher meat. Some kosher salt has been certified to meet kosher requirements by a hechsher, but this is not true for all products labelled as kosher salt.[65]

Pickling salt is ultrafine to speed dissolving to make brine. Gourmet salts may be used for specific tastes.

Salt in food[edit]

Salt is present in most foods, but in naturally occurring foodstuffs such as meats, vegetables and fruit, it is present in very small quantities. It is often added to processed foods (such as canned foods and especially salted foods, pickled foods, and snack foods or other convenience foods), where it functions as both a preservative and a flavoring. Dairy salt is used in the preparation of butter and cheese products.[66] Before the advent of electrically powered refrigeration, salting was one of the main methods of food preservation. Thus, herring contains 67 mg sodium per 100 g, while kipper, its preserved form, contains 990 mg. Similarly, pork typically contains 63 mg while bacon contains 1,480 mg, and potatoes contain 7 mg but potato crisps 800 mg per 100 g.[12] The main sources of salt in the diet, apart from direct use of sodium chloride, are bread and cereal products, meat products and milk and dairy products.[12]

In many East Asian cultures, salt is not traditionally used as a condiment.[67] In its place, condiments such as soy sauce, fish sauce and oyster sauce tend to have a high sodium content and fill a similar role to table salt in western cultures. They are most often used for cooking rather than as table condiments.[68]

Sodium consumption and health[edit]

Table salt is made up of just under 40% sodium by weight, so a 6 g serving (1 teaspoon) contains about 2,300 mg of sodium.[69] Sodium serves a vital purpose in the human body: via its role as an electrolyte, it helps nerves and muscles to function correctly, and it is one factor involved in the osmotic regulation of water content in body organs (fluid balance).[70] Most of the sodium in the Western diet comes from salt.[71] The habitual salt intake in many Western countries is about 10 g per day, and it is higher than that in many countries in Eastern Europe and Asia.[72] The high level of sodium in many processed foods has a major impact on the total amount consumed.[73] In the United States, 75% of the sodium eaten comes from processed and restaurant foods, 11% from cooking and table use and the rest from what is found naturally in foodstuffs.[74]

Because consuming too much sodium increases risk of cardiovascular diseases,[71] health organizations generally recommend that people reduce their dietary intake of salt.[71][75][76][77] High sodium intake is associated with a greater risk of stroke, total cardiovascular disease and kidney disease.[3][72] In regard to the US-typical diet, a reduction in sodium intake by 1 g per day may reduce cardiovascular disease by about 30 percent.[78][71] In adults and children with no acute illness, a decrease in the intake of sodium from the typical high levels reduces blood pressure.[76][79] A low sodium diet results in a greater improvement in blood pressure in people with hypertension.[80][81]

The World Health Organization recommends that adults should consume less than 2,000 mg of sodium (which is contained in 5 g of salt) per day.[75] Guidelines by the United States recommend that people with hypertension, African Americans, and middle-aged and older adults should limit consumption to no more than 1,500 mg of sodium per day and meet the potassium recommendation of 4,700 mg/day with a healthy diet of fruits and vegetables.[71][82]

While reduction of sodium intake to less than 2,300 mg per day is recommended by developed countries,[71] one review recommended that sodium intake be reduced to at least 1,200 mg (contained in 3 g of salt) per day, as a further reduction in salt intake the greater the fall in systolic blood pressure for all age groups and ethinicities.[76] Another review indicated that there is inconsistent/insufficient evidence to conclude that reducing sodium intake to lower than 2,300 mg per day is either beneficial or harmful.[83]

One of the two most prominent dietary risks for disability in the world is eating too much sodium.[84]

Chemicals production[edit]

Salt is used, directly or indirectly, in the production of many chemicals, which consume most of the world's production.[85]

Non-dietary uses[edit]

Only about 6% of the salt manufactured in the world is used in food. Of the remainder, 12% is used in water conditioning processes, 8% goes for de-icing highways and 6% is used in agriculture. The rest (68%) is used for manufacturing and other industrial processes,[86] and sodium chloride is one of the largest inorganic raw materials used by volume. Its major chemical products are caustic soda and chlorine, which are separated by the electrolysis of a pure brine solution. These are used in the manufacture of PVC, plastics, paper pulp and many other inorganic and organic compounds. Salt is also used as a flux in the production of aluminium. For this purpose, a layer of melted salt floats on top of the molten metal and removes iron and other metal contaminants. It is also used in the manufacture of soaps and glycerine, where it is added to the vat to precipitate out the saponified products. As an emulsifier, salt is used in the manufacture of synthetic rubber, and another use is in the firing of pottery, when salt added to the furnace vaporises before condensing onto the surface of the ceramic material, forming a strong glaze.[87]

When drilling through loose materials such as sand or gravel, salt may be added to the drilling fluid to provide a stable "wall" to prevent the hole collapsing. There are many other processes in which salt is involved. These include its use as a mordant in textile dying, to regenerate resins in water softening, for the tanning of hides, the preservation of meat and fish and the canning of meat and vegetables.[87][88][89]

Chlor-alkali industry[edit]

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[edit]

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 myriad other chemicals. In the Mannheim process and in the Hargreaves process, it is used for the production of sodium sulfate and hydrochloric acid.

Standard[edit]

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[edit]

Sodium chloride is heavily used, so even relatively minor 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.[90] Salt is also used to increase the curing of concrete in cemented casings.[85]

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[clarification needed] them. One of its main roles is to provide the positive ion charge to promote the absorption of negatively charged ions of dyes.[85]

It is also 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.[85]

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.[85][90]

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.[85]

Sodium chloride is sometimes used as a cheap and safe desiccant because of its hygroscopic properties, making salting an effective method of food preservation historically; the salt draws water out of bacteria through osmotic 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.

Water softening[edit]

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.[85][90]

Road salt[edit]

Phase diagram of water-NaCl mixture.

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 (concentrated solution 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 de-icing, mixtures of brine and salt are used, sometimes with additional agents such as calcium chloride and/or magnesium chloride. The use of salt or brine becomes ineffective below −10 °C (14 °F).

Mounds of road salt for use in winter.

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.[85]

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 the eutectic point of −22.4 °C (−8.3 °F) can be reached with about 25 wt% of salt.[91]

Environmental effects[edit]

Road salt ends up in fresh water bodies and could harm aquatic plants and animals by disrupting their osmoregulation ability.[92] 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.[85] A 2009 study found that approximately 70 percent of the road salt being applied in the Minneapolis-St Paul metro area is retained in the local watershed.[93]

Food industry and agriculture[edit]

Many microorganisms 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.

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.[85]

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 (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, e.g. in potato chips, pretzels, cat and dog food.[85]

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.

Medicine[edit]

Sodium chloride is used together with water as one of the primary solutions for intravenous therapy. Nasal Spray usually contains Saline in it.

Firefighting[edit]

A class D fire extinguisher for various metals

Sodium chloride is the principal extinguishing agent in fire extinguishers (Met-L-X, Super D) used on combustible metal fires such as magnesium, potassium, sodium, and NaK alloys (Class D). Thermoplastic powder is added to the mixture, along with waterproofing (metal stearates) and anti-caking materials (tricalcium phosphate) 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 as a cartridge-operated unit, although stored pressure versions are now popular. Common sizes are 30 pounds (14 kg) portable and 350 pounds (160 kg) wheeled.

Cleanser[edit]

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[edit]

Defect-free NaCl crystals have an optical transmittance of about 90% for infrared light, specifically between 200 nm and 20 µm. They were therefore used in optical components (windows and prisms) operating in that 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, vacuum sealed assembly areas or for short-term uses such as prototyping. Nowadays materials like zinc selenide (ZnSe), which are stronger mechanically and are less sensitive to moisture, are used instead of NaCl for the infrared spectral range.

Chemistry[edit]

SEM image of a grain of table salt

Salt is mostly sodium chloride, the ionic compound with the formula NaCl, representing equal proportions of sodium and chlorine. Sea salt and freshly mined salt (much of which is sea salt from prehistoric seas) also contain small amounts of trace elements (which in these small amounts are generally good for plant and animal health). Mined salt is often refined in the production of table salt; it is dissolved in water, purified via precipitation of other minerals out of solution, and re-evaporated. During this same refining process it is often also iodized. Salt crystals are translucent and cubic in shape; they normally appear white but impurities may give them a blue or purple tinge. The molar mass of salt is 58.443 g/mol, its melting point is 801 °C (1,474 °F) and its boiling point 1,465 °C (2,669 °F). Its density is 2.17 grams per cubic centimetre and it is readily soluble in water. When dissolved in water it separates into Na+ and Cl ions and the solubility is 359 grams per litre.[94] From cold solutions, salt crystallises as the dihydrate NaCl·2H2O. Solutions of sodium chloride have very different properties from those of pure water; the freezing point is −21.12 °C (−6.02 °F) for 23.31 wt% of salt, and the boiling point of saturated salt solution is around 108.7 °C (227.7 °F).[91]

Solid sodium chloride[edit]

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 (octahedral voids) 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 (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. Thermal 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 with doping.[95]

Aqueous solutions[edit]

Solubility of NaCl in various solvents
(g NaCl / 1 kg of solvent at 25 °C, 77 °F)[96]
Water 360
Formamide 94
Glycerin 83
Propylene glycol 71
Formic acid 52
Liquid ammonia 30.2
Methanol 14
Ethanol 0.65
Dimethylformamide 0.4
1-Propanol 0.124
Sulfolane 0.05
1-Butanol 0.05
2-Propanol 0.03
1-Pentanol 0.018
Acetonitrile 0.003
Acetone 0.00042

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.[97] Solutions of sodium chloride have very different properties from pure water. The freezing point is −21.12 °C (−6.02 °F) for 23.31 wt% of salt, and the boiling point of saturated salt solution is near 108.7 °C (227.7 °F).[91] From cold solutions, salt crystallises as the dihydrate NaCl·2H2O.

View of one slab of NaCl(H2O)2 (red = O, white = H, green = Cl, purple = Na).[98]

pH of sodium chloride solutions[edit]

pH of sodium chloride solutions is not 7 but increases from 5.6 to 8.4 as NaCl concentration increases from 0 to 5 mol/L.[99]

Unexpected stable stoichiometric variants[edit]

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; five new compounds were predicted (e.g., Na3Cl, Na2Cl, Na3Cl2, NaCl3, and NaCl7). 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.[100]

Occurrence[edit]

Small particles of sea salt are the dominant cloud condensation nuclei far out at sea, which allow the formation of clouds in otherwise non-polluted air.[101]

Production[edit]

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.[85] In 2010, world production was estimated at 270 million tonnes, the top five producers (in million tonnes) being China (60.0), United States (45.0), Germany (16.5), India (15.8) and Canada (14.0).[102] Salt is also a byproduct of potassium mining.

Food grade salt accounts for only a small part of salt production in industrialized countries (7% in Europe),[103] although worldwide, food uses account for 17.5% of total production.[104]

In 2013, total world production of salt was 264 million tonnes, the top five producers being China (71 million), the United States (40 million), India (18 million), Germany (12 million) and Canada (11 million).[105]

Brine from salt wells is boiled to produce salt at Bo Kluea, Nan Province, Thailand
Salt mounds in Salar de Uyuni, Bolivia

The manufacture of salt is one of the oldest chemical industries.[106] A major source of salt is seawater, which has a salinity of approximately 3.5%. This means that there are about 35 grams (1.2 oz) of dissolved salts, predominantly sodium (Na+
) and chloride (Cl
) ions, per kilogram (2.2 lbs) of water.[107] The world's oceans are a virtually inexhaustible source of salt, and this abundance of supply means that reserves have not been calculated.[88] The evaporation of seawater is the production method of choice in marine countries with high evaporation and low precipitation rates. Salt evaporation ponds are filled from the ocean and salt crystals can be harvested as the water dries up. Sometimes these ponds have vivid colours, as some species of algae and other micro-organisms thrive in conditions of high salinity.[108]

Elsewhere, salt is extracted from the vast sedimentary deposits which have been laid down over the millennia from the evaporation of seas and lakes. These are either mined directly, producing rock salt, or are extracted in solution by pumping water into the deposit. In either case, the salt may be purified by mechanical evaporation of brine. Traditionally, this was done in shallow open pans which were heated to increase the rate of evaporation. More recently, the process is performed in pans under vacuum.[89] The raw salt is refined to purify it and improve its storage and handling characteristics. This usually involves recrystallization during which a brine solution is treated with chemicals that precipitate most impurities (largely magnesium and calcium salts). Multiple stages of evaporation are then used to collect pure sodium chloride crystals, which are kiln-dried.[109] Some salt is produced using the Alberger process, which involves vacuum pan evaporation combined with the seeding of the solution with cubic crystals, and produces a grainy-type flake.[110] The Ayoreo, an indigenous group from the Paraguayan Chaco, obtain their salt from the ash produced by burning the timber of the Indian salt tree (Maytenus vitis-idaea) and other trees.[111]

One of the largest salt mining operations in the world is at the Khewra Salt Mine in Pakistan. The mine has nineteen storeys, eleven of which are underground, and 400 km (250 mi) of passages. The salt is dug out by the room and pillar method, where about half the material is left in place to support the upper levels. Extraction of Himalayan salt is expected to last 350 years at the present rate of extraction of around 385,000 tons per annum.[112]

Biological functions[edit]

The long held belief that a high-salt diet raises the risk of cardio-vascular disease is coming under scrutiny.[113] More recently, dietary salt was demonstrated to attenuate nitric oxide production. Nitric oxide (NO) contributes to vessel homeostasis by inhibiting vascular smooth muscle contraction and growth, platelet aggregation, and leukocyte adhesion to the endothelium.[114]

See also[edit]

References[edit]

 This article incorporates public domain material from the United States Geological Survey document: "Salt" (PDF). 

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External links[edit]