Sodium chloride

• 1. Introduction

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  Sodium chloride is made at a rate of roughly 250 million tonnes a year, going by figures from 2008. The biggest uses are not culinary. They are chemical manufacturing and the de-icing of roads. Chemists write the compound as NaCl, an ionic pairing of sodium and chloride in a one-to-one ratio. It is transparent or translucent, brittle, and quick to draw moisture from the air. In rock form it is the mineral halite. Everyone else just calls it salt. This documentary follows the compound out of the salt shaker and into places few listeners would expect. What do factories build from it? Why do winter road crews spread it by the truckload, and what does that cost rivers and bridges? How does it hold back microbes, and how are its atoms arranged inside a single grain?

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• 2. The Chemical Backbone

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  Three kinds of electrolysis cell, called mercury, diaphragm and membrane cells, turn salt water into chlorine, hydrogen and sodium hydroxide. This is the chloralkali process, and sodium chloride is where it begins. Each cell type separates the chlorine from the sodium hydroxide in its own way. The electrolysis uses a great deal of energy, so researchers are developing other technologies. Even small efficiency gains can bring large economic returns. The chlorine goes into PVC thermoplastics, disinfectants and solvents. The sodium hydroxide is used across many industries, including paper, soap and aluminum.

  A second family of reactions starts with the same ingredient. In the Solvay process, salt is converted into sodium carbonate and calcium chloride. Sodium carbonate then feeds the making of glass, sodium bicarbonate and dyes, along with a myriad of other chemicals. The Mannheim process takes salt in a different direction and yields sodium sulfate and hydrochloric acid. Yet the second-largest appetite for salt has nothing to do with chemistry. It belongs to the roads.

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• 3. Winter Roads

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  Before snow even falls, road crews can spray brine, which is salt dissolved in water. This anti-icing step stops snow and ice from bonding to the pavement. It also means far less salt is needed once the snowfall arrives. After a storm, crews switch to mixtures of brine and solid salt, sometimes with calcium chloride or magnesium chloride added. Salt is spread both from grit bins and from winter service vehicles.

  In the United Kingdom, salt for de-icing comes mostly from a single mine in Winsford, in Cheshire. Under 100 parts per million of sodium ferrocyanide is mixed in. This anticaking agent lets stockpiled rock salt pour freely from gritting vehicles. In recent years the same additive has also turned up in table salt.

  In the United States, a sugar-beet processing byproduct was mixed into rock salt as another trick. The sugary carbohydrate solution made the salt cling to road surfaces about 40% better than loose rock salt alone. Because it stayed put, the treatment did not need repeating, which saved time and money.

  Physical chemistry sets a limit on how well any of this works. A water-salt mixture reaches its lowest freezing point at 23.31 percent salt by weight. Freezing near that concentration is so slow, though, that in practice about 25 percent salt is needed to reach the eutectic point.

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• 4. The Environmental Toll

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  A 2009 study of the Minneapolis-St Paul metro area found that about 70% of the road salt applied there stayed in the local watershed. Salt that reaches fresh water can harm aquatic plants and animals by disrupting their ability to regulate their internal salt balance. Across North America and Europe, rising salinity in fresh waterways, together with extra leaching of metals, is an ongoing problem. Spring rains and thaws usually dilute the sodium where salt was spread. Even so, evidence of environmental loading has been found at peak usage.

  Highways suffer too. Salt is linked to corrosion of bridge decks, vehicles, reinforcing bar and wire, and unprotected steel in road construction. Runoff, spray from passing vehicles and windblown salt also reach roadside soil, vegetation, surface water and groundwater.

  On the coast, the problem is different. Salt everywhere in the air and spray causes trouble for any coating, because trapped salts wreck adhesion. Naval authorities and shipbuilders therefore check salt levels on surfaces during construction. The IMO rule most often used allows at most 50 milligrams per square metre of soluble salts, measured as sodium chloride. The check itself is made with a Bresle test.

  Some agencies are already trying substitutes for road salt, including beer, molasses and beet juice. Airlines rely more on glycol and sugar than on salt-based solutions to de-ice aircraft.

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• 5. Kitchen and Farm

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  Bacon, fish and cabbage are three foods that salt has long been used to preserve. The method works through osmosis. Salt pulls water out of microorganisms' cells and stops them from reproducing, and reproduction is a major cause of spoilage. Better drying agents exist, but few are safe for people to eat, which is why salt made such a cheap and safe desiccant.

  Preservation is only one of its jobs in food. Salt enhances flavor, binds and tenderizes, and controls fermentation, texture and color. In bacon, ham and other processed meats it helps color develop. In sausages it helps form a gel of meat, fat and moisture. The food industry's use ranks, from highest to lowest, as other food processing, meat packers, canning, baking, dairy and grain mill products.

  In dairy, salt controls color, fermentation and texture in cheese. In canning it acts as a flavor enhancer, preservative, carrier for other ingredients, dehydrating agent and enzyme inhibitor. Bakers add it to slow fermentation in bread dough and to strengthen gluten. Salt also seasons potato chips, pretzels, and cat and dog food.

  Vets use a warm saturated salt solution to make animals vomit. Gardeners can water plants with 1 to 3 grams of salt per liter, which is generally safe for most plants as a fertilizer.

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• 6. Odd Jobs

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  When a drill bit hits a salt formation underground, crews add salt to the drilling fluid until it is saturated. That keeps the fluid from dissolving the salt layer it is cutting through. In oil and gas wells, salt also thickens the drilling fluid to hold back high gas pressures. It helps concrete cure in cemented casings as well.

  Dye houses use salt in a brine rinse to separate organic contaminants. It encourages dyestuffs to precipitate out and sharpens colors in the dye bath. It does this by supplying positive ions that help negatively charged dye ions get absorbed. Paper mills use it to make sodium chlorate, which is reacted with sulfuric acid and a reducing agent such as methanol to make the bleach chlorine dioxide.

  Tanners add salt to animal hides to inhibit microbes and draw moisture back in. Rubber makers need it to produce buna, neoprene and white rubber. Road builders mix salt into soil to firm up the foundation under highways.

  Hard water carries calcium and magnesium ions that fight with soap and leave scale in pipes. Water-softening units remove those ions with ion-exchange resins, and the resins are regenerated with salt.

  In dry-powder Class D extinguishers, salt is the main agent against fires in metals such as magnesium, zirconium, titanium and lithium. It forms a crust that excludes oxygen. In medicine, salt and water make one of the primary solutions for intravenous therapy, and nasal sprays often contain saline. People have used salt to scrub household surfaces since at least medieval times.

  Salt crystals once made infrared windows and lenses. They pass at least 90% of infrared light across wavelengths from 0.2 to 18 micrometres, through a 1 millimetre thickness. But the crystals are soft and absorb water from air, gradually growing a frost, so zinc selenide and chalcogenide glasses are more widely used.

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• 7. Crystal and Solution

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  Each ion in a salt crystal is surrounded by six ions of the opposite charge, sitting at the corners of a regular octahedron. The larger chloride ions, at 167 picometres, form a cubic array. The smaller sodium ions, at 116 picometres, fill the gaps between them. The arrangement is known as the rock salt structure, and many other compounds share it. Solid salt melts at 801 degrees Celsius, and the liquid boils at 1465 degrees. From cold solutions below freezing, it crystallises with water as hydrohalite. In 2023, researchers found that under pressure it can also form hydrates holding 8.5 and 13 water molecules.

  The pull between sodium and chloride ions is so strong that only highly polar solvents like water dissolve salt well. In water, each sodium ion holds eight water molecules at a distance of 250 picometres. Each chloride ion is surrounded by an average of six. A saturated solution boils near 108.7 degrees Celsius. Sauerkraut brine is about 2.5 percent salt, seawater 3.5 percent and pickles 5 percent. At 20 degrees Celsius, the saturation point is 26.4 percent. The solution stays close to neutral, at about pH 7, because chloride is so weakly basic.

  Solvents vary enormously. At 25 degrees Celsius, a kilogram of water holds 360 grams of salt, but a kilogram of ethanol holds only 0.65 grams. Acetone holds just 0.00042 grams.

  And in 2013, the neat one-to-one ratio was challenged. Researchers predicted five new sodium-chlorine compounds, including Na3Cl and NaCl7. Some have since been confirmed at high pressure, among them cubic and orthorhombic NaCl3.

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• 8. Supply and Sky

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  In 2017, world salt production was estimated at 280 million tonnes. China led with 68.0 million tonnes, ahead of the United States at 43.0 and India at 26.0. Germany and Canada each produced 13.0 million. Most salt is made by evaporating seawater or brine from wells and salt lakes. Mining rock salt is also a major source, and salt comes out as a byproduct of potassium mining too.

  In the earth's crust, salt occurs as the mineral halite, also called rock salt. A tiny amount floats in the atmosphere as suspended sea salt particles. Far out at sea, those particles are the dominant cloud condensation nuclei. They let clouds form in air that is otherwise unpolluted.

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