Sodium hydroxide

• 1. Introduction

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  Sodium hydroxide, better known as caustic soda or lye, is a white solid built from sodium cations and hydroxide anions. In 2022, factories around the world turned out roughly 83 million tons of it. Yet many people know it only through a strange sensation. Touch a solution of it with bare hands, which nobody should do, and the skin feels slippery. The reason is that oils on the skin, such as sebum, are being converted into soap. That slipperiness says a lot about the compound. It is a strongly corrosive base that breaks down fats and proteins at ordinary temperatures. Chemistry students meet it beside plain water and hydrochloric acid, as one of the simplest ways to demonstrate the pH scale. So why would a substance so hostile to living tissue turn up in pretzels, paper, drinking water and drain cleaner? Who first learned to make it, and how do factories produce it today?

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• 2. The Soap Makers' Recipe

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  Late in the 13th century, an Arab book titled Al-mukhtara' fi funun min al-suna' wrote down a soap recipe with sodium hydroxide hidden inside it. The title translates as Inventions from the Various Industrial Arts. It was compiled by al-Muzaffar Yusuf ibn 'Umar ibn 'Ali ibn Rasul, a king of Yemen who died in 1295.

  The recipe began with al-qily, the Arabic name for ash from saltwort plants, which are rich in sodium. This ash was an impure form of sodium carbonate, and the Arabic word lies behind our word alkali. The maker mixed it with quicklime, which is calcium oxide. Water was then passed through the mixture again and again until a solution of sodium hydroxide came out. European soap makers later followed the same method.

  Traditional cold process soap making still depends on the compound. In the nineteenth century it was made in hard rather than liquid form, because a solid was easier to store and transport. Soap itself divides the same way. Hard bar soap uses sodium hydroxide, while liquid soaps use potassium hydroxide. Sodium hydroxide wins on price, and a smaller quantity of it is needed. The plant ash that started the story would not stay the raw material for long.

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• 3. Making It By The Million

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  In 1791, the French chemist and surgeon Nicolas Leblanc, who lived from 1742 to 1806, patented a process for mass-producing sodium carbonate. His artificial product replaced natural soda ash, the impure carbonate obtained from the ashes of sodium-rich plants.

  That carbonate fed an older method called causticizing. Makers treated sodium carbonate with calcium hydroxide, also known as slaked lime. The reaction exploits a difference in solubility, because sodium hydroxide dissolves while calcium carbonate does not. The carbonate came from the Leblanc process in the early 19th century and from the Solvay process in the late 19th century. Both routes were later superseded entirely.

  The replacement was the chloralkali process, which makes sodium hydroxide in a single step. By the 20th century, electrolysis of sodium chloride had become the main method. Factories first produce a 32% solution, then evaporate it to 50%. Chlorine gas is the chief by-product. Evaporating the remaining water yields solid sodium hydroxide, which is most commonly sold as flakes, prills and cast blocks.

  In 1998, total world production was around 45 million tonnes. North America and Asia each contributed about 14 million tonnes, and Europe about 10 million. By 2022, demand was estimated at 51 million tonnes. In the United States, the leading producer is Olin, with annual output near 5.7 million tonnes. Its sites are at Freeport in Texas, Plaquemine and St. Gabriel in Louisiana, McIntosh in Alabama, Charleston in Tennessee, Niagara Falls in New York, and Bécancour in Canada. Oxychem, Westlake, Shintech and Formosa are other major American producers, and all of them use the chloralkali process.

  Sodium metal dropped into water also yields sodium hydroxide, along with hydrogen gas and heat that often ignites a flame. Teachers use it to show how reactive the alkali metals are. Commercially it appears only inside the mercury cell version of the chloralkali process, where sodium amalgam is reacted with water.

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• 4. Heat, Thickness And Hydrates

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  Pure sodium hydroxide melts at 318 degrees Celsius without decomposing and boils at 1388 degrees Celsius. It is a colorless crystalline solid, highly soluble in water and less so in polar solvents such as ethanol and methanol. It will not dissolve in ether or other non-polar solvents.

  Dropping the solid into water releases a surge of heat. Dissolving it is strongly exothermic, much like the hydration of sulfuric acid, and the heat makes splashing a real safety threat. The solution that results is usually colorless and odorless. The solid itself readily absorbs moisture and carbon dioxide from the air.

  A 50% solution has a viscosity of 78 millipascal-seconds. Water measures 1.0 and olive oil 85 at room temperature, so concentrated sodium hydroxide flows almost like oil. Like any liquid, it thins as it warms. That thickness affects how the chemical is applied and stored.

  In 1893, Spencer Umfreville Pickering described in detail the tangled solubility diagram of the hydrates, which bind different numbers of water molecules. The known forms run from a heptahydrate that appears at about -28 degrees Celsius to a monohydrate that appears above 12 degrees. The monohydrate has a density of 1.829 grams per cubic centimeter. It is often what suppliers sell as sodium hydroxide, so published data may describe it rather than the anhydrous compound.

  Supercooling makes the hydrates unpredictable. Cool a solution with a 1:2 mole ratio of sodium hydroxide to water, which is 52.6% sodium hydroxide by mass, and the monohydrate normally begins to crystallize at about 22 degrees Celsius. Yet the solution can be supercooled to -15 degrees, where it may suddenly crystallize as the dihydrate. When solutions below 18.4% are cooled, water ice forms first and leaves the sodium hydroxide dissolved.

  The alpha form of the tetrahydrate has a density of 1.33 grams per cubic centimeter. It melts at 7.55 degrees Celsius into a liquid of density 1.392, so it floats on its own melt like ice on water. The beta form is metastable. Below -20 degrees it often switches to the alpha form within minutes, and the solid swells by 6.5%.

  Inside the crystals, the monohydrate follows a layered pattern. Each sodium atom is surrounded by six oxygen atoms, three from hydroxide ions and three from water molecules. Both the monohydrate and the anhydrous compound form orthorhombic crystals.

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• 5. Reactions And Aluminium

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  Hydrochloric acid turns sodium hydroxide into sodium chloride and water. Every neutralization of a strong acid by this base gives off heat. Chemists can use such reactions for titrations, but sodium hydroxide cannot serve as a primary standard. It absorbs moisture and carbon dioxide from the air, so its exact amount in a sample is hard to trust.

  Sulfur dioxide and other acidic oxides also react with it. Industry uses this to scrub harmful acidic gases produced when coal burns, keeping them out of the atmosphere.

  Glass suffers slowly in aluminium-free company too. At ambient temperatures, aqueous sodium hydroxide reacts with glass to form soluble silicates. Glass joints and stopcocks therefore tend to freeze shut. Long exposure to hot solutions damages flasks and glass-lined reactors and frosts the glass.

  Iron is different. It does not dissolve in base, because it is not amphoteric, so sodium hydroxide leaves it alone at room temperature. Above 500 degrees Celsius, molten sodium hydroxide can react with fine iron filings to give iron(III) oxide, sodium metal and hydrogen gas.

  Aluminium is far less safe. In 1986 in the United Kingdom, an aluminium road tanker was mistakenly used to carry a 25% sodium hydroxide solution. The reaction produced hydrogen gas that pressurized the contents and damaged tankers. The products are hydrogen and sodium aluminate. Metalworkers can use the same chemistry for etching, for removing anodizing, or for turning a polished surface into a satin-like finish, though an unprotected surface may degrade afterward.

  The Bayer process turns this behavior into a refining method. Sodium hydroxide dissolves alumina from bauxite ore, and impurities such as iron oxides stay behind as a highly alkaline red mud. The alumina then becomes the raw material for aluminium made through the Hall-Héroult electrolytic process. Zinc and lead also dissolve in concentrated solutions, forming sodium zincate and sodium plumbate.

  Other metals react the opposite way. Because most transition metal hydroxides are insoluble, sodium hydroxide can precipitate them from solution. Copper gives a blue solid, iron(II) a green one, and iron(III) a yellow or brown one. Water treatment plants make gelatinous aluminium hydroxide from aluminium sulfate with sodium hydroxide or bicarbonate, and it traps fine particles so they can be filtered out.

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• 6. Lye In The Kitchen

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  Bagels are often boiled in a lye solution before baking, which gives their crust a shine. German pretzels are poached in either a boiling sodium carbonate solution or a cold sodium hydroxide solution, and that step contributes to their distinctive crust. German lye rolls are glazed with sodium hydroxide before they go into the oven, which makes them crisp.

  In Chinese cooking, lye water is an essential ingredient in the crust of traditional baked moon cakes. Most yellow Chinese noodles are made with it, and people often assume the color comes from egg. One variety of zongzi uses lye water to add a sweet flavor.

  Century eggs depend on the chemistry too, because sodium hydroxide gels the egg whites. In the Philippines, the dessert kutsinta uses a small quantity of lye water to give rice flour batter a jelly-like texture. A similar kakanin called pitsi-pitsi or pichi-pichi uses grated cassava instead of rice flour. Norway has its own lye dish in lutefisk.

  Hominy shows how far the idea reaches. Dried maize kernels are soaked in lye water and expand considerably. They can be fried into corn nuts or dried and ground into grits. They also lead to masa, the flour behind Mexican corn tortillas and tamales. Nixtamal is similar but uses calcium hydroxide instead of sodium hydroxide.

  Food makers also use the compound to peel fruits and vegetables chemically, to process chocolate and cocoa, to make caramel coloring, to scald poultry, and to thicken ice cream. Olives are often soaked in it to soften them. It carries the food additive number E524. Food grade sodium hydroxide is hard to buy in small quantities, so home cooks often substitute sodium carbonate.

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• 7. Pulp, Oil And Drains

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  Around 56% of all sodium hydroxide is used by industry, and about a quarter of that goes to making paper. In the kraft process, sodium hydroxide and sodium sulfide form the white liquor that separates lignin from cellulose fibers. Later bleaching stages for the brown pulp, including oxygen delignification and oxidative extraction, need a strongly alkaline environment with a pH above 10.5. The same chemistry helps produce artificial textile fibres such as rayon.

  Oil drilling relies on it as well. Added to drilling mud in bentonite systems, it raises alkalinity and viscosity and neutralizes acid gases such as hydrogen sulfide and carbon dioxide met underground. Refiners also treat poor quality crude oil with it to remove sulfurous impurities, a process called caustic washing. The toxic waste is difficult to deal with, so many countries have banned the process. In 2006, Trafigura used it and then dumped the waste in Ivory Coast.

  Biodiesel makers use sodium hydroxide as a catalyst when methanol reacts with triglycerides. Only the anhydrous form works, because water would turn the fat into soap tainted with methanol. It is also cheaper than potassium hydroxide, since it is produced from common salt. In cement mixes, plasticisers containing it help homogenise the mix, reduce the water needed and improve workability.

  Homes meet the chemical in drain openers, sold as dry crystals or a thick gel. The alkali converts grease into water-soluble soap and glycerol, and it breaks down proteins such as hair. The heat from dissolving speeds the work. Oven cleaners use it, and a soak solution can degrease stainless steel and glass bakeware.

  Parts washers show a larger shift. Sodium hydroxide detergents replaced many solvent-based systems in the early 1990s, after the Montreal Protocol outlawed trichloroethane. They spray heated, pressurized solution against dirty parts and are considered an environmental improvement over solvent cleaning.

  Some hair relaxers contain it, but the high rate and intensity of chemical burns led makers to use other alkalis in consumer products. Professionals still use sodium hydroxide relaxers. Painters once used it as the most common stripper on wooden objects, though it can raise the grain and stain the wood. In low concentrations it balances pH in facial cleansers, creams, lotions and makeup. Water suppliers sometimes add it to raise pH, which makes water less corrosive to plumbing and keeps lead and copper from dissolving into it.

  One retired use belongs to medicine. A few drops of sodium hydroxide turned the blood of carbon monoxide poisoning patients a vermilion color. Today CO oximetry does that job.

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• 8. Burns And Misuse

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  A few drops of sodium hydroxide solution can decompose proteins and lipids in living tissue through amide hydrolysis and ester hydrolysis. The result is a chemical burn, and contact with the eyes may cause permanent blindness. Even the solid is corrosive if water vapor is present. Rubber gloves, safety clothing and eye protection are essential. For spills on skin, first aid means irrigation with large quantities of water for at least ten to fifteen minutes. The heat of dissolving can also cause heat burns or ignite flammable materials.

  That power over protein has an industrial application. Farms once used a sealed chamber in which a carcass was mixed with sodium hydroxide and water. The mixture broke the chemical bonds holding the flesh together and slowly produced a dark brown liquid. The only solids left were bone hulls, which could be crushed between fingertips. Animal disposal contractors also use it to decompose roadkill dumped in landfills.

  Its cheapness and availability have attracted criminals. The Italian serial killer Leonarda Cianciulli used it to turn dead bodies into soap. In Mexico, a man who worked for drug cartels admitted disposing of over 300 bodies with it.

  Storage demands care. Laboratories keep it in bottles, cargo handlers use intermediate bulk containers, and manufacturing or wastewater plants may have stationary tanks holding up to 100,000 gallons. Compatible materials include polyethylene, carbon steel, polyvinyl chloride, stainless steel and fiberglass reinforced plastic with a resistant liner.

  For wildlife, the picture is less severe. Toxicity to fish falls around 20-200 milligrams per liter and tracks the rise in pH. Because the chemical is quickly neutralized and does not accumulate, its effect on the environment is usually easy to handle.

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