Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations Na+
and hydroxide anions OH−
Sodium hydroxide is a highly caustic base and alkali that decomposes proteins at ordinary ambient temperatures and may cause severe chemical burns. It is highly soluble in water, and readily absorbs moisture and carbon dioxide from the air. It forms a series of hydrates NaOH·nH
2O. The monohydrate NaOH·H
2O crystallizes from water solutions between 12.3 and 61.8 °C. The commercially available "sodium hydroxide" is often this monohydrate, and published data may refer to it instead of the anhydrous compound. As one of the simplest hydroxides, it is frequently utilized alongside neutral water and acidic hydrochloric acid to demonstrate the pH scale to chemistry students.
Sodium hydroxide is used in many industries: in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents, and as a drain cleaner. Worldwide production in 2004 was approximately 60 million tonnes, while demand was 51 million tonnes.
|Preferred IUPAC name
|Systematic IUPAC name
3D model (JSmol)
|E number||E524 (acidity regulators, ...)|
|UN number||1824, 1823|
|Molar mass||39.9971 g mol−1|
|Appearance||White, waxy, opaque crystals|
|Melting point||318 °C (604 °F; 591 K)|
|Boiling point||1,388 °C (2,530 °F; 1,661 K)|
|418 g/L (0 °C) |
1110 g/L (20 °C)
3370 g/L (100 °C)
|Solubility||soluble in glycerol |
negligible in ammonia
insoluble in ether
slowly soluble in propylene glycol
|Solubility in methanol||238 g/L|
|Solubility in ethanol||<<139 g/L|
|Vapor pressure||<2.4 kPa (at 20 °C)|
|Basicity (pKb)||-0.56 (NaOH(aq) = Na+ + OH–) |
Refractive index (nD)
Heat capacity (C)
|59.66 J/mol K|
Std enthalpy of
Gibbs free energy (ΔfG˚)
|Safety data sheet||External MSDS|
|GHS signal word||Danger|
|P280, P305+351+338, P310|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|40 mg/kg (mouse, intraperitoneal)|
LDLo (lowest published)
|500 mg/kg (rabbit, oral)|
|US health exposure limits (NIOSH):|
|TWA 2 mg/m3|
|C 2 mg/m3|
IDLH (Immediate danger)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Pure sodium hydroxide is a colorless crystalline solid that melts at 318 °C without decomposition. It is highly soluble in water, with a lower solubility in polar solvents such as ethanol and methanol. NaOH is insoluble in ether and other non-polar solvents.
Similar to the hydration of sulfuric acid, dissolution of solid sodium hydroxide in water is a highly exothermic reaction where a large amount of heat is liberated, posing a threat to safety through the possibility of splashing. The resulting solution is usually colorless and odorless. As with other alkaline solutions, it feels slippery with skin contact due to the process of saponification that occurs between NaOH and natural skin oils.
Sodium hydroxide can form several hydrates NaOH·nH
2O, which result in a complex solubility diagram that was described in detail by S. U. Pickering in 1893. The known hydrates and the approximate ranges of temperature and concentration (mass percent of NaOH) of their saturated water solutions are:
Early reports refer to hydrates with n = 0.5 or n = 2/3, but later careful investigations failed to confirm their existence.
The only hydrates with stable melting points are NaOH·H
2O (65.10 °C) and NaOH·3.5H
2O (15.38 °C). The other hydrates, except the metastable ones NaOH·3H
2O and NaOH·4H
2O (β) can be crystallized from solutions of the proper composition, as listed above. However, solutions of NaOH can be easily supercooled by many degrees, which allows the formation of hydrates (including the metastable ones) from solutions with different concentrations.
For example, when a solution of NaOH and water with 1:2 mole ratio (52.6% NaOH by mass) is cooled, the monohydrate normally starts to crystallize (at about 22 °C) before the dihydrate. However, the solution can easily be supercooled down to -15 °C, at which point it may quickly crystallize as the dihydrate. When heated, the solid dihydrate might melt directly into a solution at 13.35 °C; however, once the temperature exceeds 12.58 °C. it often decomposes into solid monohydrate and a liquid solution. Even the n = 3.5 hydrate is difficult to crystallize, because the solution supercools so much that other hydrates become more stable.
A second stable eutectic composition is 45.4% (mass) of NaOH, that solidifies at about 4.9 °C into a mixture of crystals of the dihydrate and of the 3.5-hydrate.
The α form of the tetrahydrate has density 1.33 g/cm3. It melts congruously at 7.55 °C into a liquid with 35.7% NaOH and density 1.392 g/cm3, and therefore floats on it like ice on water. However, at about 4.9 °C it may instead melt incongruously into a mixture of solid NaOH·3.5H
2O and a liquid solution.
The β form of the tetrahydrate is metastable, and often transforms spontaneously to the α form when cooled below −20 °C. Once initiated, the exothermic transformation is complete in a few minutes, with a 6.5% increase in volume of the solid. The β form can be crystallized from supercooled solutions at −26 °C, and melts partially at −1.83 °C.
The "sodium hydroxide" of commerce is often the monohydrate (density 1.829 g/cm3). Physical data in technical literature may refer to this form, rather than the anhydrous compound.
The monohydrate crystallizes in the space group Pbca, with cell dimensions a = 1.1825, b = 0.6213, c = 0.6069 nm. The atoms are arranged in a hydrargillite-like layer structure /O Na O O Na O/... Each sodium atom is surrounded by six oxygen atoms, three each from hydroxyl anions HO−
and three from water molecules. The hydrogen atoms of the hydroxyls form strong bonds with oxygen atoms within each O layer. Adjacent O layers are held together by hydrogen bonds between water molecules.
In general, such neutralization reactions are represented by one simple net ionic equation:
This type of reaction with a strong acid releases heat, and hence is exothermic. Such acid-base reactions can also be used for titrations. However, sodium hydroxide is not used as a primary standard because it is hygroscopic and absorbs carbon dioxide from air.
Sodium hydroxide also reacts with acidic oxides, such as sulfur dioxide. Such reactions are often used to "scrub" harmful acidic gases (like SO2 and H2S) produced in the burning of coal and thus prevent their release into the atmosphere. For example,
Glass reacts slowly with aqueous sodium hydroxide solutions at ambient temperatures to form soluble silicates. Because of this, glass joints and stopcocks exposed to sodium hydroxide have a tendency to "freeze". Flasks and glass-lined chemical reactors are damaged by long exposure to hot sodium hydroxide, which also frosts the glass. Sodium hydroxide does not attack iron since iron does not have amphoteric properties (i.e., it only dissolves in acid, not base). A few transition metals, however, may react vigorously with sodium hydroxide.
In 1986, an aluminium road tanker in the UK was mistakenly used to transport 25% sodium hydroxide solution, causing pressurization of the contents and damage to the tanker. The pressurization was due to the hydrogen gas which is produced in the reaction between sodium hydroxide and aluminium:
Unlike sodium hydroxide, the hydroxides of most transition metals are insoluble, and therefore sodium hydroxide can be used to precipitate transition metal hydroxides. The following colours are observed: blue-copper, green-iron(II), yellow/brown-iron(III). Zinc and lead salts dissolve in excess sodium hydroxide to give a clear solution of Na2ZnO2 or Na2PbO2.
Aluminium hydroxide is used as a gelatinous flocculant to filter out particulate matter in water treatment. Aluminium hydroxide is prepared at the treatment plant from aluminium sulfate by reacting it with sodium hydroxide or bicarbonate.
Sodium hydroxide can be used for the base-driven hydrolysis of esters (as in saponification), amides and alkyl halides. However, the limited solubility of sodium hydroxide in organic solvents means that the more soluble potassium hydroxide (KOH) is often preferred. Touching sodium hydroxide solution with the bare hands, while not recommended, produces a slippery feeling. This happens because oils on the skin such as sebum are converted to soap. Despite solubility in propylene glycol it is unlikely to replace water in saponificaction due to propylene glycol primary reaction with fat before reaction between sodium hydroxide and fat.
Sodium hydroxide is industrially produced as a 50% solution by variations of the electrolytic chloralkali process. Chlorine gas is also produced in this process. Solid sodium hydroxide is obtained from this solution by the evaporation of water. Solid sodium hydroxide is most commonly sold as flakes, prills, and cast blocks.
In 2004, world production was estimated at 60 million dry metric tonnes of sodium hydroxide, and demand was estimated at 51 million tonnes. In 1998, total world production was around 45 million tonnes. North America and Asia each contributed around 14 million tonnes, while Europe produced around 10 million tonnes. In the United States, the major producer of sodium hydroxide is the Dow Chemical Company, which has annual production around 3.7 million tonnes from sites at Freeport, Texas, and Plaquemine, Louisiana. Other major US producers include Oxychem, PPG, Olin, Pioneer Companies, Inc. (PIONA, which was purchased by Olin), and Formosa. All of these companies use the chloralkali process.
Historically, sodium hydroxide was produced by treating sodium carbonate with calcium hydroxide in a metathesis reaction. (Sodium hydroxide is soluble while calcium carbonate is not.) This process was called causticizing.
Sodium hydroxide is also produced by combining pure sodium metal with water. The byproducts are hydrogen gas and heat, often resulting in a flame, making this a common demonstration of the reactivity of alkali metals in academic environments; however, it is not commercially viable, as the isolation of sodium metal is typically performed by reduction or electrolysis of sodium compounds including sodium hydroxide.
Sodium hydroxide is a popular strong base used in industry. Around 56% of sodium hydroxide produced is used by industry, 25% of which is used in the paper industry. Sodium hydroxide is also used in the manufacture of sodium salts and detergents, pH regulation, and organic synthesis. It is used in the Bayer process of aluminium production. In bulk, it is most often handled as an aqueous solution, since solutions are cheaper and easier to handle.
Sodium hydroxide is used in many scenarios where it is desirable to increase the alkalinity of a mixture, or to neutralize acids.
For example, in the petroleum industry, sodium hydroxide is used as an additive in drilling mud to increase alkalinity in bentonite mud systems, to increase the mud viscosity, and to neutralize any acid gas (such as hydrogen sulfide and carbon dioxide) which may be encountered in the geological formation as drilling progresses.
Poor quality crude oil can be treated with sodium hydroxide to remove sulfurous impurities in a process known as caustic washing. As above, sodium hydroxide reacts with weak acids such as hydrogen sulfide and mercaptans to yield non-volatile sodium salts, which can be removed. The waste which is formed is toxic and difficult to deal with, and the process is banned in many countries because of this. In 2006, Trafigura used the process and then dumped the waste in Africa.
Sodium hydroxide is also widely used in pulping of wood for making paper or regenerated fibers. Along with sodium sulfide, sodium hydroxide is a key component of the white liquor solution used to separate lignin from cellulose fibers in the kraft process. It also plays a key role in several later stages of the process of bleaching the brown pulp resulting from the pulping process. These stages include oxygen delignification, oxidative extraction, and simple extraction, all of which require a strong alkaline environment with a pH > 10.5 at the end of the stages.
In a similar fashion, sodium hydroxide is used to digest tissues, as in a process that was used with farm animals at one time. This process involved placing a carcass into a sealed chamber, then adding a mixture of sodium hydroxide and water (which breaks the chemical bonds that keep the flesh intact). This eventually turns the body into a liquid with coffee-like appearance, and the only solid that remains are bone hulls, which could be crushed between one's fingertips. Sodium hydroxide is frequently used in the process of decomposing roadkill dumped in landfills by animal disposal contractors. Due to its low cost and availability, it has been used to dispose of corpses by criminals. Italian serial killer Leonarda Cianciulli used this chemical to turn dead bodies into soap. In Mexico, a man who worked for drug cartels admitted disposing of over 300 bodies with it. Sodium hydroxide is a dangerous chemical due to its ability to hydrolyze protein. If a dilute solution is spilled on the skin, burns may result if the area is not washed thoroughly and for several minutes with running water. Splashes in the eye can be more serious and can lead to blindness.
Strong bases attack aluminium. Sodium hydroxide reacts with aluminium and water to release hydrogen gas. The aluminium takes the oxygen atom from sodium hydroxide, which in turn takes the oxygen atom from the water, and releases the two hydrogen atoms, The reaction thus produces hydrogen gas and sodium aluminate. In this reaction, sodium hydroxide acts as an agent to make the solution alkaline, which aluminium can dissolve in. This reaction can be useful in etching, removing anodizing, or converting a polished surface to a satin-like finish, but without further passivation such as anodizing or alodining the surface may become degraded, either under normal use or in severe atmospheric conditions.
In the Bayer process, sodium hydroxide is used in the refining of alumina containing ores (bauxite) to produce alumina (aluminium oxide) which is the raw material used to produce aluminium metal via the electrolytic Hall-Héroult process. Since the alumina is amphoteric, it dissolves in the sodium hydroxide, leaving impurities less soluble at high pH such as iron oxides behind in the form of a highly alkaline red mud.
Other amphoteric metals are zinc and lead which dissolve in concentrated sodium hydroxide solutions to give sodium zincate and sodium plumbate respectively.
Sodium hydroxide is traditionally used in soap making (cold process soap, saponification). It was made in the nineteenth century for a hard surface rather than liquid product because it was easier to store and transport.
For the manufacture of biodiesel, sodium hydroxide is used as a catalyst for the transesterification of methanol and triglycerides. This only works with anhydrous sodium hydroxide, because combined with water the fat would turn into soap, which would be tainted with methanol. NaOH is used more often than potassium hydroxide because it is cheaper and a smaller quantity is needed.
Food uses of sodium hydroxide include washing or chemical peeling of fruits and vegetables, chocolate and cocoa processing, caramel coloring production, poultry scalding, soft drink processing, and thickening ice cream. Olives are often soaked in sodium hydroxide for softening; Pretzels and German lye rolls are glazed with a sodium hydroxide solution before baking to make them crisp. Owing to the difficulty in obtaining food grade sodium hydroxide in small quantities for home use, sodium carbonate is often used in place of sodium hydroxide.
Specific foods processed with sodium hydroxide include:
Sodium hydroxide is frequently used as an industrial cleaning agent where it is often called "caustic". It is added to water, heated, and then used to clean process equipment, storage tanks, etc. It can dissolve grease, oils, fats and protein-based deposits. It is also used for cleaning waste discharge pipes under sinks and drains in domestic properties. Surfactants can be added to the sodium hydroxide solution in order to stabilize dissolved substances and thus prevent redeposition. A sodium hydroxide soak solution is used as a powerful degreaser on stainless steel and glass bakeware. It is also a common ingredient in oven cleaners.
A common use of sodium hydroxide is in the production of parts washer detergents. Parts washer detergents based on sodium hydroxide are some of the most aggressive parts washer cleaning chemicals. The sodium hydroxide-based detergents include surfactants, rust inhibitors and defoamers. A parts washer heats water and the detergent in a closed cabinet and then sprays the heated sodium hydroxide and hot water at pressure against dirty parts for degreasing applications. Sodium hydroxide used in this manner replaced many solvent-based systems in the early 1990s when trichloroethane was outlawed by the Montreal Protocol. Water and sodium hydroxide detergent-based parts washers are considered to be an environmental improvement over the solvent-based cleaning methods.
Sodium hydroxide is used in the home as a type of drain opener to unblock clogged drains, usually in the form of a dry crystal or as a thick liquid gel. The alkali dissolves greases to produce water soluble products. It also hydrolyzes the proteins such as those found in hair which may block water pipes. These reactions are sped by the heat generated when sodium hydroxide and the other chemical components of the cleaner dissolve in water. Such alkaline drain cleaners and their acidic versions are highly corrosive and should be handled with great caution.
Sodium hydroxide is used in some relaxers to straighten hair. However, because of the high incidence and intensity of chemical burns, manufacturers of chemical relaxers use other alkaline chemicals in preparations available to average consumers. Sodium hydroxide relaxers are still available, but they are used mostly by professionals.
A solution of sodium hydroxide in water was traditionally used as the most common paint stripper on wooden objects. Its use has become less common, because it can damage the wood surface, raising the grain and staining the colour.
Sodium hydroxide is sometimes used during water purification to raise the pH of water supplies. Increased pH makes the water less corrosive to plumbing and reduces the amount of lead, copper and other toxic metals that can dissolve into drinking water.
Sodium hydroxide has been used for detection of carbon monoxide poisoning, with blood samples of such patients turning to a vermilion color upon the addition of a few drops of sodium hydroxide. Today, carbon monoxide poisoning can be detected by CO oximetry.
Sodium hydroxide is used in some cement mix plasticisers. This helps homogenise cement mixes, preventing segregation of sands and cement, decreases the amount of water required in a mix and increases workability of the cement product, be it mortar, render or concrete.
See: Sodium hydroxide test for flavonoids
EMPA researchers are experimenting with concentrated sodium hydroxide (NaOH) as the thermal storage or seasonal reservoir medium for domestic space-heating. If water is added to solid or concentrated sodium hydroxide (NaOH), heat is released. The dilution is exothermic - chemical energy is released in the form of heat. Conversely, by applying heat energy into a dilute sodium hydroxide solution the water will evaporate so that the solution become more concentrated and thus store the supplied heat as latent chemical energy.]
Like other corrosive acids and alkalis, drops of sodium hydroxide solutions can readily decompose proteins and lipids in living tissues via amide hydrolysis and ester hydrolysis, which consequently cause chemical burns and may induce permanent blindness upon contact with eyes. Solid alkali can also express its corrosive nature if there is water, such as water vapor. Thus, protective equipment, like rubber gloves, safety clothing and eye protection, should always be used when handling this chemical or its solutions. The standard first aid measures for alkali spills on the skin is, as for other corrosives, irrigation with large quantities of water. Washing is continued for at least ten to fifteen minutes.
Lithium battery cells, if ingested, cause serious injuries, even if not crushed. The damage is caused, not by the contents of the battery, but by the electric current it creates, which causes sodium hydroxide to build up and burn through the oesophagus and into major blood vessels, which can cause fatal bleeding.
Sodium hydroxide was first prepared by soap makers.:p45 A procedure for making sodium hydroxide appeared as part of a recipe for making soap in an Arab book of the late 13th century: Al-mukhtara` fi funun min al-suna` (Inventions from the Various Industrial Arts), which was compiled by al-Muzaffar Yusuf ibn `Umar ibn `Ali ibn Rasul (d. 1295), a king of Yemen. The recipe called for passing water repeatedly through a mixture of alkali (Arabic: al-qily, where qily is ash from saltwort plants, which are rich in sodium ; hence alkali was impure sodium carbonate) and quicklime (calcium oxide, CaO), whereby a solution of sodium hydroxide was obtained. European soap makers also followed this recipe. When in 1791 the French chemist and surgeon Nicolas Leblanc (1742–1806) patented a process for mass-producing sodium carbonate, natural "soda ash" (impure sodium carbonate that was obtained from the ashes of plants that are rich in sodium):p36 was replaced by this artificial version.:p46 However, by the 20th century, the electrolysis of sodium chloride had become the primary method for producing sodium hydroxide.