Sodium hydroxide

Sodium hydroxide, also known aslyeandcaustic soda,is aninorganic compoundwith the formulaNaOH. It is a white solidionic compoundconsisting ofsodiumcationsNaandhydroxideanionsOH.
Sodium hydroxide is a highlycorrosivebaseandalkalithat decomposeslipidsandproteinsat ambienttemperaturesand may cause severechemical burns. It is highly soluble inwater, and readily absorbsmoistureandcarbon dioxidefrom theair. It forms a series ofhydratesNaOH·nH2O.The monohydrateNaOH·H2Ocrystallizes 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 theanhydrouscompound.
As one of the simplest hydroxides, sodium hydroxide is frequently used alongside neutralwaterand acidichydrochloric acidto demonstrate the pH scale to chemistry students.
Sodium hydroxide is used in many industries: in the making ofwood pulpandpaper,textiles,drinking water,soapsanddetergents, and as adrain cleaner. Worldwide production in 2022 was approximately 83 million tons.
Pure sodium hydroxide is a colorless crystalline solid that melts at 318 °C (604 °F) without decomposition and boils at 1,388 °C (2,530 °F). It is highly soluble in water, with a lower solubility inpolarsolventssuch asethanolandmethanol.Sodium hydroxide is insoluble inetherand other non-polar solvents.
Similar to the hydration of sulfuric acid,dissolutionof solid sodium hydroxide in water is a highlyexothermic reactionwhere 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 ofsaponificationthat occurs betweenNaOHand natural skin oils.
Concentrated (50%) aqueous solutions of sodium hydroxide have a characteristicviscosity, 78 mPa·s, that is much greater than that of water (1.0 mPa·s) and near that of olive oil (85 mPa·s) at room temperature. The viscosity of aqueousNaOH, as with any liquid chemical, is inversely related to its temperature, i.e., its viscosity decreases as temperature increases, and vice versa. The viscosity of sodium hydroxide solutions plays a direct role in its application as well as its storage.
Sodium hydroxide can form several hydratesNaOH·nH2O, which result in a complex solubility diagram that was described in detail bySpencer Umfreville Pickeringin 1893.The known hydrates and the approximate ranges of temperature and concentration (mass percent of NaOH) of theirsaturatedwater solutions are:
Early reports refer to hydrates withn= 0.5 orn= 2/3, but later careful investigations failed to confirm their existence.
The only hydrates with stable melting points areNaOH·H2O(65.10 °C) andNaOH·3.5H2O(15.38 °C). The other hydrates, except the metastable onesNaOH·3H2OandNaOH·4H2O(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 then= 3.5 hydrate is difficult to crystallize, because the solution supercools so much that other hydrates become more stable.
A hot water solution containing 73.1% (mass) of NaOH is aeutecticthat solidifies at about 62.63 °C as an intimate mix of anhydrous and monohydrate crystals.
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 third stable eutectic has 18.4% (mass) of NaOH. It solidifies at about −28.7 °C as a mixture of water ice and the heptahydrateNaOH·7H2O.
When solutions with less than 18.4% NaOH are cooled, watericecrystallizes first, leaving the NaOH in solution.
The α form of the tetrahydrate has density 1.33 g/cm. It melts congruously at 7.55 °C into a liquid with 35.7% NaOH and density 1.392 g/cm, and therefore floats on it like ice on water. However, at about 4.9 °C it may instead melt incongruously into a mixture of solidNaOH·3.5H2Oand 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/cm). Physical data in technical literature may refer to this form, rather than the anhydrous compound.
NaOH and its monohydrate form orthorhombic crystals with the space groups Cmcm (oS8) and Pbca (oP24), respectively. The monohydrate cell dimensions are a = 1.1825, b = 0.6213, c = 0.6069nm. The atoms are arranged in ahydrargillite-like layer structure, with each sodium atom surrounded by six oxygen atoms, three each from hydroxide ions 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 byhydrogen bondsbetween water molecules.
Sodium hydroxide reacts with protic acids to produce water and the corresponding salts. For example, when sodium hydroxide reacts withhydrochloric acid,sodium chlorideis formed:
In general, suchneutralizationreactions are represented by one simple net ionic equation:
This type of reaction with a strong acid releases heat, and hence isexothermic. Suchacid–base reactionscan also be used fortitrations. However, sodium hydroxide is not used as aprimary standardbecause it ishygroscopicand absorbscarbon dioxidefrom air.
Sodium hydroxide also reacts withacidic oxides, such assulfur dioxide. Such reactions are often used to "scrub" harmful acidic gases (likeSO2andH2S) 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 solublesilicates. Because of this, glass joints andstopcocksexposed to sodium hydroxide have a tendency to "freeze".Flasksand glass-linedchemical reactorsare damaged by long exposure to hot sodium hydroxide, which also frosts the glass. Sodium hydroxide does not attackironat room temperature, since iron does not haveamphotericproperties (i.e., it only dissolves in acid, not base).
Nevertheless, at high temperatures (e.g. above 500 °C), iron can reactendothermicallywith sodium hydroxide to formiron(III) oxide,sodiummetal, andhydrogengas.This is due to the lowerenthalpy of formationof iron(III) oxide (−824.2 kJ/mol) compared to sodium hydroxide (−500 kJ/mol) and positive entropy change of the reaction, which implies spontaneity at high temperatures (ΔST > ΔH,ΔG < 0) and non-spontaneity at low temperatures (ΔST < ΔH,ΔG > 0). Consider the following reaction between molten sodium hydroxide and finely divided iron filings:
A fewtransition metals, however, may react quite vigorously with sodium hydroxide under milder conditions.
In 1986, an aluminiumroad tankerin the UK was mistakenly used to transport 25% sodium hydroxide solution,causing pressurization of the contents and damage to tankers. The pressurization is due to the hydrogen gas which is produced in the reaction between sodium hydroxide and aluminium:
Unlike sodium hydroxide, which is soluble, the hydroxides of most transition metals are insoluble, and therefore sodium hydroxide can be used toprecipitatetransition metal hydroxides. The following colours are observed:
Zinc and lead salts dissolve in excess sodium hydroxide to give a clear solution ofNa2ZnO2orNa2PbO2.
Aluminium hydroxideis used as a gelatinousflocculantto filter out particulate matter inwater treatment. Aluminium hydroxide is prepared at the treatment plant fromaluminium sulfateby reacting it with sodium hydroxide or bicarbonate.
Sodium hydroxide can be used for the base-drivenhydrolysis of esters(also calledsaponification),amidesandalkyl halides.However, the limited solubility of sodium hydroxide in organic solvents means that the moresolublepotassium hydroxide(KOH) is often preferred. Touching a sodium hydroxide solution with bare hands, while not recommended, produces a slippery feeling. This happens because oils on the skin such assebumare converted to soap.
Despite solubility inpropylene glycolit is unlikely to replace water in saponification due to propylene glycol's primary reaction with fat before reaction between sodium hydroxide and fat.
Sodium hydroxide is industrially produced, first as a 32% solution, and then evaporated to a 50% solution by variations of the electrolyticchloralkali process.Chlorine gasis 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 2022, world production was estimated at 83 million dry tonnes of sodium hydroxide, and demand was estimated at 51 million tonnes.In 1998, total world production was around 45 milliontonnes. 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 isOlin, which has annual production around 5.7 milliontonnesfrom sites atFreeport, Texas;Plaquemine, Louisiana;St. Gabriel, Louisiana;McIntosh, Alabama;Charleston, Tennessee;Niagara Falls, New York; andBécancour, Canada. Other major US producers includeOxychem,Westlake, Shintek, andFormosa. All of these companies use thechloralkali process.
Historically, sodium hydroxide was produced by treatingsodium carbonatewithcalcium hydroxide(slaked lime) in ametathesis reactionwhich takes advantage of the fact that sodium hydroxide is soluble, while calcium carbonate is not. This process was called causticizing.
The sodium carbonate for this reaction was produced by theLeblanc processin the early 19th century, or theSolvay processin the late 19th century. The conversion of sodium carbonate to sodium hydroxide was superseded entirely by thechloralkali process, which produces sodium hydroxide in a single process.
Sodium hydroxide is also produced by combining pure sodium metal with water. The byproducts are hydrogen gas and heat, often resulting in a flame.
This reaction is commonly used for demonstrating the reactivity of alkali metals in academic environments; however, it is not used commercially aside from a reaction within themercury cell chloralkali processwheresodium amalgamis reacted with water.
Sodium hydroxide is a popular strongbaseused in industry. Sodium hydroxide is used in the manufacture of sodium salts and detergents, pH regulation, and organic synthesis. In bulk, it is most often handled as anaqueous solution,since solutions are cheaper and easier to handle.
Sodium hydroxide is used in many scenarios where it is desirable to increase thealkalinityof a mixture, or to neutralize acids. For example, in the petroleum industry, sodium hydroxide is used as an additive indrilling mudto increasealkalinityinbentonitemud systems, to increase the mudviscosity, and to neutralize anyacid gas(such ashydrogen sulfideandcarbon dioxide) which may be encountered in thegeological formationas drilling progresses. Another use is insalt spray testingwhere pH needs to be regulated. Sodium hydroxide is used with hydrochloric acid to balance pH. The resultant salt, NaCl, is the corrosive agent used in the standard neutral pH salt spray test.
Poor qualitycrude oilcan be treated with sodium hydroxide to removesulfurousimpurities in a process known ascaustic washing. Sodium hydroxide reacts with weak acids such ashydrogen sulfideandmercaptansto 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,Trafiguraused the process and thendumped the waste in Ivory Coast.
Other common uses of sodium hydroxide include:
Sodium hydroxide is also widely used in pulping of wood for making paper or regenerated fibers. Along withsodium sulfide, sodium hydroxide is a key component of the white liquor solution used to separateligninfromcellulosefibersin thekraft process. It also plays a key role in several later stages of the process ofbleaching the brown pulpresulting from the pulping process. These stages includeoxygendelignification,oxidativeextraction, 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 a dark brown color,and the only solids that remain are bone hulls, which can be crushed between one's fingertips.
Sodium hydroxide is frequently used in the process of decomposingroadkilldumped in landfills by animal disposal contractors.Due to its availability and low cost, it has been used by criminals to dispose of corpses. Italianserial killerLeonarda Cianciulliused 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 attackaluminium. Sodium hydroxide reacts with aluminium and water to release hydrogen gas. The aluminium takes an oxygen atom from sodium hydroxide, which in turn takes an oxygen atom from water, and releases two hydrogen atoms. The reaction thus produceshydrogengas andsodium aluminate. In this reaction, sodium hydroxide acts as an agent to make the solution alkaline, which aluminium can dissolve in.
Sodium aluminate is an inorganic chemical that is used as an effective source ofaluminium hydroxidefor many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given asNaAlO2,Na3AlO3,Na[Al(OH)4],Na2O·Al2O3orNa2Al2O4. Formation of sodium tetrahydroxoaluminate(III) or hydrated sodium aluminate is given by:
This reaction can be useful inetching, removing anodizing, or converting a polished surface to a satin-like finish, but without furtherpassivationsuch asanodizingoralodiningthe surface may become degraded, either under normal use or in severe atmospheric conditions.
In theBayer 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 via theelectrolyticHall-Héroult process. Since the alumina isamphoteric, it dissolves in the sodium hydroxide, leaving impurities less soluble at highpHsuch asiron oxidesbehind in the form of a highly alkalinered mud.
Other amphoteric metals are zinc and lead which dissolve in concentrated sodium hydroxide solutions to givesodium zincateandsodium plumbaterespectively.
Sodium hydroxide is traditionally used in soap making (cold processsoap,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 ofbiodiesel, sodium hydroxide is used as acatalystfor thetransesterificationof methanol and triglycerides. This only works withanhydroussodium hydroxide, because combined with water the fat would turn intosoap, which would be tainted withmethanol. NaOH is used more often thanpotassium hydroxidebecause it is cheaper and a smaller quantity is needed. Due to production costs, NaOH, which is produced using common salt is cheaper than potassium hydroxide.
Sodium hydroxide is an ingredient used in someskin careandcosmeticproducts, such as facial cleansers, creams, lotions, and makeup. It is typically used in low concentration as apH balancer, due its highly alkaline nature.
Food uses of sodium hydroxide include washing or chemical peeling offruitsandvegetables,chocolateandcocoaprocessing,caramel coloringproduction,poultryscalding,soft drinkprocessing, and thickeningice cream.Olivesare often soaked in sodium hydroxide for softening;pretzelsand Germanlye rollsare 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 carbonateis often used in place of sodium hydroxide.It is known asE numberE524.
Specific foods processed with sodium hydroxide include:
Sodium hydroxide is frequently used as an industrialcleaning agentwhere it is often called "caustic". It is added to water, heated, and then used to clean process equipment, storage tanks, etc. It can dissolvegrease,oils,fatsandprotein-based deposits. It is also used for cleaning waste discharge pipes under sinks and drains in domestic properties.Surfactantscan 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 onstainless steeland glass bakeware. It is also a common ingredient in oven cleaners.
A common use of sodium hydroxide is in the production ofparts washerdetergents. 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 1990swhentrichloroethanewas outlawed by theMontreal 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 ofdrain openersto unblock clogged drains, usually in the form of a dry crystal or as a thick liquid gel. The alkali dissolvesgreasesto producewater solubleproducts. It alsohydrolyzesproteins, such as those found inhair, which may block water pipes. These reactions are sped by theheat generatedwhen sodium hydroxide and the other chemical components of the cleaner dissolve in water. Suchalkaline drain cleanersand theiracidic versionsare highlycorrosiveand should be handled with great caution.
Sodium hydroxide is used in somerelaxerstostraighten hair. However, because of the high incidence and intensity of chemical burns, manufacturers of chemical relaxers use other alkaline chemicals in preparations available to 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 duringwater purificationto 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 ofcarbon monoxide poisoning, with blood samples of such patients turning to avermilioncolor upon the addition of a few drops of sodium hydroxide.Today, carbon monoxide poisoning can be detected byCO 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.
Like othercorrosiveacidsandalkalis, a few drops of sodium hydroxide solutions can readily decomposeproteinsandlipidsinliving tissuesviaamide hydrolysisandester hydrolysis, which consequently causechemical burnsand may induce permanentblindnessupon contact with eyes.Solid alkali can also express its corrosive nature if there is water, such as water vapor. Thus,protective equipment, likerubber gloves,safety clothingandeye 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.
Moreover,dissolutionof sodium hydroxide is highlyexothermic, and the resulting heat may cause heat burns or ignite flammables. It also produces heat when reacted with acids.
Sodium hydroxide is mildly corrosive toglass, which can cause damage toglazingor causeground glass jointsto bind.Sodium hydroxide is corrosive to several metals, likealuminiumwhich reacts with the alkali to produce flammablehydrogengas on contact.
Careful storage is needed when handling sodium hydroxide for use, especially bulk volumes. Following proper NaOH storage guidelines and maintaining worker/environment safety is always recommended given the chemical's burn hazard.
Sodium hydroxide is often stored in bottles for small-scale laboratory use, withinintermediate bulk containers(medium volume containers) for cargo handling and transport, or within large stationary storage tanks with volumes up to 100,000 gallons for manufacturing or waste water plants with extensive NaOH use. Common materials that are compatible with sodium hydroxide and often utilized for NaOH storage include: polyethylene (HDPE, usual,XLPE, less common),carbon steel,polyvinyl chloride(PVC),stainless steel, andfiberglass reinforced plastic(FRP, with a resistant liner).
Sodium hydroxide must be stored in airtight containers to preserve itsnormalityas it will absorb water and carbon dioxide from the atmosphere.
Sodium hydroxide was first prepared by soap makers.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 ofalkali(Arabic:al-qily, whereqilyis ash fromsaltwortplants, which are rich in sodium; hencealkaliwas impuresodium 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 surgeonNicolas Leblanc(1742–1806) patented aprocess for mass-producing sodium carbonate, natural "soda ash" (impure sodium carbonate that was obtained from the ashes of plants that are rich in sodium)was replaced by this artificial version.However, by the 20th century, theelectrolysis of sodium chloridehad become the primary method for producing sodium hydroxide.