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definitions - Sodium_hydroxide

sodium hydroxide (n.)

1.a strongly alkaline caustic used in manufacturing soap and paper and aluminum and various sodium compounds

Sodium Hydroxide (n.)

1.(MeSH)A highly caustic substance that is used to neutralize acids and make sodium salts. (From Merck Index, 11th ed)

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synonyms - Sodium_hydroxide

Sodium Hydroxide (n.) (MeSH)

Caustic Soda  (MeSH)

sodium hydroxide (n.)

caustic soda

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Sodium hydroxide

Sodium hydroxide
CAS number 1310-73-2 YesY
PubChem 14798
ChemSpider 14114 YesY
UNII 55X04QC32I YesY
EC number 215-185-5
UN number 1823
KEGG D01169 YesY
MeSH Sodium+Hydroxide
ChEBI CHEBI:32145 YesY
RTECS number WB4900000
Gmelin Reference 68430
Jmol-3D images Image 1
Molecular formula NaOH
Molar mass 39.9971 g mol-1
Appearance White, waxy, opaque crystals
Odor odorless
Density 2.13 g/cm3
Melting point

318 °C, 591 K, 604 °F

Boiling point

1388 °C, 1661 K, 2530 °F

Solubility in water 111 g/100 mL (at 20 °C)
Solubility in methanol 23.8 g/100 mL
Solubility in ethanol <<13.9 g/100 mL
Vapor pressure <2.4 kPa (at 20 °C)
Acidity (pKa) 13
Refractive index (nD) 1.3576
MSDS External MSDS
GHS pictograms The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
EU Index 011-002-00-6
EU classification Corrosive C
R-phrases R35
S-phrases (S1/2), S26, S37/39, S45
NFPA 704
NFPA 704.svg
Related compounds
Other anions Sodium hydrosulfide
Other cations Caesium hydroxide

Lithium hydroxide
Potassium hydroxide
Rubidium hydroxide

 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Sodium hydroxide, also known as lye and caustic soda,[2][3] with the molecular formula NaOH is a highly caustic metallic base which is a white solid available in pellets, flakes, granules, and as a 50% saturated solution.

It is soluble in water, ethanol and methanol. This alkali is deliquescent and readily absorbs moisture and carbon dioxide in air.

Sodium hydroxide is used in many industries, mostly as a strong chemical base 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.[4] Although molten sodium hydroxide also possesses similar properties, the high temperature required limits applications.



  Physical properties

Pure sodium hydroxide is usually a white solid which may be pellets, flakes or granules, though sometimes it is in form of a 50% saturated solution. It is very soluble in water with a lower solubility in ethanol and methanol but it 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 in which a large amount of heat is liberated posing potential danger as the corrosive solution may splash out. The resulting solution is usually colourless and odorless with slippery feeling upon contact in common with other alkalis.

ΔH°dissolution for aqueous dilution is −44.45 kJ / mol; from aqueous solutions at 12.3–61.8 °C, it crystallizes in monohydrate, with a melting point 65.1 °C and density of 1.829 g/cm3.[citation needed] The standard enthalpy change of formation (ΔH°form) of anhydrous solid NaOH is −425.61 kJ / mol.[5]

  Chemical properties

With acids

Sodium hydroxide reacts with protic acids to give water and the corresponding salts. For example, with hydrochloric acid, sodium chloride is formed:

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

In general such neutralization reactions are represented by one simple net ionic equation:

OH(aq) + H+(aq) → H2O(l)

This type of reaction with a strong acid releases heat, and hence is referred to as 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.

With acidic oxides

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,

2 NaOH + CO2Na2CO3 + H2O
With amphoteric metals and oxides

Sodium hydroxide slowly reacts with glass to form sodium silicate, so glass joints and stopcocks exposed to NaOH have a tendency to "freeze". Flasks and glass-lined chemical reactors are damaged by long exposure to hot sodium hydroxide, and the glass becomes frosted. 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,[6] 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:

2 Al + 2 NaOH + 2 H2O → 2 NaAlO2 + 3 H2

Unlike NaOH, the hydroxides of most transition metals are insoluble, and therefore sodium hydroxide can be used to precipitate transition metal hydroxides.

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 NaOH.

Al2(SO4)3 + 6 NaOH → 2 Al(OH)3 + 3 Na2SO4

NaOH can be used for the base-driven hydrolysis of esters (as in saponification), amides and alkyl halides. However, the limited solubility of NaOH in organic solvents means that the more soluble KOH is often preferred.


In the laboratory, with careful control of conditions, sodium metal can be isolated from the electrolysis of the molten monohydrate in a low temperature version of the Castner process, according to the following reaction:

4 NaOH·H2O(l) → 4 Na(l) + O2(g) + 6 H2O(g)

The monohydrate does not need to be heated in order to melt, as the process produces enough heat due to ohmic heating. However, it must be initiated with a small quantity of liquid water to create an electrically conductive electrolyte. As the system's temperature increases, the monohydrate will start to melt at about 65 °C as stated above. Only when the temperature reaches about 100 °C can sodium be isolated. Below this temperature, the water produced will react with the sodium, above this point, any water formed will be driven off in the vapour phase, creating an essentially anhydrous reaction. While this process has some advantages over other electrolytic processes, it is not preferred by most chemists for several reasons: a marginal quantity of sodium produced boils at the electrode interface, the vapour thus given off consists primarily of fumed sodium oxide, which tends to settle on any surface in close proximity with corrosive consequences.


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.[4]

In 2004, world production was estimated at 60 million dry metric tonnes of sodium hydroxide, and demand was estimated at 51 million tonnes.[4] In 1998, total world production was around 45 million tonnes. North America and Asia collectively 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 (which was purchased by Olin), Inc. (PIONA), and Formosa. All of these companies use the chloralkali process.[7]

Of historic interest is the Leblanc process, which produced sodium carbonate, followed by roasting to create carbon dioxide and sodium oxide, which readily absorbs water to create sodium hydroxide. This method is still occasionally used. It helped establish sodium hydroxide as an important commodity chemical. The Leblanc process was superseded by the Solvay process in the late 19th century.

Na2CO3 → Na2O + CO2
Na2O + H2O → 2 NaOH

Sodium hydroxide may be formed by the metathesis reaction between calcium hydroxide (also known as lime) and sodium carbonate (also known as soda ash):[8]

Ca(OH)2 + Na2CO3 → CaCO3 + 2 NaOH

  Chloralkali electrolysis

  Basic membrane cell used in the electrolysis of brine.

Sodium hydroxide is produced (along with chlorine and hydrogen) via the chloralkali process. This involves the electrolysis of an aqueous solution of sodium chloride. The sodium hydroxide builds up at the cathode, where water is reduced to hydrogen gas and hydroxide ion:

2 Na+ + 2 H2O + 2 e → H2 + 2 NaOH

More accurately:

2 Na+Cl + 2 H2O + 2 e → H2 + 2 Cl + 2 NaOH

The Cl ions are oxidized to chlorine gas at the anode.

To produce NaOH it is necessary to prevent reaction of the NaOH with the chlorine. This is typically done in one of three ways, of which the membrane cell process is economically the most viable.

  • Mercury cell process (also called the Castner-Kellner process); sodium ions are reduced to sodium metal, which forms an amalgam with a mercury cathode; this sodium is then reacted with water to produce NaOH. There have been concerns about mercury releases, although modern plants claim to be safe in this regard.[9]
  • Diaphragm cell process; uses a steel cathode, and the reaction of NaOH with Cl2 is prevented using a porous diaphragm, often made of asbestos fibers. In the diaphragm cell process the anode area is separated from the cathode area by a permeable diaphragm. The brine is introduced into the anode compartment and flows through the diaphragm into the cathode compartment. A diluted caustic brine leaves the cell. The sodium hydroxide must usually be concentrated to 50% and the salt removed. This is done using an evaporative process with about three tonnes of steam per tonne of sodium hydroxide. The salt separated from the caustic brine can be used to saturate diluted brine. The chlorine contains oxygen and is purified by liquefaction and evaporation.[10][11]
  • Membrane cell process; similar to the diaphragm cell process, with a Nafion membrane to separate the cathode and anode reactions. Only sodium ions and a little water pass through the membrane. It produces a higher quality of NaOH. Of the three processes, the membrane cell process requires the lowest consumption of electric energy and the amount of steam needed for concentration of the caustic is relatively small (less than one tonne per tonne of sodium hydroxide).[10][12]


  Canister of sodium hydroxide.

Sodium hydroxide is the principal strong base used in the chemical industry. In bulk it is most often handled as an aqueous solution, since solutions are cheaper and easier to handle. Sodium hydroxide, a strong base, is responsible for most of these applications. Another strong base such as potassium hydroxide is likely to yield positive results as well.

56 % of sodium hydroxide produced is used by the chemical industry, with 25 % of the same total used by the paper industry. Sodium hydroxide is also used for the manufacture of sodium salts and detergents, for pH regulation, and for organic synthesis. It is used in the Bayer process of aluminium production.[4]

Sodium hydroxide is used in many scenarios where it is desirable to increase the alkalinity of a mixture, or to neutralize acids.

For example, sodium hydroxide is used as an additive in drilling mud to increase alkalinity in bentonite mud systems, to increase the mud viscosity, and to neutralise any acid gas (such as hydrogen sulfide and carbon dioxide) which may be encountered in the geological formation as drilling progresses.

In the same industry, 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 give the 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.[13][14]

  Paper making

Sodium hydroxide was also widely used in making paper. Along with sodium sulfide, NaOH 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.

  Tissue digestion

In a similar fashion, sodium hydroxide is used to digest tissues, such 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 coffee-like[15][16] liquid, and the only solid that remains are bone hulls, which could be crushed between one's fingertips.[17] Sodium hydroxide is frequently used in the process of decomposing roadkill dumped in landfills by animal disposal contractors.[16]

  Dissolving amphoteric metals and compounds

Strong bases attack aluminium. Sodium hydroxide reacts with aluminium and water to release hydrogen gas. The aluminium takes the oxygen atom from sodium hydroxide (NaOH), 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.

  Esterification and transesterification reagent

Sodium hydroxide is traditionally used in soap making (cold process soap, saponification).[18] 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. It is used more often than potassium hydroxide because it is cheaper and a smaller quantity is needed.

  Cleaning agent

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 the 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 detergent 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[citation needed] 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.

  Food preparation

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; or, if soaked longer, for transformation into black olives. 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.[19]

Specific foods processed with sodium hydroxide include:

  • The Pinoy or Filipino dessert (kakanin) called kutsinta uses a bit of lye water to help give the rice flour batter a jelly like consistency.
  • A similar process is also used in the kakanin known as pitsi-pitsi or pichi-pichi (pit-chi-pit-chi) except that the mixture uses grated cassava instead of rice flour.
  • The Scandinavian delicacy known as lutefisk (from lutfisk, "lye fish").
  • Hominy is dried maize (corn) kernels reconstituted by soaking in lye-water. These expand considerably in size and may be further processed by frying to make corn nuts or by drying and grinding to make grits. Nixtamal is similar, but uses calcium hydroxide instead of sodium hydroxide.
  • Sodium hydroxide is also the chemical that causes gelling of egg whites in the production of Century eggs.
  • German pretzels are poached in a boiling sodium carbonate solution or cold sodium hydroxide solution before baking, which contributes to their unique crust.
  • Lye-water is an essential ingredient in the crust of the traditional baked Chinese moon cakes.
  • Most yellow coloured Chinese noodles are made with lye-water but are commonly mistaken for containing egg.
  • Some methods of preparing olives involve subjecting them to a lye-based brine.[20]

  Domestic uses

  Hardware store grade NaOH to be used as drain cleaner
  Paint stripping with caustic soda

Sodium hydroxide is used in the home as a drain cleaning agent for clearing clogged drains, usually in the form of a dry crystal or as a thick liquid gel. Sodium hydroxide saponifies the triglycerides found in fat into water soluble fatty acid salts and hydrolyzes the amide bonds in protein, such as those found in hair. These reactions are sped by the heat generated when sodium hydroxide and the other chemical components of the cleaner dissolve in water. Such drain cleaners and their acidic versions are highly caustic and should be handled with care.

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.


  Chemical burn caused by sodium hydroxide solution photographed 44 hours after exposure.

Like other corrosive acids and alkalis, drops of sodium hydroxide solution can decompose proteins and lipids in skin, eyes or other living tissues via amide hydrolysis and ester hydrolysis which consequently causes chemical burn and may induce permanent blindness if it contacts eyes.[2][3] Solid alkali may also express its corrosive nature if there is water so protective equipment such as rubber gloves, safety clothing and eye protection should always be used when handling the material or its solutions. Solutions containing sodium hydroxide with percentage by mass of equal or more than 2% are said to be labelled as "corrosive".[citation needed]

Moreover, dissolution of sodium hydroxide is highly exothermic, and the resulting heat may cause heat burns or ignite flammables. It also produces heat when reacted with acids.

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.

Sodium hydroxide is corrosive to some metals, e.g. aluminum, which produces flammable hydrogen gas on contact. Sodium hydroxide is also mildly corrosive to glass, which can cause damage to glazing or freezing of ground glass joints. Careful storage is needed.

  See also


  1. ^ a b "Sodium Hydroxide - Compound Summary". http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=14798&loc=ec_rcs. Retrieved June 12, 2012. 
  2. ^ a b c "Material Safety Datasheet". http://www.certified-lye.com/MSDS-Lye.pdf. 
  3. ^ a b c "Material Safety Datasheet 2". http://www.hillbrothers.com/msds/pdf/sodium-hydroxide-10-50-liq.pdf. 
  4. ^ a b c d Cetin Kurt, Jürgen Bittner (2005), "Sodium Hydroxide", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, DOI:10.1002/14356007.a24_345.pub2 
  5. ^ P.Atkins and J.de Paula Physical Chemistry, 8th edn. (W.H. Freeman 2006) p.1000
  6. ^ Stamell, Jim (2001), EXCEL HSC Chemistry, Pascal Press, pp. 199, ISBN 978-1-74125-299-6 
  7. ^ Kirk-Othmer Encyclopedia of Chemical Technology 5th edition (online, account needed), John Wiley & Sons. Accessed November 21, 2005.
  8. ^ Chemistry:Precision and Design.Verne Biddle, Gregory Parker.A Beka Book, Inc.
  9. ^ "Chlorine Online Diagram of mercury cell process". Euro Chlor. Archived from the original on 2006-09-02. http://web.archive.org/web/20060902010053/http://www.eurochlor.org/animations/mercury-cell.asp. Retrieved 2006-09-15. 
  10. ^ a b "Euro Chlor-How is chlorine made?". Euro Chlor. http://www.eurochlor.org/makingchlorine. Retrieved 2006-09-15. 
  11. ^ "Chlorine Online Diagram of diaphragm cell process". Euro Chlor. http://www.eurochlor.org/animations/diaphragm-cell.asp. Retrieved 2006-09-15. 
  12. ^ "Chlorine Online Diagram of membrane cell process". Euro Chlor. http://www.eurochlor.org/animations/membrane-cell.asp. Retrieved 2006-09-15. 
  13. ^ Sample, Ian (16 September 2009). "Trafigura case: toxic slop left behind by caustic washing". The Guardian. http://www.guardian.co.uk/environment/2009/sep/16/trafigura-case-toxic-slop. Retrieved 2009-09-17. 
  14. ^ "Trafigura knew of waste dangers". BBC Newsnight. 16 September 2009. http://news.bbc.co.uk/1/hi/programmes/newsnight/8259765.stm. Retrieved 2009-09-17. 
  15. ^ Ayres, Chris (27 February 2010) Clean green finish that sends a loved one down the drain Times Online. Retrieved 2010-03-08.
  16. ^ a b Thacker, H. Leon; Kastner, Justin (August 2004). Carcass Dispoal: A Comprehensive Review. Chapter 6. National Agricultural Biosecurity Center, Kansas State University, 2004. Retrieved 2010-03-08
  17. ^ Roach, Mary (2004). Stiff: The Curious Lives of Human Cadavers, New York: W.W. Norton & Company. ISBN 0-393-32482-6.
  18. ^ Morfit, Campbell (1856). A treatise on chemistry applied to the manufacture of soap and candles. Parry and McMillan. http://books.google.com/?id=XVBDAAAAIAAJ. 
  19. ^ "Hominy without Lye". National Center for Home Food Preservation. http://www.uga.edu/nchfp/publications/uga/hominy_without_lye.html. 
  20. ^ "Olives: Safe Methods for Home Pickling (application/pdf Object)". ucanr.org. 2010 [last update]. http://ucanr.org/freepubs/docs/8267.pdf. Retrieved January 22, 2012. 


  • Euro Chlor-How is chlorine made? Chlorine Online
  • Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Butterworth–Heinemann. ISBN 0080379419. 
  • Heaton, A. (1996) An Introduction to Industrial Chemistry, 3rd edition, New York:Blackie. ISBN 0-7514-0272-9.

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