Editing Sodium hydroxide (section)

==Uses==
{{see also|Hydrodesulfurization}}Sodium hydroxide is a popular strong [[base (chemistry)|base]] used 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 an [[aqueous solution]],<ref>{{cite web|url=http://www.ppg.com/chemicals/chloralkali/products/Documents/CausticSodamanual2008.pdf|title=Document 2 - CausticSodamanual2008.pdf|year=2013|access-date=July 17, 2014|archive-date=March 19, 2015|archive-url=https://web.archive.org/web/20150319003215/http://www.ppg.com/chemicals/chloralkali/products/Documents/CausticSodamanual2008.pdf|url-status=dead}}</ref> 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. Another use is in [[salt spray test]]ing where 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 quality [[crude oil]] can be treated with sodium hydroxide to remove [[sulfur]]ous impurities in a process known as ''caustic washing''. 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 [[2006 Côte d'Ivoire toxic waste dump|dumped the waste in Ivory Coast]].<ref name="Guardian">{{cite news|url=https://www.theguardian.com/environment/2009/sep/16/trafigura-case-toxic-slop|title=Trafigura case: toxic slop left behind by caustic washing|last=Sample|first=Ian|date=16 September 2009 |newspaper=The Guardian|access-date=2009-09-17}}</ref><ref name="bbc">{{cite news|url=http://news.bbc.co.uk/1/hi/programmes/newsnight/8259765.stm|title=Trafigura knew of waste dangers|date=16 September 2009|publisher=BBC Newsnight|access-date=2009-09-17}}</ref>

Other common uses of sodium hydroxide include:
* for making soaps and detergents. Sodium hydroxide is used for hard bar soap, while [[potassium hydroxide]] is used for liquid soaps.<ref>{{cite web|title=A Guide to Caustic Chemicals Used in Soap Making {{!}} Brenntag|url=https://www.brenntag.com/en-us/industries/household-industrial-institutional/chemicals-in-liquid-soap/|access-date=2020-10-03|website=www.brenntag.com|language=en}}</ref><ref>{{cite web|date=2016-09-06|title=Sodium Hydroxide {{!}} Uses, Benefits, and Chemical Safety Facts|url=https://www.chemicalsafetyfacts.org/sodium-hydroxide/|access-date=2020-10-03|website=ChemicalSafetyFacts.org|language=en-US}}</ref> Sodium hydroxide is used more often than [[potassium hydroxide]] because it is cheaper and a smaller quantity is needed.
* as drain cleaners that convert pipe-clogging fats and grease into soap, which dissolves in water
* for making artificial textile fibres such as [[rayon]]
* in the manufacture of [[paper]]. Around 56% of sodium hydroxide produced is used by industry, 25% of which is used in the paper industry.
* in purifying [[Bauxite|bauxite ore]] from which [[aluminium]] metal is extracted. This is known as the [[Bayer process]].
* de-greasing metals
* [[oil refining]]
* making [[dye]]s and [[bleach]]es
* in water treatment plants for pH regulation
* to treat bagels and pretzel dough, giving the distinctive shiny finish

===Chemical pulping===
{{main|Pulp (paper)}}
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]] [[fiber]]s in the [[kraft process]]. It also plays a key role in several later stages of the process of [[Bleaching of wood pulp|bleaching the brown pulp]] resulting from the pulping process. These stages include [[oxygen]] delignification, [[oxidation|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, 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,<ref name="Ayres">Ayres, Chris (27 February 2010) [https://www.thetimes.com/article/clean-green-finish-that-sends-a-loved-one-down-the-drain-9qdc6bjbfl8 Clean green finish that sends a loved one down the drain] Times Online. Retrieved 2013-02-20.</ref><ref name="carcassdisposal">Thacker, H. Leon; Kastner, Justin (August 2004). [https://krex.k-state.edu/dspace/bitstream/handle/2097/662/Chapter6.pdf ''Carcass Disposal: A Comprehensive Review. Chapter 6''] {{Webarchive|url=https://web.archive.org/web/20170202034816/https://krex.k-state.edu/dspace/bitstream/handle/2097/662/Chapter6.pdf |date=2017-02-02 }}. National Agricultural Biosecurity Center, Kansas State University, 2004. Retrieved 2010-03-08</ref> and the only solids that remain are bone hulls, which can be crushed between one's fingertips.<ref name="Roach">Roach, Mary (2004). ''Stiff: The Curious Lives of Human Cadavers'', New York: W.W. Norton & Company. {{ISBN|0-393-32482-6}}.</ref>

Sodium hydroxide is frequently used in the process of decomposing [[roadkill]] dumped in landfills by animal disposal contractors.<ref name="carcassdisposal" /> Due to its availability and low cost, it has been used by criminals to dispose of corpses. Italian [[serial killer]] [[Leonarda Cianciulli]] used this chemical to turn dead bodies into soap.<ref>{{cite news |url= https://www.bbc.co.uk/news/magazine-27257822 |title= Sodium:Getting rid of dirt – and murder victims |date= 3 May 2014 |work= [[BBC News]]}}</ref> In Mexico, a man who worked for drug cartels admitted disposing of over 300 bodies with it.<ref>{{cite news | url = https://www.washingtonpost.com/wp-dyn/content/article/2009/01/26/AR2009012602190.html | author = William Booth | title = 'Stewmaker' Stirs Horror in Mexico | newspaper = [[Washington Post]] | date = January 27, 2009}}</ref>

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.<ref>{{cite web|url=http://www.atsdr.cdc.gov/MMG/MMG.asp?id=246&tid=45|archive-url=https://web.archive.org/web/20100528070612/http://www.atsdr.cdc.gov/MMG/MMG.asp?id=246&tid=45|url-status=dead|archive-date=May 28, 2010|title=ATSDR – Medical Management Guidelines (MMGs): Sodium Hydroxide|website=www.atsdr.cdc.gov}}</ref>

===Dissolving amphoteric metals and compounds===
Strong bases attack [[aluminium]]. 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 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.

:{{chem2|2 Al + 2 }} {{chem2|NaOH + 2 H2O}} → 2 {{chem2| NaAlO2}} + {{chem2|3 H2}}

Sodium aluminate is an inorganic chemical that is used as an effective source of [[aluminium hydroxide]] for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as {{chem2|NaAlO2}}, {{chem2|Na3AlO3}}, {{chem2|Na[Al(OH)4]}}, {{chem2|Na2O*Al2O3}} or {{chem2|Na2Al2O4}}. Formation of sodium tetrahydroxoaluminate(III) or hydrated sodium aluminate is given by:<ref>{{cite web|last=PubChem|title=Aluminium sodium tetrahydroxide|url=https://pubchem.ncbi.nlm.nih.gov/compound/166673|access-date=2020-10-03|website=pubchem.ncbi.nlm.nih.gov|language=en}}</ref>

:{{chem2|2 Al + 2 NaOH + 6 H2}}O {{chem2|→ 2 Na[Al(OH)4] + 3 H2}}

This reaction can be useful in [[etching]], removing anodizing, or converting a polished surface to a satin-like finish, but without further [[Passivation (chemistry)|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 via the [[electrolysis|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]]).<ref name="Morfit">{{cite book|title =A treatise on chemistry applied to the manufacture of soap and candles|first= Campbell|last=Morfit|author-link=Campbell Morfit|publisher=Parry and McMillan|year = 1856|url =https://archive.org/details/atreatiseonchem00morfgoog}}</ref> 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. Due to production costs, NaOH, which is produced using common salt is cheaper than potassium hydroxide.<ref>{{cite web|title=Side by Side Comparison: Potassium Hydroxide and Sodium Hydroxide - Similarities, Differences and Use Cases|url=https://info.noahtech.com/blog/sicomparison-potassium-hydroxide-and-sodium-hydroxide|access-date=2020-10-03|website=info.noahtech.com|language=en-us|archive-date=2020-10-10|archive-url=https://web.archive.org/web/20201010090833/https://info.noahtech.com/blog/sicomparison-potassium-hydroxide-and-sodium-hydroxide|url-status=dead}}</ref>

===Skincare ingredient===
Sodium hydroxide is an ingredient used in some [[skin care]] and [[Cosmetics|cosmetic]] products, such as facial cleansers, creams, lotions, and makeup. It is typically used in low concentration as a [[PH|pH balancer]], due its highly alkaline nature.<ref>{{cite web |url= https://www.cir-safety.org/sites/default/files/inorganic%20hydroxides.pdf |title= Safety Assessment of Inorganic Hydroxides as Used in Cosmetics |work=cir-safety.org | year=2015 |access-date=May 12, 2024}}</ref>

===Food preparation===
Food uses of sodium hydroxide include washing or chemical peeling of [[fruits]] and [[vegetables]], [[chocolate]] and [[Cocoa mass|cocoa]] processing, [[caramel coloring]] production, [[poultry]] scalding, [[soft drink]] processing, and thickening [[ice cream]].<ref>{{cite web |url= http://www.rsc.org/chemistryworld/podcast/CIIEcompounds/transcripts/NaOH.asp |title=Sodium Hydroxide |work=rsc.org | year=2014 |access-date=November 9, 2014}}</ref> [[Olive]]s are often soaked in sodium hydroxide for softening; [[pretzel]]s and German [[lye roll]]s 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.<ref name="Hominy">{{cite web| url=http://www.uga.edu/nchfp/publications/uga/hominy_without_lye.html| title=Hominy without Lye| publisher=National Center for Home Food Preservation| access-date=2019-01-15| archive-date=2011-09-11| archive-url=https://web.archive.org/web/20110911000538/http://www.uga.edu/nchfp/publications/uga/hominy_without_lye.html| url-status=dead}}</ref> It is known as [[E number]] E524.

Specific foods processed with sodium hydroxide include:
* German [[pretzel]]s 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.
* One variety of [[zongzi]] uses lye water to impart a sweet flavor.
* Sodium hydroxide causes gelling of egg whites in the production of [[century egg]]s.
* Some methods of preparing olives involve subjecting them to a lye-based brine.<ref>{{cite web |url=http://ucanr.org/freepubs/docs/8267.pdf |title=Olives: Safe Methods for Home Pickling (application/pdf Object) |work=ucanr.org |year=2010 |access-date=January 22, 2012 |archive-date=January 17, 2012 |archive-url=https://web.archive.org/web/20120117053826/http://ucanr.org/freepubs/docs/8267.pdf |url-status=dead }}</ref>
* The Filipino dessert ({{langx|fil|kakanin}}) called {{lang|fil|[[kutsinta]]}} uses a small quantity 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 {{lang|fil|[[pitsi-pitsi]]}} or {{lang|fil|pichi-pichi}} except that the mixture uses grated [[cassava]] instead of rice flour.
* The [[Norway|Norwegian]] dish known as [[lutefisk]] ({{langx|no|lutfisk|lit=lye fish}}).
* [[Bagel]]s are often boiled in a lye solution before baking, contributing to their shiny crust.
* [[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]]. Hominy is used to create [[masa]], a popular flour used in Mexican cuisine to make [[corn tortillas]] and [[tamales]]. [[Nixtamal]] is similar, but uses [[calcium hydroxide]] instead of sodium hydroxide.

===Cleaning agent===
{{Main|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 process equipment, storage tanks, etc. It can dissolve [[grease (lubricant)|grease]], [[oils]], [[fat]]s 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]] [[detergent]]s. 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{{Citation needed|date=May 2010}} when [[1,1,1-Trichloroethane|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.

[[File:NaOH - drain-cleaner.jpg|thumb|left|upright|[[Hardware store]]s grade sodium hydroxide to be used as a type of [[Chemical drain cleaners#Alkaline drain openers|drain cleaner]].]]
[[File:Paint stripping with caustic soda.jpg|thumb|upright|Paint stripping with caustic soda]]
Sodium hydroxide is used in the home as a type of [[chemical drain cleaners#Alkaline drain openers|drain openers]] to unblock clogged drains, usually in the form of a dry crystal or as a thick liquid gel. The alkali reacts with [[fat|greases]] to produce [[water soluble]] soap and [[glycerol]]. It also [[hydrolysis|hydrolyzes]] [[proteins]], such as those found in [[hair]], which may block waste water pipes. Dissolving sodium hydroxide in water is an [[exothermic]] reaction producing considerable quantities of heat which assists in speeding up the reactions with grease and other organic matter. Such [[chemical drain cleaners#Alkaline drain openers|alkaline drain cleaners]] and their [[chemical drain cleaners#Acidic drain openers|acidic versions]] are highly [[corrosive]] and should be handled with great caution.

=== Relaxer ===
Sodium hydroxide is used in some [[relaxer]]s 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 consumers. Sodium hydroxide relaxers are still available, but they are used mostly by professionals.

=== Paint stripper ===
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.

===Water treatment===
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.<ref>{{cite web|url=http://articles.extension.org/pages/32302/drinking-water-treatment-ph-adjustment|title=Drinking Water Treatment – pH Adjustment|year=2011|access-date=June 23, 2016|archive-date=August 10, 2018|archive-url=https://web.archive.org/web/20180810194639/http://articles.extension.org/pages/32302/drinking-water-treatment-ph-adjustment|url-status=dead}}</ref><ref>{{cite web|url=http://www.water-research.net/index.php/drinking-water-issues-corrosive-water-lead-copper-aluminum-zinc-and-more|author=Brian Oram, PG|title=Drinking Water Issues Corrosive Water (Lead, Copper, Aluminum, Zinc and More)|year=2014|access-date=June 23, 2016|archive-date=July 1, 2016|archive-url=https://web.archive.org/web/20160701214918/http://www.water-research.net/index.php/drinking-water-issues-corrosive-water-lead-copper-aluminum-zinc-and-more|url-status=dead}}</ref>

===Historical uses===
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.<ref>[https://books.google.com/books?id=2df7-uOC8ZwC&pg=PA168 Page 168] in: ''The Detection of poisons and strong drugs''.
Author: Wilhelm Autenrieth.
Publisher: P. Blakiston's son & Company, 1909.</ref> Today, carbon monoxide poisoning can be detected by [[CO oximetry]].

=== In cement mixes, mortars, concrete, grouts ===
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.