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
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
cations and
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. ...
anions .
Sodium hydroxide is a highly
caustic
Caustic most commonly refers to:
* Causticity, a property of various corrosive substances
** Sodium hydroxide, sometimes called ''caustic soda''
** Potassium hydroxide, sometimes called ''caustic potash''
** Calcium oxide, sometimes called ''caust ...
base and
alkali that decomposes
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s at ordinary ambient
temperatures and may cause severe
chemical burns. It is highly soluble in
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
, and readily absorbs
moisture and
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
from the
air. It forms a series of
hydrates .
The monohydrate 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, sodium hydroxide is frequently used alongside neutral
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
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
Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distribu ...
,
textiles,
drinking water
Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, ...
,
soap
Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are us ...
s and
detergents, and as a
drain cleaner. Worldwide production in 2004 was approximately 60 million tons, while demand was 51 million tons.
Properties
Physical properties
Pure sodium hydroxide is a colorless crystalline solid that melts at without decomposition, and with a boiling point of . It is highly soluble in water, with a lower solubility in
polar
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
*Polar climate, the cli ...
solvents such as
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
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.
Viscosity
Concentrated (50%) aqueous solutions of sodium hydroxide have a characteristic
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the int ...
, 78 m
Pa·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 aqueous NaOH, as with any liquid chemical, is inversely related to its service 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.
Hydrates
Sodium hydroxide can form several hydrates , 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:
*Heptahydrate, : from −28 °C (18.8%) to −24 °C (22.2%).
*Pentahydrate, : from −24 °C (22.2%) to −17.7 (24.8%).
*Tetrahydrate, , α form: from −17.7 (24.8%) to +5.4 °C (32.5%).
*Tetrahydrate, , β form: metastable.
*Trihemihydrate, : from +5.4 °C (32.5%) to +15.38 °C (38.8%) and then to +5.0 °C (45.7%).
*Trihydrate, : metastable.
*Dihydrate, : from +5.0 °C (45.7%) to +12.3 °C (51%).
*Monohydrate, : from +12.3 °C (51%) to 65.10 °C (69%) then to 62.63 °C (73.1%).
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 (65.10 °C) and (15.38 °C). The other hydrates, except the metastable ones and (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 hot water solution containing 73.1% (mass) of NaOH is a
eutectic that 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 heptahydrate .
[M. Conde Engineering:]
Solid-Liquid Equilibrium (SLE) and Vapour-Liquid Equilibrium (VLE) of Aqueous NaOH
. Online report, accessed on 2017-04-29.
When solutions with less than 18.4% NaOH are cooled, water
ice crystallizes first, leaving the NaOH in solution.
The α form of the tetrahydrate has density 1.33 g/cm
3. It melts congruously at 7.55 °C into a liquid with 35.7% NaOH and density 1.392 g/cm
3, 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 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/cm
3). Physical data in technical literature may refer to this form, rather than the anhydrous compound.
Crystal structure
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 = 0.6069
nm. The atoms are arranged in a
hydrargillite-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 by
hydrogen bonds between water molecules.
Chemical properties
Reaction with acids
Sodium hydroxide reacts with protic acids to produce water and the corresponding salts. For example, when sodium hydroxide reacts with
hydrochloric acid,
sodium chloride is formed:
:
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
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
from air.
Reaction with acidic oxides
Sodium hydroxide also reacts with
acidic oxides, such as
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic a ...
. Such reactions are often used to "
scrub" harmful acidic gases (like and ) produced in the burning of coal and thus prevent their release into the atmosphere. For example,
:
Reaction with metals and oxides
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
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
at room temperatures, since iron does not have
amphoteric properties (i.e., it only dissolves in acid, not base).
Nevertheless, at high temperatures (e.g. above 500 °C), iron can react
endothermically with sodium hydroxide to form
iron(III) oxide,
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
metal, and
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
gas. This is due to the lower
enthalpy of formation of iron(III) oxide (−824.2 kJ/mol) compared to sodium hydroxide (-500 kJ/mol) and positive entropy change of reaction, which imply spontaneity at high temperatures (, ) and non-spontaneity at low temperatures (, ). Consider the following reaction between molten sodium hydroxide and finely divided iron filings:
:
A few
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s, however, may react vigorously with sodium hydroxide under milder conditions.
In 1986, an aluminium
road tanker
A tank truck, gas truck, fuel truck, or tanker truck (American English) or tanker (British English) is a motor vehicle designed to carry liquids or gases on roads. The largest such vehicles are similar to railroad tank cars, which are also des ...
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:
:
Precipitant
Unlike sodium hydroxide, which is soluble, 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:
*Copper - blue
*Iron(II) - green
*Iron(III) - yellow / brown
Zinc and lead salts dissolve in excess sodium hydroxide to give a clear solution of or .
Aluminium hydroxide
Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic ...
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.
:
:
Saponification
Sodium hydroxide can be used for the base-driven
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
of
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...
s (as in
saponification),
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...
s 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 a sodium hydroxide solution with bare hands, while not recommended, produces a slippery feeling. This happens because oils on the skin such as
sebum
A sebaceous gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands occur in the greatest nu ...
are converted to soap.
Despite solubility in propylene glycol it is unlikely to replace water in saponification due to propylene glycol's primary reaction with fat before reaction between sodium hydroxide and fat.
Production
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 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 Olin, which has annual production around 5.7 million
tonnes from sites at
Freeport, Texas, and
Plaquemine, Louisiana, St Gabriel, Louisiana, McIntosh, Alabama, Charleston, Tennessee, Niagara Falls, New York, and Becancour, Canada. Other major US producers include
Oxychem,
Westlake, Shintek and
Formosa. All of these companies use the
chloralkali process.
[''Kirk-Othmer Encyclopedia of Chemical Technology''](_blank)
5th edition, John Wiley & Sons.
Historically, sodium hydroxide was produced by treating
sodium carbonate with
calcium hydroxide in a
metathesis reaction which takes advantage of the fact that sodium hydroxide is soluble, while calcium carbonate is not. This process was called causticizing.
:
This process was superseded by the
Solvay process in the late 19th century, which was in turn supplanted by the
Leblanc process and then
chloralkali process which is in use today.
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 commercially viable, as the isolation of sodium metal is typically performed by reduction or electrolysis of sodium compounds including sodium hydroxide.
Uses
Sodium hydroxide is a popular strong
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, 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
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the int ...
, and to neutralize any
acid gas Acid gas is a particular typology of natural gas or any other gas mixture containing significant quantities of hydrogen sulfide (H2S), carbon dioxide (CO2), or similar acidic gases. A gas is determined to be acidic or not after it is mixed with w ...
(such as
hydrogen sulfide and
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
) which may be encountered in the
geological formation as drilling progresses.
Another use is in
Salt spray testing 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
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 Ivory Coast.
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. Sodium hydroxide is used more often than
potassium hydroxide because it is cheaper and a smaller quantity is needed.
*as drain cleaners that contain sodium hydroxide convert fats and grease that can clog pipes into soap, which dissolves in water ''(see
cleaning agent)''.
*for making artificial textile fibres (such as
Rayon).
*in the manufacture of
paper
Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distribu ...
. Around 56% of sodium hydroxide produced is used by industry, 25% of which is used in the paper industry ''(see
chemical pulping
Paper chemicals designate a group of chemicals that are used for paper manufacturing, or modify the properties of paper. These chemicals can be used to alter the paper in many ways, including changing its color and brightness, or by increasing i ...
)''.
*in purifying
bauxite ore from which
aluminium metal is extracted. This is known as
Bayer process
The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide) and was developed by Carl Josef Bayer. Bauxite, the most important ore of aluminium, contains only 30–60% aluminium oxide (Al2O3), th ...
''(see
dissolving amphoteric metals and compounds)''.
*in de-greasing metals,
oil refining, and making
dye
A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...
s and
bleaches.
*in water treatment plants for pH regulation.
*to treat bagels and pretzel dough, giving the distinctive shiny finish.
Chemical pulping
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
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
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
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
delignification,
oxidative
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
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,
[Ayres, Chris (27 February 2010]
Clean green finish that sends a loved one down the drain
Times Online. Retrieved 2013-02-20.[Thacker, H. Leon; Kastner, Justin (August 2004)]
''Carcass Disposal: A Comprehensive Review. Chapter 6''
National Agricultural Biosecurity Center, Kansas State University, 2004. Retrieved 2010-03-08 and the only solids that remain are bone hulls, which can be crushed between one's fingertips.
[Roach, Mary (2004). ''Stiff: The Curious Lives of Human Cadavers'', New York: W.W. Norton & Company. .]
Sodium hydroxide is frequently used in the process of decomposing
roadkill dumped in landfills by animal disposal contractors.
Due to its availability and low cost, it has been used by criminals to dispose of corpses. Italian
serial killer
A serial killer is typically a person who murders three or more persons,A
*
*
*
* with the murders taking place over more than a month and including a significant period of time between them. While most authorities set a threshold of three ...
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.
Dissolving amphoteric metals and compounds
Strong bases attack
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
. 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
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
gas and
sodium 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 of
aluminium hydroxide
Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic ...
for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as , , , or . Formation of sodium tetrahydroxoaluminate(III) or hydrated sodium aluminate is given by:
:
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
The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide) and was developed by Carl Josef Bayer. Bauxite, the most important ore of aluminium, contains only 30–60% aluminium oxide (Al2O3), th ...
, 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
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of w ...
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).
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
Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are us ...
, 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.
Food preparation
Food uses of sodium hydroxide include washing or chemical peeling of
fruits and
vegetables,
chocolate and
cocoa
Cocoa may refer to:
Chocolate
* Chocolate
* ''Theobroma cacao'', the cocoa tree
* Cocoa bean, seed of ''Theobroma cacao''
* Chocolate liquor, or cocoa liquor, pure, liquid chocolate extracted from the cocoa bean, including both cocoa butter an ...
processing,
caramel coloring production,
poultry
Poultry () are domesticated birds kept by humans for their eggs, their meat or their feathers. These birds are most typically members of the superorder Galloanserae (fowl), especially the order Galliformes (which includes chickens, qu ...
scalding,
soft drink processing, and thickening
ice cream
Ice cream is a sweetened frozen food typically eaten as a snack or dessert. It may be made from milk or cream and is flavoured with a sweetener, either sugar or an alternative, and a spice, such as cocoa or vanilla, or with fruit such as ...
.
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.
It is known as
E number E524.
Specific foods processed with sodium hydroxide include:
*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.
*One variety of
zongzi uses lye water to impart a sweet flavor.
*Sodium hydroxide is also the chemical that causes gelling of egg whites in the production of
Century eggs.
*Some methods of preparing olives involve subjecting them to a lye-based brine.
*The Filipino dessert ( fil, kakanin) called 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 or except that the mixture uses grated
cassava instead of rice flour.
*The
Norwegian dish known as
lutefisk ( no, lutfisk, lit=lye fish).
*
Bagel
A bagel ( yi, בײגל, translit=beygl; pl, bajgiel; also spelled beigel) is a bread roll originating in the Jewish communities of Poland. It is traditionally shaped by hand into a roughly hand-sized ring from yeasted wheat dough that is fi ...
s are often boiled in a lye solution before baking, contributing to their shiny crust.
*
Hominy is dried
maize
Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American English, North American and Australian English), is a cereal grain first domesticated by indigenous peoples of Mexico, indigenous ...
(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
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
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
-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
Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...
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 Trichloroethane (CHCl) may refer to either of two isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in sp ...
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
A drain cleaner is method for unblocking sewer pipes or clogged wastewater drains. The term may also refer to a mechanical device such as a plumber's snake, drain auger, toilet plunger, or similar device. Occasionally, the term is applied to a plu ...
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
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solub ...
products. It also
hydrolyzes
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysis ...
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.
Relaxer
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 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.
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. 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.
Experimental
Flavonoids
See:
Sodium hydroxide test for flavonoids
Summer-winter heat storage
After decades of research,
EMPA researchers and others are experimenting with concentrated sodium hydroxide (NaOH) as the thermal storage or seasonal reservoir medium for power plants and 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 becomes more concentrated and thus stores the supplied heat as
latent chemical energy.
Neutron moderator
Seaborg Technologies is working on a reactor design in which NaOH is used as a neutron moderator,
Safety
Like other
corrosive acids and
alkalis, drops of sodium hydroxide solutions can readily decompose
proteins and
lipids
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
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
Personal protective equipment (PPE) is protective clothing, helmets, goggles, or other garments or equipment designed to protect the wearer's body from injury or infection. The hazards addressed by protective equipment include physical, ...
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.
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.
Sodium hydroxide is also mildly corrosive to
glass
Glass is a non- crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenchin ...
, which can cause damage to
glazing or cause
ground glass joint
Ground glass joints are used in laboratories to quickly and easily fit leak-tight apparatus together from interchangeable commonly available parts. For example, a round bottom flask, Liebig condenser, and oil bubbler with ground glass joints ...
s to bind. Sodium hydroxide is corrosive to several metals, like
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
which reacts with the alkali to produce flammable
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
gas on contact:
Storage
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, within
intermediate bulk container
Intermediate bulk containers (also known as IBC tank, IBC tote, IBC, or pallet tank) are industrial-grade containers engineered for the mass handling, transport, and storage of liquids, semi-solids, pastes, or solids. The two main categories of I ...
s (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
Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...
, and
fiberglass reinforced plastic (FRP, with a resistant liner).
Sodium hydroxide must be stored in airtight containers to preserve its
normality as it will absorb water from the atmosphere.
History
Sodium hydroxide was first prepared by soap makers.
[Thorpe, Thomas Edward, ed., ''A Dictionary of Applied Chemistry'' (London, England: Longmans, Green, and Co., 1913), vol. 5]
/ref> A procedure for making sodium hydroxide appeared as part of a recipe for making soap in an Arab book of the late 13th century: (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: , where is ash from Glasswort, 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) was replaced by this artificial version. However, by the 20th century, the electrolysis of sodium chloride had become the primary method for producing sodium hydroxide.[O'Brien, Thomas F.; Bommaraju, Tilak V. and Hine, Fumio (2005) ''Handbook of Chlor-Alkali Technology'', vol. 1. Berlin, Germany: Springer. Chapter 2: History of the Chlor-Alkali Industry, p. 34. ]
See also
* Acid and base
* HAZMAT Class 8 Corrosive Substances
* List of cleaning agents
References
Bibliography
*
External links
International Chemical Safety Card 0360
*Euro Chlor-How is chlorine made
Chlorine Online
CDC – Sodium Hydroxide – NIOSH Workplace Safety and Health Topic
*Data sheets
Technical charts (page 33—41)
for enthalpy, temperature and pressure
Sodium Hydroxide MSDS
Certified Lye MSDS
Hill Brothers MSDS
*[https://web.archive.org/web/20150508191719/http://www.inclusive-science-engineering.com/inorganic-chemical-caustic-soda-production-process-description-and-flowsheet/ Caustic soda production in continuous causticising plant by lime soda process]
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